Re: [討論] 無意間爬知識+看到

[john0312 (Chen John L)]

對於不同的CD格式, 我們首先要看的, 是數位資訊如何的存在這些載體當中.

CD 是採用 44.1kHz/16bit Stereo Pulse Code Modulation
SHM-CD 是 2.8224 MHz Direct Stream Digital ( Please refer to that
oversampling thing )
BluRay disk則不一定, BluRay支援許多格式(PCM, Digital Dolby, DTS... etc), 而且
有一些還是有損壓縮, 而實際使用的格式才能決定他的品質.

也就是說, 從這些光碟裡面讀出來的資料不一定相同. 我猜測BluRay裡面你們聽到的應該
是192kHz/24bit PCM, 因為其他幾乎都是有損壓縮格式. 如果他採用有損壓縮格式, 音質
必定會下降.

另外, 這三種格式的物理結構也有差距. 例如說BluRay的資料層較接近於表面, 所以他比
較容易被刮傷. 這些都會影響音質.

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再來討論CD to CD variation. 感謝louis0407兄提出的兩篇論文

I've checked the Audio Engineering Society, and discovered that the two
abovementioned paper actually cost twenty USD each. Therefore I doubt if you
have actually read them before. I've to admit that I myself have not read
them before as well, as I am not that rich to buy that paper. However, I have
found later papers that are done by the same authors as the two
abovementioned paper (Ian Dennis and Julian Dunn). The paper discussed the
various possibilities that will cause two numerically identical Compact Disc
to sound different.

First of all, if the same digital data is feed to the DAC, it will produce
the same analog waveform everytime, no matter where the digital data is from.
However, there's one exception to the above statement, that is the power
supply.  We have to be aware that the two stepper motor used in the CD-ROM
drive is inductive load, and due to the fast changing current, will cause the
positive power rail to flunctuate if the power supply is not well designed.
If the positive power rail flunctuates, it will be coupled into the internal
Zener Diode voltage reference of the DAC. However, in modern CD Player
design, the power supply for inductive loads ( the two motors ), the digital
loads and the analog loads are completely seperated, that means it will not
cause any observable change in the DAC output.

Therefore, whatsoever that causes variation between CDs will cause the input
to the DAC to change as well. In the paper, the authors addressed several
issues, such as the different ways that a PCM signal could have been encoded
in the CD-ROM, which might lead to differences between the master recording
and the pressed CDs. As I've mentioned before, in the old times, because
hardwares are not capable of that much calculation, they do not check if the
data they read is erroneous, which leads to errors ( the so called jitters )
being passed onto the DAC, and leads to degradation of audio quality.

In the paper, the author also carried out one mass listening test, and one
"golden ear" listening test. However, neither of the two listening test is
able to show that the difference between CDs, if any, is audiable, even to
expert listener, or the so called "golden ears".

In conclusion, if we are able to ensure that the motor noise doesn't get
coupled into the sensitive analog circuits, and if use error correcting
softwares to ensure that the data read from the CD-ROM drive is correct, we
can be assured that there is no variation between audio CDs that are
numerically equivalent.

EDIT:
The paper is at:
http://www.prismsound.com/m_r_downloads/cdinvest.pdf

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◆ From: 220.133.76.65
※ 編輯: john0312        來自: 220.133.76.65        (07/08 11:37)
推 SeaSprite:大推!!
→ SeaSprite:懶的看全文可以看最後一段或最後一句 但推薦全讀
推 louis0407:我確實沒看那三篇的全文 這一說起來 是我不夠負責
→ louis0407:不過這三篇論文是我在看一些討論時 裡面提到的 那篇討論
→ louis0407:的主題是為何cd聽起來會不同 換句話說 他們是建立在
→ louis0407:同曲目cd聽起來會不同(即使數位資料一致)這個共識下
→ louis0407:其中有提到一些相關的AES論文 因此我拿了幾篇 本意只是
→ louis0407:說明確實有這樣的學術討論 但只看過幾段引用就亂貼
→ louis0407:確實不甚恰當 這點我應該檢討
推 louis0407:至於你的結論 你為何不親自嘗試? 還是你已做過聽覺測試?
→ louis0407:我的看法是我不質疑這件事的存在 但是我不清楚why
推 louis0407:順便建議一下 能把jitter,offset還有error分開嗎 這樣
→ louis0407:會更容易釐清觀念(或者說分歧) 對我來說 jitter和error
→ louis0407:是不該混在一起互相替代的 雖然他們確實相關
推 louis0407:剛又想到 想請教john兄 您對轉盤端jitter對後端da影響
→ louis0407:的看法是? 是直接否定jitter這完意對聲音的影響還是
→ louis0407:針對cd壓制品質對聲音的影響在質疑?? 感謝
推 padro:我特別強調載體材質差異 就是對louis兄所說的有疑問阿XD