Re: [討論] 無意間爬知識+看到
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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※ 編輯: john0312 來自: 220.133.76.65 (07/08 11:37)
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