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.
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.
برگردیم به مطلب تورستن در مورد انتقال دیتا از فایل به DAC :
|Beyond bit-perfect: The importance of the Player Software And MAC OS X Playback Integer Mode|
Damien PLISSON, Audirvana developer
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:
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:
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]
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.
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
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.
[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.