این DAC 4 شرکت EAR احتمالا نیاز به یه رابط SPDIF داره. من بین همه این رابط ها از WaveLink HS خوشم اومده که در زیر میبینید :
اول این لینک هارو دقیق بخونید خیلی خوبن :
http://www.stereophile.com/content/pdm-pwm-delta-sigma-1-bit-dacs#uvK84ec8zX22B853.97
http://www.thewelltemperedcomputer.com/index.html
http://www.mojo-audio.com/blog/
Thorsten Loesch
http://www.audiostream.com/content/qa-thorsten-loesch-amrifi#lj9GAqmbHhKALb2T.97
Gordon Rankin
http://www.audiostream.com/content/qa-wavelength-audios-gordon-rankin#H5fQlcaW35sB3Gl2.97
http://www.6moons.com/audioreviews/wavelength2/brick_2.html
John Swenson
http://www.audiostream.com/content/qa-john-swenson-part-1-what-digital#mh5Q2RTVxyOBCJRK.97
http://www.audiostream.com/content/qa-john-swenson-part-2-are-bits-just-bits
http://www.audiostream.com/content/qa-john-swenson-part-3-how-bit-perfect-software-can-affect-sound
Steve Nugent
http://www.audiostream.com/content/qa-steve-nugent-empirical-audio
Benjamin Zwickel
http://www.mojo-audio.com/blog/computer-audio-misconceptions/
Pedja Rogic
https://www.audialonline.com/topics/
http://www.monoandstereo.com/2008/04/interview-with-pedja-rogic-of-audial.html
این روزها در مورد DAC و موضوع حذف نویز و جیتر از PC میخونم. تورستن و گوردون و ژان و استو بیشترین تحقیقات رو تو زمینه گرفتن بهترین نتیجه از PC داشتند با ارتباط USB . دو مورد هست یک ایزوله کردن (Galvanic USB Isolation) گالوانیک نویز PC از DAC که هر چقدر ایزوله تر باشه بهتره اما با امکانات این صنعت (دو راه یکی کوپل نوری که جیتر بیشتری داره و دیگری استفاده از چیپ های Analog Device مثل ADUM4160 که هم محدودیت 96کیلوهرتز دارند و هم جیتر بیشتر) عملا هر چقدر ایزولیشن بیشتر بشه جیتر هم بیشتر میشه و موضوع مهم دیگر برای صدای خوب حذف و کاهش جیتر که اونم کار راحتی نیست.
First isolation is a good thing to have. I run a CG filter from my main power and put all the computer stuff on one side and all the analog on another. That way the switching computer noise has 2 filters to jump thru before it gets to the analog side.
I isolate at the I2S level. We found over the years that this was the best place for it. We reclock all the signals on the DAC side so the jitter is very low.
Using the Analog Devices ADUM series… well requires some engineering. First it is not setup to drive cables. These chips were meant to be on the board of the isolating endpoint. They also don’t sound that good… not sure why… maybe the same reason some USB cables don’t sound good. They are limited in speed to Full Speed or 24/96.
Most of the current asynchronous or even adaptive have no isolation. Anything that is self powered cannot have isolation…. well SPDIF converters can because they can have a transformer or optical output. But then again as pointed out above SPDIF leads to more jitter. This does not mean they are bad. These just may require a little more setup and adjustments than those that are.
I really would not get hung up on this as much as getting something that sounds good to you. Visit a dealer or have a dealer send you something to hear. Don’t buy by numbers or what people say here. Nobody listens the same or has the same system, room etc…
have fun!
Gordon
فعلا با نتایج من تو اینترنت بهترین حالت Computer Audio گرفتن دیتا از asynchronous usb کامپیوتر هست.
DAC بهتره و شاید لازمه که True Multi-Bit باشه یعنی از سیستم R-2R استفاده کنه و نه از سیستم Delta Sigma و حتی اگر عبارت Multi-Bit رو قبل Delta Sigma دیدید بدونید اون دک مالتی بیت واقعی نیست.
https://www.hifi.ir/wp-content/uploads/2016/07/Best-Performance.pdf
http://www.mojo-audio.com/blog/optimizing-os-x-yosemite/
So you have an Apple computer running OS X Yosemite and you want to optimize performance. I’ve got you covered. If you prefer to run an older version of OS X, I have another optimization blog that covers Snow Leopard through Mountain Lion.
Less is more…
It’s obvious Steve Jobs is dead. Forget about the fact that when Yosemite was first released it had so many bugs and conflicts you would think it was engineered by Microsoft. Apple was on the cutting edge of professional audio and video production, photography, and CAD, but has now redirected their marketing efforts toward the average social networking, iCloud using, multiple mobile device consumer.
Yosemite emphasizes a plethora of “eye candy” aesthetic features, automated multi-device and multi-platform integration, and organizational apps such as Mission Control, Dashboard, and Widgets.
The more of these optional processes you remove, the more you’ll improve Yosemite’s performance.
My company, Mojo Audio, manufactures and sells high-end computerized music and video servers for the home theater enthusiast and recording/editing professional. Though this blog was written to assist our customers in optimizing the performance of Yosemite running on their upgraded Mac Mini media servers, these same optimizations will work on any Mac running Yosemite.
I’ll walk you through exactly what you need to do to disable memory resident programs that automatically load by default when Yosemite boots up. And I’ll also show you how to turn off optional wireless control interfaces, such as infrared, WiFi, and Bluetooth.
When you disable these optional processes and reallocate system resources, your computer does less switching, swapping, interrupting, and error correcting. Freeing up system resources and turning off unused wireless control interfaces results in noticeably more efficient performance which translates to a more fluid and coherent audio and video presentation.
Warning! I recommend that before making any changes or updating software in any computer that you backup your drives. Using “cloning” software, such as Carbon Copy Cloner, creates a bootable backup.
It’s a perfect use of your time . . . you can clone your drives while you review the rest of this guide.
It’s often easier to start with a clean slate than to figure out how to turn off and undo things we may have unknowingly turned on.
You did remember to back up your data and boot drives, right?
When you do a clean install you’re going to erase everything, so now’s a good time to make and test (yes, test) your system backups.
Warning! When doing a clean install do not activate any automated features. Activate only one web browser, preferably Safari. Don’t activate any optional features or plug-ins, such as Google Chrome. BTW, Chrome and Yosemite don’t play nice together, so I wouldn’t recommend using Chrome under any circumstances.
Here is a link to a guide on how to do a clean install.
Now that you’re back to clean, and nothing optional has been added, I’ll show you how to turn off a series of default resident processes that automatically boot up with Yosemite.
Safari does all sorts of things automatically to enhance your browsing experience. Of course, anything automatic consumes system resources. One option would be installing software, such as ClicktoPlugin, that prevents Safari from automatically loading plugs-ins.
Software like ClicktoPlugin not only speeds up browsing, but also reduces fan speed and extends battery life. It also improves performance of audio and video streaming.
The way it works is that it replaces any plug-in object with an unobtrusive placeholder that can be clicked on to load the embedded content of the plug-in. Now plug-ins only activate manually.
Another option you have instead of or in addition to using a plug-in management Application would be to uninstall Adobe Flash. Without Flash and plug-ins, Safari performs better.
Remove any files, folders, or infrequently used Dock icons. These convenience and organizational options slow down performance by requiring the system to constantly redraw them on the screen.
For a minimized Dock, the only icons you need are Finder and Trash. The rest of the icons can be deleted. Some people like to keep Launchpad in their Dock for easier access to Applications.
To keep icons in your Dock, opposite click on the icon and select Keep in Dock. If you use an Application all the time and want it to open whenever you restart your computer, just opposite click on the icon and select Open at Login.
If you are optimizing for audio or video, you want your player and/or recording software to automatically open at Login. For desktop use, this could be accounting software, CAD software, and/or a web browser.
Applications automatically opening when the computer restarts is very important to restore function for media servers or home automation.
Close all Applications
Before applying any optimizations, make certain the only application running is Finder.
The Force Quit window will allow you to view all applications that are running.
Go into the Apple Menu, drop-down menu, and select Force Quit.
Click on and highlight an application. Press the Force Quit button in the lower right corner. Repeat until the only application shown is Finder.
Disabling “eye candy” such as Transparency and turning off automated features such as Widgets will improve Yosemite performance.
Reduce Transparency:
Go into the Apple Menu, select System Preferences, and click on Accessibility.
In the Accessibility window, check the Reduce Transparency box.
Change Minimize Windows to Scale:
Any effect that is used when minimizing windows unnecessarily consumes system resources.
Go into the Apple Menu, select System Preferences, and click on Dock. Change the Minimize windows from Genie effect to Scale effect.
Uncheck the Animate opening applications box. You can check or uncheck any of these features. Remember: less is more.
Disable Widgets:
Widgets are cool looking and convenient, but they use quite a bit of system resources. Not updating Widgets significantly speeds up rebooting and improves performance. Click on the Widgets toggle in the upper right corner.
Uncheck, deactivate, and remove as many items as possible from both the Today and the Notifications panels.
Disable the Extensions that you don’t need:
Go into the Apple Menu, select System Preferences, and click on Extension.
Go to the Today options in the left column and uncheck any or all extensions you don’t require.
Disable sleep, Power Nap, and screen saver:
Go into the Apple Menu, select System Preferences, and click on Energy Saver.
Uncheck everything except Prevent computer from sleeping automatically and Start up automatically after a power failure. Move the Turn display off after slider to the far left set to Never.
Disable Notifications:
Go into the Apple Menu, select System Preferences, and click on Notifications.
In the left column click on and highlight the Do not disturb option and uncheck as many boxes as possible.
In the left column click on and highlight each of the features (Calendar, Face Time, Mail, Messages, etc) and uncheck as many boxes as possible. Change to 1 recent item for each feature.
Speed up new Finder window generation:
In the Finder pull down menu select Preferences.
Set the New Finder windows show to the place you go to most often. This could be Desktop, Documents, Music or your user home folder.
You can also control several other aesthetic features by checking or unchecking these features.
Removing unused languages:
Go into the Apple Menu, select System Preferences, and click on Languages & Region.
Make certain that only the language(s) you use are listed. Highlight and delete unused languages and add any languages you may need using the + and – buttons.
Removing the presentation order for Spotlight:
Go into the Apple Menu, select System Preferences, and click on Spotlight.
In the Spotlight control window, select the Search Results tab.
You’ll see a list of Applications with check boxes next to them. Uncheck all of the boxes. There is no point in leaving “Music” for a music or media server because your library/player software doesn’t access through Spotlight.
Disabling Login Items:
Ideally you want only one Application to automatically open when you login to your computer. If you use your Mac as a dedicated music server, you would want only your player software to open at Login. If you are a professional, you would want your accounting software, CAD software, etc. to automatically open at Login. At the same time, you want to disable any other applications that may automatically want to open at Login.
Go into the Apple Menu, select System Preferences, and click on Users & Groups. To streamline the Login items click on the Login Items tab at the top.
Note: This is the same window you can add guests and users with restricted access. For example, you could allow your children to log on to your media server and play recordings from your NAS drive, but not have access to change any of your administrative settings.
You’ll see all the Applications you’ve set to automatically open at Login in the main window. You can remove any optional automated fluff Apps, such as iTunes Helper, to improve system performance.
I recommended that you upgrade iTunes with a high-performance music/media player Application.
Disabling Automatic Software Update:
Go into the Apple Menu, select System Preferences, and click on App Store. Uncheck the Automatic Updates box.
Note: This is the same place you can manually check for updates.
I cannot recommend more strongly that before updating you always make and test (yes, test) a backup of your drives.
Often updates have bugs or conflicts with drivers. Having a bootable backup allows you to easily return to your former working system, when as is quite common, an update screws up your computer : P
Repairing disk permissions:
For optimal system performance, Repair Disk Permissions after you do OS X updates, do Application updates, or transfer large quantities of data. This is a good habit to get into.
You can find Repair Disk Permissions in your Disk Utility Application.
Activate your Launchpad. Disk Utility is located in your Other folder.
Note: use the Escape key on your keyboard to return from the Other folder to the rest of your Launchpad Applications.
To Repair Disk Permissions, simply highlight the disk you want to repair.
When repairing disk permissions is done, you will see Permissions repair complete at the bottom of the Show details window.
Disabling the Time Machine automatic backup:
Go into the Apple Menu, select System Preferences, and click on Time Machine.
On the left side of the window, you’ll see a slide selector that has off at one end and on at the other. Make sure the selector is switched off.
Warning! Your drive will inevitably fail: back up often. Using a bootable backup will save you a significant amount of time when you have to reinstall your boot drive.
Disable the automatic journaling feature:
Just as before, launch Disk Utility by activating Launchpad, clicking on the Other folder, and selecting the Disk Utilities icon.
Individually select any drive on the list. Hold down the Option button on your keyboard and click on the File drop-down menu. Slide down the File drop-down menu and select Disable Journaling. If you don’t hold the Option button down while selecting the File drop-down menu, Disable Journaling will not be a selectable option.
Repeat this for all drives.
If you wish to enable the automatic journaling feature for a drive, simply repeat the above process and select Enable Journaling in the same drop-down menu.
Turning off unused wireless control interfaces, such as WiFi, Bluetooth, and Infrared, improves performance by freeing up system resources and reducing data corruption caused by EMI “noise” on sensitive internal circuitry.
Systems Mojo Audio sells with our optimized Yosemite have Infrared turned off because it’s rarely used and Bluetooth and WiFi turned on since they are often used to set up new systems. I recommend turning off as many wireless control interfaces as possible.
Warning! Turning off wireless control interfaces can disable control devices, such as your Apple Bluetooth keyboard/mouse/pad. I recommend a USB keyboard and mouse for setup and troubleshooting.
For highest performance use no wireless control interfaces:
The second-best option is to use only one wireless control interface.
Let’s start by looking at what works with what so you’ll have a better idea of which wireless control interfaces you’ll want to turn off.
Control devices associated with wireless interfaces:
The above list may differ from brand to brand or App to App and may even change over time. I recommend connecting a simple USB keyboard/mouse/pad and turning on/off wireless control interfaces one at a time and testing each of your control devices to confirm which works with which.
Note: Wireless devices that plug into USB have relatively low noise compared to WiFi or Bluetooth because they have external receivers, cause less EMI noise, and are better shielded from internal circuitry.
Turning on/off the infrared:
Go into the Apple Menu, select System Preferences, and click on Security & Privacy.
Click on the Advanced button.
Off: Check the box next to Disable remote control infrared receiver. On: Uncheck this same box to activate the infrared receiver.
If a windows Lock icon is in the locked position, you must unlock it before making any changes. To unlock any window, click on the Lock icon; you’ll be prompted to enter your administrative password.
Note: The password for any Mojo Audio optimized system is “Mojo.”
Turning on/off the WiFi control interface:
Go into the Apple Menu, select System Preferences, and click on Network.
Click on and highlight WiFi in the left column:
Off: Click on the Turn WiFi Off button. On: Click on the Turn WiFi On button.
Note: The color of the dot on the left of the networking interface goes from green to red when an interface is turned off and returns to green when turned back on.
Turning on/off the Bluetooth:
Go into the Apple Menu, select System Preferences, and click on Bluetooth. Click on the Bluetooth status button. Note the status changes.
Click on the Advanced button and additional Bluetooth options appear. Check or uncheck the options you want. Personally I uncheck them all.
Note: So that Apple keyboards/mice/pads work with our new systems we leave the Bluetooth on. I would recommend switching to a simple USB keyboard/mouse/pad and turning all the Bluetooth off.
Warning: incorrectly entering code in the Terminal Application can permanently disable your computer. It could require that you reformat and reinstall your entire boot drive.
Did you test your bootable backup? Always a good idea.
The Terminal application is a text-driven command-line interface that allows you to communicate directly with the Unix-based operating system at the core of OS X Yosemite. Unix is the lowest possible level of human interface above machine language, giving the most streamlined and direct control over Yosemite.
I’ll show you how to use Terminal to remove Apps and processes that are set running by default when Yosemite boots up .
To launch the Terminal application, activate Launchpad, go into the Other folder, and click on the Terminal icon. The Terminal interface will appear. Now you are ready to enter commands into the command-line.
Tip: To minimize the potential for error, I recommend you copy the commands and paste them into the Terminal’s command-line.
Remove Mission Control: copy/paste the following lines into Terminal’s command-line one at a time and hit the Enter key after each one:
defaults write com.apple.dock mcx-expose-disabled -bool TRUE [Enter]
killall Dock [Enter]
Remove Dashboard: copy/paste the following lines into Terminal’s command-line one at a time and hit the Enter key after each one:
defaults write com.apple.dashboard mcx-disabled -boolean YES [Enter]
killall Dock [Enter]
Remove Spotlight: copy/paste the following into Terminal’s command-line and hit the Enter key:
sudo launchctl unload -w /System/Library/LaunchDaemons/com.apple.metadata.mds.plist [Enter]
In order for the following command to take effect you’ll be required to enter your administrative password and hit the Enter key.
Note: The password for Mojo Audio optimized systems is: “Mojo.”
Remove Automatic Termination: copy/paste the following into Terminal’s command-line and hit the Enter key:
defaults write -g NSDisableAutomaticTermination -bool TRUE [Enter]
Finder is the main organizer in Yosemite. Unlike most applications which you can quit or Force Quit, the default settings in Yosemite will allow you to relaunch but not to actually quit Finder.
Resetting Yosemite to quit Finder: copy/paste the following into Terminal’s command-line one at a time and hit the Enter key after each one:
defaults write com.apple.finder QuitMenuItem 1 [Enter]
killall Finder [Enter]
Important! Quit the Terminal Application.
To maintain optimal performance, each time you reboot your computer, and each time you use Finder, you will need to follow the instructions below to manually quit the Finder Application. Since Yosemite needs at least one Application to be open before you can quit Finder, you’ll need to open your player or primary software first.
Click on the Finder icon in the dock.
Select the Finder column in the top drop-down menu next to the Apple Menu in the upper left corner of your screen.
Slide down to the bottom row and select Quit Finder.
You’ll know Finder is not running if the dot under the Finder icon in the dock is gone.
Reactivate Dashboard: copy/paste the following into Terminal’s command-line one at a time and hit the Enter key after each one:
defaults write com.apple.dashboard mcx-disabled -boolean NO [Enter]
killall Dock [Enter]
Reactivate Mission Control: copy/paste the following into Terminal’s command-line one at a time and hit the Enter key after each one:
defaults write com.apple.dock mcx-expose-disabled -bool FALSE [Enter}
killall Dock [Enter]
Reactivate Spotlight: copy/paste the following into Terminal’s command-line and hit the Enter key:
sudo launchctl load -w /System/Library/LaunchDaemons/com.apple.metadata.mds.plist [Enter]
In order for the following command to take effect you will be required to enter your administrative password and then hit the Enter key.
Reactivate Automatic Termination: copy/paste the following into Terminal’s command-line and hit the Enter key:
defaults write -g NSDisableAutomaticTermination -bool FALSE [Enter]
Reactivate Finder: copy/paste the following into Terminal’s command-line one at a time and hit the Enter key after each one:
defaults write com.apple.finder QuitMenuItem 0 [Enter]
killall Finder [Enter]
Important! Quit the Terminal Application.
To maintain the highest level of performance and security, you must periodically do a few tasks manually:
If you like what you read in this blog and are interested in getting more free tips and tricks, sign up for Mojo Audio’s Audiofiles blog. Also, sign up for our e-newsletter to get more useful info as well as coupons, special offers, and first looks at new products.
Plus, don’t forget to “like us” on Facebook.
Enjoy!
Benjamin Zwickel
Read Moreاین متن عالیه :
http://www.thewelltemperedcomputer.com/KB/USB.html
http://www.empiricalaudio.com/computer-audio/technical-papers/
Universal Serial Bus (USB) is a serial bus standard to interface devices. USB was designed to allow many peripherals to be connected using a single standardized interface socket and to improve the plug-and-play capabilities by allowing devices to be connected and disconnected without rebooting the computer (hot swapping). Other convenient features include providing power to low-consumption devices without the need for an external power supply and allowing many devices to be used without requiring manufacturer specific, individual device drivers to be installed.
A nice story about the development of the USB interface: The D/A diaries: A personal memoir of engineering heartache and triumph by Hitoshi Kondoh.
USB is a computer bus like any other but for some reason it inspires people to make all kind of funny products.
USB wedding ring
USB audio is very popular.
One of the reasons is that USB audio is part of the USB standard and as a consequence native mode drivers are available in all the popular OS (Win, OSX and Linux).
Connecting a USB audio device is a matter of plug&play.
USB audio is a flexible solution as any PC offers USB.
If you use a laptop this is probably the way to go if you want to improve on the on-board sound card.
The audio is routed to the USB.
This is a matter of choosing the USB audio device in your media player.
The on-board sound card is bypassed; in fact you don’t need a sound card at all.
The USB audio device is your (outboard) sound card.
Today the resolution of USB audio ranges from 16 bit/ 32 kHz to 32 bit/ 384 kHz.
A lot of DACs are still limited to 16 bit/ 48 kHz max.
The data transfer from the PC to the DAC can be done in adaptive or in asynchronous mode.
In adaptive mode the DAC adjust its timing to the rate the data is pouring in.
In asynchronous mode the DAC keeps its timing constant and controls the amount of data send by the PC. By design asynchronous mode eliminates input jitter.
A lot of people think USB audio is limited to 16 bits/48 kHz max.
A lot of (cheap and sometimes not so cheap) USB DACs are indeed limited to this resolution.
This is because the manufacturer decided to use a simple and cheap of the shelf hardware solution.
Another common misunderstanding is the specification of the bus (USB 1,2 or 3) and the USB audio standard (1 or 2).
This standard allows for 24 bits/96 kHz max.
The standard itself doesn’t impose any limitation on sample rate.
Class 1 is tied to USB 1 Full Speed = 12 MHz
Every millisecond a package is send.
Maximum package size is 1024 bytes.
2 channel * 24 bit * 96000 Hz sample rate= 4608000 bits/s or 576 Byte/ms
This fits in the 1024 byte limit.
Any higher popular sample rate e.g. 176 kHz needs 1056 bytes so in excess of the maximum package size.
All operating systems (Win, OSX, and Linux) support USB Audio Class 1 natively.
This means you don’t need to install drivers, it is plug&play.
All support 2 channel audio with 24 bit words and 96 kHz sample rate
It is downwards compatible with class 1.
USB Audio Class 2 additionally supports 32 bit and all common sample rates > 96 kHz
Class 2 uses High Speed (480 MHz). This requires USB 2 or 3.
As the data rate of High Speed is 40 X Full speed, recording a 60 channel using 24 bits at 96 kHz (132 Mbit/s) is not a problem.
From mid-2010 on USB audio class 2 drivers are available in OSX 10.6.4 and Linux.
Both support sample rates up to 384 kHz.
It is unclear if Microsoft is going to support USB Audio 2.
You need a third party USB class 2 driver on Windows.
Companies like Thesycon or Centrance have developed a USB Class 2 Audio driver for Windows.
Using High Speed USB for playback there are no limits in resolution.
The data send over the USB must be transformed to a format a DAC (the chip doing the DA conversion) does understand. This can be SPDIF or I2S.
This is the task of the receiver chip.
Adaptive mode 16 bit units often use the Cmedia or TI (PCM270x) based chip sets. These are not programmable and usually only support 16 bit and 32, 44.1, 48 kHz sample rate.
An example of how this chip-set performs compared with asynchronous USB can be found below.
24 bit adaptive mode DACs needs a programmable design (TAS1020 or other USB Audio Controller).
This chip enables 24 bit/ 96 kHz over USB.
Chips like the TAS1020 are limited to full speed.
You can’t do high speed as needed for USB audio class 2.
96 kHz is the upper limit when using native mode USB Audio Class 1 drivers.
A USB audio class 2 or a custom driver is needed to run 176/ 192 kHz and higher.
An example is the Tenor TE8802L by Galaxy Far East Corp.
Today you can buy complete USB-receiver modules like XMOS.
Basically a USB to I2S or SPDIF converter.
A couple of these interfaces can be found here.
Data is exchanged over USB using one of the four possible modes:
Transfer modes explained in detail.
When the computer sends the audio stream to an USB port, if first reads the data from the hard disk and caches blocks of the data in memory.
It is then spooled from memory to the output port in a continuous stream (Isochronous mode).
Frames are sent out every millisecond.
This happens whether there is any data in the frame or not.
The rate at which the frames go out is determined by a oscillator driving the USB bus.
This rate is independent of everything else going on in the PC.
In principle this guarantees a constant flow of the frames.
In practice the frames might not be filled properly with data because some program simply hogs the CPU or the PCI.
Anti virus polling the internet at high priority are a well known example.
Isochronous transfer can be done with three possible types of synchronization modes in the USB audio device.
There must be some kind of synchronization between the PC and the DAC to avoid buffer under/overrun.
The clock driving the DAC is directly derived from the 1 kHz frame rate.
This mode was used by the early USB audio devices.
They were limited to 48 kHz and pretty jittery.
In this mode the timing is generated by a separate clock.
A control circuit (sample rate guesser) measures the average rate of the data coming over the bus and adjusts the clock to match that.
Since the clock is not directly derived from a bus signal it is far less sensitive to bus jitter than synchronous mode, but what is going on the bus still can affect it.
It’s still generated by a PLL that takes its control from the circuits that see the jitter on the bus.
In this mode an external clock is used to clock the data out of the buffer and a feedback stream is setup to tell the host how much data to send.
A control circuit monitors the status of the buffer and tells the host to increase the amount of data if the buffer is getting too empty or to decrease if it’s getting too full.
Since the readout clock is not dependent on anything going on with the bus, it can be fed directly from a low jitter oscillator, no PLL need apply.
This mode can be made to be very insensitive to bus jitter.
The warm reception in the audiophile world of asynchronous USB as developed and promoted by Wavelength inspired other brands to offer asynchronous USB DACs .
Asynchronous mode is not better by design but by implementation because you can implement a top quality (low jitter) clock in the DAC.
There is actually a good example of this case of its the implementation of the clock thats important, not the asyncness itself that is important. The recent inexpensive Musiland devices use an asynchronous protocol but then use a frequency synthesizer to generate the local clock rather than use a fixed frequency oscillator. The result is jitter that is actually worse than some of the better adaptive implementations!
John Swenson
The best way to get the most out of a dac chip is to put 2 audio oscillators right next to the dac chip. Buffer the oscillators and send them back to the USB controller to use to create the I2S (or other audio data stream L/R justified, DSP whatever) and this will give you the best response and the lowest jitter.
What many companies are doing is using the Frequency Synthesizer to create the audio oscillators. Basically these are frequency multipliers that can create any frequency and in the case of the TAS1020 down to 4Hz resolution. The problem with a Frequency Synthesizer is that the jitter can be as much as 100x that of a fixed oscillator. When enabling the oscillator in the TAS1020 also adds noise to the audio data stream because of the noise it fixes to the power supplies.
So choose wisely what you buy and ask the correct questions.
It’s not about the code… though all of ours is different, it may have an effect on the sound. But more so it has to do with the hardware and how that functions.
Thanks
J. Gordon Rankin
Not everybody agree that asynchronous is better.
Centrance, manufacturer of adaptive mode solutions, is one of them.
Some manufacturers may lead you to believe that Asynchronous USB transfers are superior to Adaptive USB transfers. This no more true than saying that you “must” hold the fork in your left hand. If you know what you are doing, you will feed yourself with either hand.
Async USB provides a simpler way to implement a low jitter DAC relative to adaptive mode USB. For the cost of a small number of lines of firmware code, you reduce the amount and complexity hardware needed and potentially reduce the cost of the hardware needed for a high quality result. Most thinking engineers appreciate simplicity and the potential for low cost designs that deliver the goods.
You can find plenty of bovine excrement in the marketing of all kinds of high-end gear. Marketing products using buzz words without supporting detail or test results works when the audience is technically ignorant.
Asynchronous USB looks to be the perfect solution.
You configure your PC for bit-perfect output and the DAC takes care of the timing totally independent of the timing of the PC.
But there are posts on the Internet claiming that even in case of an async USB DAC what is happening upstream is still affecting sound quality.
Almost all recent offerings of quality DACs have asynchronous USB input.
One issue with USB is that it sends regular bursts of info like the start of frame packet – “The SOF packet consisting of an 11-bit frame number is sent by the host every 1ms ± 500ns on a full speed bus or every 125 µs ± 0.0625 µs on a high speed bus”. If the timing of this shifts or is variable, this could elicit a different & variable reaction from the USB receiver & translate into a different & varying jitter or noise spectrum. Making the PC end as solid & stable as possible without undue processing could be one factor in ameliorating this variation. It might not be the low level of jitter that we notice but the variation in jitter – that’s one reason why I say that the measurements we currently run seem not to be capable of picking up these issues or we are not directing them to the correct target for measuring.
Some companies don’t use USB audio in isochronous transfer mode.
They implement their own solution using bulk mode transfer.
Bulk mode is asynchronous by design.
As it is bulk mode,
In case of isochronous mode it is exactly the reverse.
As long as the DAC is the only one connected to an internal hub, bandwidth is in general not the problem using USB high speed mode.
Inherent to a vendor specific solution is that he either supports your OS or not.
The advantage of USB audio is that it is natively supported by Win, OSX and Linux.
However in case of USB audio class 2 on Win you need a third party driver too.
Anyway this solution does audio over the USB without using the USB audio of the operating system.
Jim Lesurf did a nice experiment.
He measured the analog out of a DAC Magic when feed by its own adaptive mode USB and by a asynchronous USB to SPDIF converter (Halide).
The differences between adaptive (USB direct) and asynchronous (Halide) are clear.
According to the author not only measurable but also audible. [5]
Archimago [11] measured the jitter performance of a adaptive mode USB and a async mode.
adaptive mode USB
Asynchronous mode USB (CM6631A
Indeed, the jitter performance improves with asynchronous USB
Cable length between full speed devices is limited to 5 meters. For a low speed device the limit is 3 meters.
As the signal degrades proportional to the length of the cable, a short cable is often recommended.
Other says this can put a source of RFI (the PC) to close to the USB-DAC.
As file based audio is gaining momentum and many believe asynchronous USB the way to go there is a growing market for audiophile grade USB cables.
The question of course is why a cable can have any impact on sound quality.
Some say that improved jitter performance of a cable can make a difference.
Others say that the reason we use asynchronous USB is exactly to have zero input jitter at the DAC so all what is happening upstream is irrelevant cable included.[1]
Audiophile USB cables are becoming as controversial as high-end power cords.
The Limitations of digital audio processors and cables create timing errors known as jitter, which remove portions of the audio signal and replace them with noise and distortion. Cables tend to round off the square waveforms of the signal, making them less clear to the processor, thus increasing jitter. This rounding effect varies greatly among cables and a truly superior digital audio cable can make great improvements in sound quality.
http://www.wireworldcable.com/categories/usb_cables.html
Another manufacturer talking some marketing bull shit?
They do have a point.
Digital is indeed sending fully analogue electrons over a wire.
And indeed, the block pulse degrades with the length.
A good digital cable is one who minimizes this degradation.
The USB 2.0 specification lists a maximum cable length of 5 meters (~15 feet). This is marginal with the best of cables, and many “audiophile grade” cables will run into problems even with far shorter lengths. There have been many credible reports of improved sound quality with some cables, but these have almost all been in systems using Class 1 Audio, with a maximum data rate of 12 MHz. When the data rate is boosted by a factor of 40x to 480 MHz, there are very few “audiophile” cable companies that have the tools and experience to ensure good results.
Computer Audio Playback Overview – Ayre
This is an easy test.
Connect your high speed USB device, e.g. a hard disk using your audiophile grade USB cable. If hi-speed mode (480 Mbps instead of 12 Mbps, the old USB 1 standard) fails, it is a bad DIY digital cable not even compliant with USB 2 standards.
A clear and well written step by step guide to setup and USB DAC using XP, Vista or OSX can be found at the Ayre website.
Setup for Win7.
Setup for Vista.
You can check if your asynchronous USB DAC is really asynchronous.
Audio devices supporting asynchronous transfer mode should have an extra ‘endpoint descriptor’ with
bmAttributes = 0x5 (USB_ENDPOINT_TYPE_ASYNCHRONOUS).
Svyr has more
A simple way to find out is to use the Thesycon USB Descriptor Dumper
Some users complain about dropouts when playing USB audio.
This might be due to different devices sharing the same USB-Hub.
If your audio and your graphics card are on the same hub, the bandwidth required by the graphics might cause the audio to stutter.
Anti-virus programs polling the internet with high priority might interrupt the audio too.
Trouble shooting USB audio is covered here.
نتایج من تا این لحظه (بعد از 30 ساعت مطالعه) این بوده در عمل بهترین گزینه USB هست (دقت کنید فقط USB 2 و USB 3 خوب هستند و گزینه USB1 مناسب نیست) بصورت Asynchronous و استفاده از یه کابل خوب مثل پیوریست و اینکه استفاده از ری کلاکر و واسط های حذف نویز اصلا خوب نیست. بهتره از مک استفاده بشه با نرم افزار بیت پرفکت Audirvana با تنظیمات مربوط. کل دیتای فایل موزیک باید تو حافظه باشه.
بعضی ها معتقدند بهتره از Toslink اپتیک برای ایزوله کردن استفاده کنیم که من حس میکنم راه حل USB (با پیشرفت بیشتر USB در آینده) راه حل بهتری خواهد بود. خود تورستن هم میگه این راه حل خیلی Jitter داره و اصلا خوب نیست.
راه دیگه هم Firewire هست که عملا نه DAC سازان رفتند سمتش و نه تو مک بوک خیلی بهش توجهی شده. این راه حل خوبه اما خیلی برای ماها عملی نیست و بیشتر بدرد استودیوها میخوره و برای پهنای باند بالا خوبه. راه حل PC با کارت صدای AES یا Firewire و سیستم لینوکس هم خیلی در موردش به نتایجی کسی نرسید که بخواد توضیحی به ما بده. عملا بهترین حالت استفاده از پورت یو اس بی MAC هست با Audirvana. خوبی دیگر Usb نسبت به Firewire در سادگی و نداشتن درایور هم هست.
هارد چه SSD چه معمولی خیلی فرقی روی ماجرا ندارند اما SSD کمی بهتره. برق یا باطری روی کامپیوتر مهمه و خودم ترجیح میدم از یک Power Supply خوب استفاده کنم.
موضوعی که مهمه اینه که تو انتخاب DAC باید به نکات زیر دقت کنیم :
دقت کنید نقش کابل USB برای کاهش نویز از PC به DAC بسیار مهمه.
از سایت رومی این لینک رو دیدم که مصاحبه های سال 2013 هست.
http://www.positive-feedback.com/Issue41/ca_intro.htm
Larry Moore and Eric Hider of Ultra Fi Audio Designs
Andreas Koch of Playback Designs
Steve Nugent of Empirical Audio
Gordon Rankin of Wavelength Audio
Jon Reichbach of Sonic Studio/Amarra
Vinnie Rossi of Red Wine Audio
John Stronczer of Bel Canto Designs
Daniel Weiss of Weiss Digital Audio
Vincent Sanders and John Hughes of VRS Audio Solutions
Charles Hansen of Ayre Acoustics
Pete Davey of Positive Feedback Online
رومی میگه جای نظرات Dan Lavry, Ed Meitner, Michael Ritter and Pflash Pflaumer هم خالیه. شرکت های Lavry Engineering و EMM Labs و berkeley audio design .
Read Moreنتایج ما از یک سورس دیجیتال خوب تو میکرو :
در مورد این DAC USB شرکت Wavelength Audio بیشتر بخونیم :
Cosecant USB DAC by Gordon Rankin
http://www.6moons.com/audioreviews/wavelength3/cosecant.html
– The Cosecant v3 connects the USB controller to a DAC module. The USB firmware to run that DAC module resides there so each module has its own developed code. The output of the module connector is sent directly to the 6GM8/ECC86 dual triode output tube, which drives the transformer-coupled output.
– The Cosecant has an external power supply with IEC connector to isolate the power from the DAC and its audio transformers.
– This is my own design and I wrote the software (1,800 lines of code for the USB controller to do what it does) like no one else does on the planet.
– The DAC featured in this review uses Asynchronous USB mode as do the Crimson. The USB interface is bidirectional, and has built-in error correction and buffering at both ends; it is an asynchronous interface. Clocking synch problems associated with SPDIF are not present with USB. The result is that the data on the disk is identical to what is leaving the DAC all the time. At start-up, the DAC tells the computer it can handle 16 bit audio at 32K, 44.1K and 48K. Since the USB receiver only has to handle these 3 frequencies, the clocking to the DAC has almost no jitter. SPDIF actually has to be synched to the exact frequency of the transport (i.e. if the transport is working at say 44.0896K instead of 44.1K the DAC has to sync to that frequency). Therefore the jitter problems of SPDIF are all but eliminated. The result is a zero error protocol to link between computer and DAC, with ultra low jitter.
– None of these DACs use any type of operational amplifiers (opamps) in their design. The DAC module is the only solid-state portion of the overall DAC.
– The output stage of each of these products is the key design element which is responsible for their overall sound.
– These DACs incorporate the TDA1543N2 (select top 5%) DAC chips with passive I/V using Shinko Tantulum resistors in a configuration that does not use analog or digital filters. Some people call these NOS DACs or what I call zero DACs. The data input is the data output without any up/oversampling or other manipulation, which seems to make for a very analog presentation.
http://www.goodsoundclub.com/Forums/ShowPost.aspx?PageIndex=1&postID=8265#8265
Well, I admit that I was clueless on the subject but I was intrigued and I consulted my engineering recourses. It turned out that what I proposed above was not exactly accurate: for instance regular computers do not use I2S bas interface…
To make the long story short: For a computer the USB port is a devise of the same hierarchy as sound card and there is not different for a computer to where output stream: to a driver on the sound card or to the driver the sit behind the USP port designation. With identical drivers used the ASE/EDU interface has fundamentally lower jitter interface, where the min jitter for USB is not even included in the USB standard. So, with identical design (more of the time used) the USB has no chance to be superior.
There is a catch however. With proper design, when USB is made not for utilitarian purposes but for high quality transfer, the USB has internal chance to be way more advanced. The ASE/EDU is forward-only interface where stream flows only in one direction with data mixed with clock marks. The USB is full duplex or bi-directional interface where stream flows in both ends. This enables designers to put a clock that might everything on the receiver side (DAC) and let this clock to manage the USB’s and even reader timing. USB sends data in burps by requests of by scheduled timing, this marks all might be managed by receiver side clock. In ASE/EDU the receiver shell recognize the timing marks and PLL or re-clock data. In USB there are no needs for PLL or re-clocking as the data arrives at the marks of the original clock. The USB in this case acts like an elephant that sticks a long trunk to another devise and sucks juice… according to my consultants this USB implementation is the most proper way to do the things. Or course no one knows HOW the USB is managed even if a DAC has USB port…
The Cat
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من باید دیتای دیجیتال فایل روی کامپیوتر رو به DAC 4 برسونم .
امروز کمی تحقیقات کردم و فعلا به این نتیجه رسیدم برای سیگنال خوب دیجیتال اگر بخوام سخت افزار بخرم باید بین 7000 تا 20000 دلار پول بدم که خیلی احمقانه است و تازه نمیدونم تو اون سخت افزار داره چه اتفاقی میفته.
ظاهرا بهترین گزینه مینی مک هست با Power Supply خطی افترمارکت و تنظیمات لازم برای Bitperfect . این Bitperfect یعنی هیچ تغییری روی فایل در حال پخش تو دیجیتال دامین ندهیم.
حالا اومدیم مینی مک رو تنطیم و با نرم افزار Audirvana موسیقی رو پخش کردیم میرسیم به ریکلاک و بافر Berekley Audio Alpha که این کارش حذف jitter سیگنال هست بدون دستکاری سیگنال دیجیتال. یه همچین چیزی باید توی خود DAC باشه (مثل AMR یا EMMlabs) اما من حدس میزنم DAC4 EAR خیلی روی این موضوع مانور نداده. البته حدس میزنم و فعلا این الفا رو درموردش تحقیق میکنم و قطعی نیست برام.
http://www.co-bw.com/Audio_OSX_Optimal_Audio.htm
http://yourfinalsystem.com/custom-modifications/yfs-music-servers
http://www.positive-feedback.com/Issue72/mojo_audio.htm
http://www.mojo-audio.com/mac-mini-upgrades/
http://www.coreaudiotechnology.com/products/music-servers/mac-mini-music-server/
http://www.positive-feedback.com/Issue54/mach2.htm
این تورستن که طراح AMR هست و کلی تو این حوزه دیجیتال سروصدا کرده مطالب جالبی نوشته در مورد اینکه چه نویز ها و مشکلاتی سر راه این انتقال سیگنال تا DAC هست.
http://www.psaudio.com/ps_how/how-to-build-a-music-server/
http://www.6moons.com/audioreviews/players/4.html
برگردیم به مطلب تورستن در مورد انتقال دیتا از فایل به DAC :
http://www.amr-audio.co.uk/html/dp777_tech-papers.html
http://www.amr-audio.co.uk/html/dp777_tech-papers_OSX-Integermode.html
| Beyond bit-perfect: The importance of the Player Software And MAC OS X Playback Integer Mode |
|
Damien PLISSON, Audirvana developer Abstract In computer audio, the player software replaces the CD drive as the transport feeding the DAC. Ensuring bit-perfect output of the original audio signal is only a pre-requisite, while minimizing jitter and RF interferences are still strongly needed. Introduction: bit-perfect as the only goal or the myth of the flat-square world In the world of digital audio, the caveats of the CD player are well known, namely the read errors and the jitter induced by its mechanical transport. 1. Sources of non-quality Assuming the output is bit-perfect, the computer as a source creates two main sources on non-quality: Software-induced jitter Digital signal is in fact an analogue waveform composed of two states separated by a voltage threshold (1 if above, 0 if under). So a slight change in the reference voltage of the source will lead to a slight temporal shift in the value change detection.
So fluctuations in the source reference voltage create jitter, as explained in details in [HawksfordDunn96]. This is the same on the receiver side with measurement threshold fluctuations from its power supply and/or ground instability. Moreover the computer can still cause this as the grounds are linked most of the time through the same signal cables. Computer load means rapidly changing power demands from the CPU and its peripherals, with peak demands that are directly related to the software behaviour. Radio-Frequency & other interferences In addition, computation, disk access, … activities mean complex current waveforms are carried on electrical lines and thus generate electromagnetic interferences. Apple computers are now made of “unibody” aluminium cases that are good protectors from inside RF interferences. But this is not sufficient as the cables connected to the computer act as antennas. And these current waveforms are also going back through the computer PSU, polluting the mains power supply. 2. The hidden audio filters of OS X As a modern operating system OS X needs to offer shared access to the devices including the audio output to all running applications. But this is done at the expense of pure sound quality: Audio mixer Fortunately when only one application is playing audio, it doesn’t affect the signal and thus is at least bit-perfect in this case. Sample rate conversion In this shared model the device sample rate is not switched to match the original signal’s, but it is this last one that is sample rate converted. Digital volume control OS X offers through its mixer volume control (e.g. the one offered in iTunes). But as it operates on the digital signal, any volume value different from 100% means loss of bit-perfect and precision loss (e.g. a volume value of 25% means 2 bits precision loss). 3. The data transfer to the DAC First way to connect to the DAC is to use the build-in TOSLINK output of the Mac. But this one should be dismissed for being too jittery for serious use. This combines the advantages of both worlds: ease of use of USB (no drivers), and stability of FireWire. This is a great step towards sound quality, but it is not decoupling completely the DAC from the computer, and the interferences, software-induced jitter still apply, starting by following the ground loops. 4. The player software impact First of all the player should ensure bit-perfect reproduction of the signal by:
Furthermore, as we have seen in section 1, the computer load (and its variations) has an impact on sound quality. Minimizing such current demands and sources of interferences is key:
5. Further optimization at driver level: Integer Mode Audio playback in OSX is usually performed through a high-level framework, the Audio Units processing graph [AppleCoreAudio]. The first optimization of an audiophile player is to bypass these overhead facilities and address directly the CoreAudio lowest layer: the Hardware Abstraction Layer. (See figure 2)
In normal mode, all data exchanges performed across the user/kernel boundary are in PCM 32-bit float format, easing the different audio streams mixing process and associated soft clipping. [AppleHAL_1] Integer mode Addressing directly the HAL [AppleHAL_2] gives the possibility to bypass the two main overhead processes of the above standard mode: Field Programmable Gate Arrays
In Integer Mode (see figure 3) the player software supplies a stream already formatted in the native DAC format, thus optimizing synchronous CPU load at the driver level. Conclusion The computer is a great music server but also a source of jitter and other RF interferences that are detrimental the sound quality, even when bit-perfect reproduction is ensured. This is what I’ve tried to get in the Audirvana player by streamlining to the maximum the real-time operations that are limited to simple data streaming in Integer Mode, while all the other processes (loading from disk, decoding, converting to DAC native format) are done Float mode offline in a preparation phase, before playback. This is called full memory play. Best results are achieved when feeding an Integer Mode, asynchronous USB DAC like the AMR DP-777 that can take advantage of all these optimization features. References [HawksfordDunn96] Bits is Bits ? in Stereophile 03/1996 Replacing the HDD by a SSD removes the directly audible mechanical noise but not the other issues as it still requests important current waveforms to transit on lengthy wires. And the OS overhead is still present. |
تو سایت Twogoodears مطلبی نوشته شده درباره ساخت یک DAC :
http://twogoodears.blogspot.com/2013/06/digital-skyrocketing.html
امکان ساخت پروژه تو لینک زیر هست خیلی شفاف توضیح داده و بد نیست همون DAC 1541 استفاده بشه .
http://www.dddac.com/dddac1794.html
Reinhard’s Quadriga, the DDDAC1794 24 Bit NOS
What about the panel?
They were, from left to right: Triodedick, Doede Douma, Jean Hiraga, Klaus Speth and André Klein.
Klaus supervising, w/Doede Douma and Dirk aka “Triodedick”, doing what they like most: “conjuring tricks”;-)
James with his girl friend, sitting and listening in the – for Hi-Fi-Fans – “center of all centers”
Jean, André, and french guest
This is Reinhard’s DDDAC1543 16bit DAC with 120 chips, also in combination with Bernd’s power
Jean Hiraga, Doede Douma
PC, and MAC, and NAS, and I-Pod, and W-Lan, and cables, and… ONLY the pix is blurred, folks;-)))
http://www.positive-feedback.com/Issue65/dac.htm
http://www.positive-feedback.com/Issue66/dsd.htm
Read More
مقاله زیر رو طراح Ayre نوشته که بنظرم خیلی خوب مطلب رو توضیح داده. قبلا هم گفتم برخی طراحان سورس دیجیتال مانند طراحان برندهای Meridian ، EMM Labs ، Spectral ، Playback Design که فیلترهای دیجیتال رو کدنویسی میکنند سعی میکنند کاری کنند پاسخ فاز بهتر بشه و سیگنال تو Time Domain وضعیت بهتری داشته باشه.
بخونید :
https://www.hifi.ir/wp-content/uploads/2010/12/Ayre_MP_White_Paper.pdf
Ayre is pleased to announce the results of an intensive research program into the audible performance of various digital filters (including “non-oversampling” filterless designs). Extensive listening tests have shown that our new custom DSP-based “Minimum Phase” filters provide a significant leap forward in both musical naturalness and ease of listening. All of our current digital products use these filters, and upgrades are also available for all Ayre disc players still in production.
The Evolution of the Digital Filter
To illustrate the performance differences between various digital filters, included are graphs of their two key parameters—their frequency response and their transient (impulse) response. These graphs allow us to trace the evolution of the digital filter,
clearly showing the changes that have led to improved sound quality.
In the beginning, there was the common linear-phase, “brickwall” digital filter. This type of digital filter is used in 99+% of all modern digital equipment, for both recording and playback, including the “Measure” position of the original Ayre disc players.
On paper, it looks nearly perfect. There is no phase shift, so it is called a “Linear Phase” filter. However, a filter can only achieve a “Linear Phase” response by introducing pre-ringing. This means that before every single musical transient, there is a “pre-echo”. In nature, there is no such thing as a “pre-echo”. All events must be “causal” in the real world—the cause must precede the effect.
A sharp, “brickwall” filter like this typically introduces about 20 cycles of pre-ringing and 20 cycles of post-ringing. It is very unnatural sounding, as the effect (pre-echo) precedes the cause (musical transient). This time-smear is interpreted by the ear-brain as both a lack of image precision in the soundstage, and also a subtle smearing of the musical sounds together.
Improving the Transient Response
The first approach to solving the problems of a conventional digital filter was to use a filter with less ringing, often known as a “slow roll-off” filter. This type of filter was used in the “Listen” position of the original Ayre disc players. By reducing the “sharpness” of the “knee” in the filter’s frequency response, the filter’s transient response is vastly improved.
Now there is only about one cycle of pre- and post-ringing. The penalty (remember, there is no such thing as a free lunch—only intelligent tradeoffs) is that there is more “leakage” (aliasing) of high frequencies above 22,050 Hz back in to the audio band. Still, this only affects very high
frequencies and the levels are low enough not to cause audible problems.
This “slow roll-off” filter reduces the time smear by a factor of ~20x compared to conventional digital filters. The net result is a much more musically natural sound, as the ear-brain is very sensitive to time-related distortions. This filter provides an outstanding compromise between frequency response and transient response, and for ten years was the mainstay of Ayre’s digital audio filters.
A natural extension to this idea is to eliminate the digital filter altogether. In theory this provides the best transient response possible from a digital playback system. There are certain audible advantages to this approach, but they are highly system-dependent and come at the expense of two separate problems.
The first is a loss of high frequencies starting at -0.75 dB at 20 kHz and reaching -3.2 dB at 20 kHz. The second is that the aliased “image” frequencies are injected into the audio signal, creating non-harmonically related distortion that increases in level as the frequency increases, reaching over 50% at 20 kHz unless additional analog filtering is employed. Careful listening tests revealed that these drawbacks outweighed the gains in improved transient response, especially given the CD format limitation with its 44.2 kHz sample rate. We therefore continued examining different approaches to digital filtering.
Eliminating the Pre-Echo
In Peter Craven’s 2004 AES paper, he proposed that the playback DAC should include a digital filter that had a corner frequency below the half-sample rate of 22,050 Hz. This would filter out any ringing (pre- and post-) that was introduced during the recording process and thereafter embedded on the disc itself. He named this an “apodizing” filter. It is a mathematical law of any filter, digital or analog, that the steeper the frequency cutoff, the more ringing it will have. In an attempt to avoid the problems of this ringing, Craven proposed using a “Minimum Phase” filter instead of the conventional “Linear Phase” filter. While this means that the phase response now varies, especially at high frequencies, there is no longer any pre-ringing. Furthermore all pre-ringing from the recording process has been filtered out, and the new playback filter only has post-ringing.
Now a giant step forward has been taken in the musical naturalness of digital audio reproduction. The unnatural pre-echoes have been completely eliminated. All of the filter’s ringing occurs after the musical transient. This is just the way that sounds occur in nature. Every sound made in the real world will have post-echoes after the original sound, so the ear-brain system more easily accepts these post-echoes from the digital filter as natural. Note that the post-ringing of a “Minimum Phase” filter is greater than that of a “Linear Phase” filter with the same
frequency response. The energy that had been contained in the pre-ringing of the “Linear Phase” filter has simply been delayed until after the transient. (Remember—there are no free lunches.) But redistributing this same total energy leads to significant gains in musical realism.
This type of digital filter is not available in off-the-shelf chips. Instead, it must be implemented in custom DSP filters. In the case of the new Ayre MP (Minimum Phase) disc players, we use sophisticated FPGAs (Field Programmable Gate Arrays) to create the desired custom filter, and the chip is easily reprogrammable should future improvements be made. This filter type is used in the “Measure” position of the new Ayre MP disc players.
The Best of Both Worlds
While the “Apodizing” filter proposed by Craven solves many of the problems with digital filters, careful listening tests conducted at Ayre showed that the multiple cycles of post-ringing still created an artificial brightness and an overall confusion to the sound. We therefore sought to combine the best aspects of Craven’s minimum-phase digital filter proposal with a slower roll-off that reduced the overall amount of ringing.
The resulting filter has no pre-echoes, and only about one cycle of post-ringing. This filter is implemented in the “Listen” position of the new Ayre MP disc players. The result is simply the most musically natural digital playback available today.
In addition to the radical improvements provided by the digital algorithms themselves, several man-months were spent conducting thorough listening tests to optimize all other aspects of the filters. The powerful customprogrammed FPGAs used in the MP series allow performance far beyond that available from off-the-shelf solutions. The mathematical calculations are conducted with 32-bit coefficients, using 64-bit accumulators to ensure the greatest degree of signal precision.
A 26x oversampling rate was determined to provide the highest level of musical realism. All of the filtering calculations are accomplished in a single pass through an FIR (Finite Impulse Response) filter. This is in contrast to conventional designs that employ a cascade of 2x FIR filters, thereby losing critical precision of the audio data as it is passed to each successive filter section. Finally, the dither algorithms for both the “Listen” and “Measure” filters were chosen on the basis of careful listening tests to provide the most realistic music reproduction possible.
The Ayre MP filter provides a significant step ahead in digital audio reproduction. The CX-7e provides this advanced technology for CDs, while the C-5xe universal disc player takes this to the limit of today’s recording technology at 292 kHz and 24 bits. In addition, this exclusive advance in digital audio reproduction is included in the Ayre QB-9 USB DAC, achieving the most natural and realistic sound available from computer-based audio systems.
http://www.stevehoffman.tv/forums/archive/index.php/t-186820.html
http://www.stereophile.com/features/106ringing/index.html
http://hifiduino.blogspot.com/2009/05/wm8741-digital-filters.html
Read Moreهمانطور که میدانید برای پخش یک دیسک مانند سی دی صوتی یا دی وی دی صوتی از دستگاهی بنام CD Player یا DVD Player استفاده میکنند. به این دستگاه میگیم سورس دیجیتال که به معنی دستگاهیست که اطلاعات دیجیتال را از روی دیسک خوانده و آنرا به یک شکل موج آنالوگ تبدیل میکند. این شکل موج آنالوگ در خروجی سورس دیجیتال ولتاژی بین 1.8 تا 2.5 ولت دارد که در صورت بالانس بودن خروجی این مقدار دو برابر میگردد. امروزه بیشتر کمپانی ها این دستگاه رو به دو بخش تقسیم کردند و هر یک را جداگانه میسازند. قسمت Transport یا دیسک خوان و قسمت DAC یا مبدل دیجیتال به آنالوگ. قسمت Transport دیسک را خوانده و اطلاعات آنرا رمزگشایی میکند و از طریق یک بافر به طبقه بعدی میدهد. خروجی Transport یک سیگنال دیجیتال هست که میتواند از طریق کابل نوری (Optical) یا کواکسیال منتقل شود. البته راه های انتقال فقط این دو راه نیست و بسته به طراحی Transport فرق میکند. با خروج سیگنال دیجیتال از Transport سیگنال از طریق کابل وارد بخش DAC میشود و این بخش سیگنال دیجیتال را بعد از پردازش تبدیل به آنالوگ میکند و با عبور از یک فیلتر پایین گذر به طبقه بعدی میفرستد.
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