為何 PS3 撥放 CD Audio 音質不佳,淺談 PQLS 和 H.A.T.S. (轉載)

wil-wil

http://www.hd.club.tw/thread-17324-1-1.html


在網路整理一部分認為合理的資料文章.

(1) 首先我們先從數位傳輸的基本,取樣頻率開始談起,等會會用到



digital_sample.jpg (41.54 KB)
取樣頻率的影響
2008-12-13 23:21



圖左是相對較低的取樣頻率,圖右是比較高的取樣頻率.取樣頻率越高,還原回來的信號也會越接近原始信號 (藍色線條表示)

(2) 接著再談到數位傳輸的格式,這邊的資料來源是:  http://audio.peufeu.com/node/7

原文節錄:
Consider a stream of samples at 44.1 kHz with 16-bit precision. This means we have 65536 different amplitude levels.
.......
Consider a signal which is entirely made of null samples with just one spike at maximum amplitude. The DAC will output this as a rectangular pulse,
1/44100 s in width.

In order to correctly reproduce this signal, we must have as much precision in the time domain as we have in the amplitude domain.


此文及後面文章會提到,數位傳輸的內容由兩大部分組成:
(a) Amplitude info. : 描述信號的大小強度,我們的取樣 bit 數越高, Amplitude 精細程度越細.

(b) Timing Info.: 傳送取樣頻率,此文提到期時 clock 是視為 Analog 方式去傳送
      The synchronisation between the sampling (ADC)  and the reconstruction (DAC) of the analogue
      output stream.
      (http://www.avforums.com/forums/blogs/welwynnick-64635/626-how-does-jitter-affect-hdmi.html)

         因此擴大機 (接收端) 要有跟 player 相同精準的 clock 來還原原始信號.
         注意: 此處指的是取樣頻率



(3) 目前的數位介面如何傳送 取樣頻率.
      (http://www.avforums.com/forums/blogs/welwynnick-64635/626-how-does-jitter-affect-hdmi.html)


(3.a) SPDIF
SPDIF carries both streams on the same channel - the timing info is embedded into the data stream, which are recovered and separated by the receiver for sending to the DACs. The data tends to corrupt the timing. (缺點)


(3.b) HDMI
HDMI does it quite differently, and carries data (amplitude info) and clock (timing info) on physically separate channels.
Unfortunately (and this is why audiophile manufacturers criticise audio on HDMI) the HDMI clock is a video clock, not an audio clock.......

HDMI 修正了 SPDIF 將 Clock 和 data 一起傳送相互干擾的問題,但是我認為,當初 HDMI 制定主要是應用在 Video 播放,因此就沒有考慮到 Prue audio (如 CD, SACD, DVD-Audio..) 撥放的情況會出現..所以就產生了 HDMI Jitter 這樣的問題..

HDMI seems to perform much worse when there is no video being carried. Playing a CD over an HDMI connection can be very poor, and spdif can sound significantly better. The difference can only be attributed to jitter on HDMI. LPCM with video on HDMI seems to be a different matter though, and can sound very good

我們都誤會 PS3了,其實原罪是 HDMI 或著該說是 User 把 HDMI 這個本來該撥 Video 的東東拿來撥 Pure Audio. 此處有第二個重點就是,如果有 Video info. (如 BD Movie 等等..那麼 Player 和 Amp 就能 Sync.,只有在 Pure audio playback 的時候才會有 HDMI Jitter這樣的問題)

4) How does HDMI 1.3a solve jitter problem?
在 HDMI 1.3a Spec 裡頭有包含 CEC (Consumer Electronic Control) 這個附錄.
在裏頭的 Sec 13.16 有提到,可以用 <Set Audio Rate> 這樣的 message 去控制 Source (Plyayer) 的 Data Rate.

A device may control the audio rate from a Source Device by sending a directly addressed
<Set Audio Rate> message.

相對的也有離開 <Set Audio Rate> 這樣 mode 的 command

The audio rate controlled state is left when the controlling device sends a <Set Audio Rate> message with [Audio Rate] = "rate Control Off", to the Source Device. The controlling device should send a <Set Audio Rate> command at least once every 2 seconds for active sensing. If a <Set Audio Rate> message is not received within 2 seconds or the status of the Source Device changes internally, then the Source Device shall quit the audio rate controlled mode.

這樣的 <Set Audio Rate> Message 其實是有不少限制,每兩秒要傳送到 player,且若 player 兩秒內沒收到 這樣的 message 將結束 rate control 狀態.

總結上述,這是HDMI 1.3a 才提出的新機制.

http://www.avforums.com/forums/blogs/welwynnick-64635/627-how-can-hdmi-avoid-jitter.html 文章裏頭也提到

The master audio clock is now in the amp, where it should have been all along (which is why I said "Most audio systems screw up digital audio in one way or another"). The timing info from the master clock is needed at the DACs to regenerate the orignal analogue audio,

The key though, was not the physical interface itself (HDMI TMDS, IEEE 1394 or ethernet or whatever) but the protocol that was run on it. I called it the A&M protocol, which comes from the CEC standard, but each manufactuer had his own name for it, like HATS or PQLS etc.

.......

In this configuration, the timing information is generated in the amp, leaving the source to supply only the digital amplitude information.  Timing info is sent back to the source to maintain flow control.

綜合上述,我整理出合理的幾個結論如下

(5) Conclusion
5.1 HDMI Jitter 只存在 Pure audio playback 的狀況下,如 SACD, DVD Audio, CD, 這些 Pure audio packet 要撥放的好,就要傳送原始的取樣頻率給 AMP.
否則AMP 可能只能去猜一個取樣頻率,自然也得不到最好的音質輸出. 有 PS3 60G 的網兄不妨試試,用 SPDIF 是可以得到比HDMI 輸出要好的結果
但是有 Video data 的 HDMI packet (如 BD Movie), 是不會有這樣的問題存在.


5.2 PQLS 和  HATS 是用來解 AMP 不知道 Source (Player)  在播放 pure audio packet 送出 Data 的 取樣頻率,合理的解釋為何只在 CD, SACD, 或是 DVD-Audio 撥放時候才會啟動這些功能.因為,只有這些pure audio 會有 HDMI jitter 的問題.


(6) Appendix
當然也有網路上的討論是講 ETK 兄提到的兩系統 Clock sync.的問題,但是反過來想如果因為線材的傳遞造成 Jitter,會影響音質
就算AMP 傳 Timing information 回來player 也解不了這樣的問題,因為相同路徑不也一樣會遇到線長誤差的 Jitter,
這樣兩端的 local clock  (Player <--> AMP) 還是永遠沒法  Sync.
這些討論如 http://forums.audioholics.com/forums/showthread.php?t=48961&page=2
個人淺見並非 PQLS 和 HATS 要去解的問題.所以放在最後.大家有空再看看就可.

[ 本帖最後由 wil-wil 於 2008-12-15 01:40 編輯 ]

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