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정확히는 위 홈페이지 에디터에게 썼구요. JEJ 는 이메일이 없길래..
제가 기술에 문외한임에도 이번에 배우게 된 내용을 쥐어짜서 다음과 같이 보냈는데, 고칠 내용이 없나 함 봐주세요.
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Hi. I'm Young Min Kim, one of audiophiles in (South) Korea.
I'm writing this e-mail for a question about an article on your homepage.
Actually I must say I'm NOT fluent about technical terms at all. But I came to understand a little bit about technical things thanks to an explosive argue about an article by your editor JEJ, in an on-line audiophile community in Korea.
Below is the article
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http://www.hometheaterhifi.com/technical-articles/427-a-secrets-technical-article.html?start=3
"I tested some 10 kHz and 20 kHz sine waves that were recorded at several word lengths (16 bit or 24 bit) and sampling frequencies (44.1 kHz, 96 kHz, 192 kHz), analyzing them in a software sequencer."
"The dots represent finite voltage values that are fed in sequence as a stream to the DAC, which then produces a stair-stepped output, after which a low-pass reconstruction filter smooths out the signal. What I want you to notice is how jagged the lines are at standard Redbook CD 16/44. The DAC and reconstruction filter's job is to make these jagged lines more sinusoidal, so that it will be like the music that was recorded, which is also sinusoidal."
Fig 1. http://www.hometheaterhifi.com/images/stories/april-2008/vinyl-vs-cd-10-khz-sine-wave-16-44---16-96---16-192---24-44---24-96---24-192-large.gif
"The dots represent finite voltage values that are fed in sequence as a stream to the DAC, which then produces a stair-stepped output, after which a low-pass reconstruction filter smooths out the signal. What I want you to notice is how jagged the lines are at standard Redbook CD 16/44. The DAC and reconstruction filter's job is to make these jagged lines more sinusoidal, so that it will be like the music that was recorded, which is also sinusoidal."
Fig.2 http://www.hometheaterhifi.com/images/stories/april-2008/vinyl-vs-cd-20-khz-sine-wave-16-44---24-96---24-192-large.gif
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and JEJ made a comment like below
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written by JEJ , October 04, 2009
A Couple of Points said - And JEJ, it's really quite misleading to show DAC output before reconstruction filtering had done its job. NO ONE hears the sine waves shown above. -
It's not misleading. You just scanned the paragraphs too quickly. I specifically mentioned in paragraph 5 - The lines connecting the dots represent the signal that is fed to the output stage before any filters are applied. - and - The filter's job is to make these jagged lines more sinusoidal, so that it will be like the music that was recorded, which is also sinusoidal.
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Here's the point.
Many guys, many of whom are engineers, are talking that the explanation above about DAC processing is quite misleading, and not based on exact concept about digital audio processing. They say that first figure in Fig. 2 shows one "generated" by a program, and the lines connecting the dots is just simply connecting the dots and shows NOT the sinusoidal waveform, NOR the reconstructed wave. And because JEJ is explaining on the comment below that "the lines connecting the dots" are fed to the DAC, and go through the fillter, JEJ is making quite a mistake about DAC processing and Nyquist theorem.
That may be the exactly the same point raised by a guy called Joshua on a comment below
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Re: Straight Lines
written by Josuah , October 08, 2009
I don't believe the graphs are true representations of the signal before the low-pass filter because the dots were connected with straight lines instead of connecting them using the mathematical formulas described on the Wikipedia page. Specifically the last bullet point under "Mathematical basis for the theorem" which I quoted earlier, with respect to the Dirac comb function.
http://en.wikipedia.org/wiki/ Nyquist–Shannon_sampling_theorem#Mathematical_basis_f
or_the_theorem
44.1kHz does not lack the required information for sound reproduction below 22kHz, but you can deconstruct/reconstruct more accurately if your process is performed at a higher sample rate. It is somewhat analogous to performing a chain of multiplication and division operations using decimal places even though your original numbers are integers. By using the decimal places, you help avoid rounding errors. This is mentioned under the "Practical considerations" section of the Wikipedia page.
http://en.wikipedia.org/wiki/ Nyquist–Shannon_sampling_theorem#Practical_considerat
ions
Dan Lavry has a nice paper about this:
http://www.lavryengineering.com/documents/Sampling_Theory.pdf
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But, Another guy in our community is having another explanation. He says that since JEJ said he tested some sine waves that were "recorded" (rather than generated), JEJ might have used some waves he himself recorded using ADC and the filter may refer to a kind of ADC filter, NOT the low-pass reconstruction filter.
most of the guys I mentioned above are apparently criticizing strongly about the explanation above. and the conversation is, actually many times somewhat.. harsh !
So, I thought I need to request the author to answer this issue.
In summary, the point is whethere
1. JEJ made a mistake about the DAC processing concept. He tested some sine waves "generated" by a program, and the dots are actually the signals fed to the DAC but the lines connecting the dots is never the signal and means nothing.
or
2'. JEJ tested some sine waves he himself recorded using an ADC or digital sound signals, and the the lines connecting the dots represent the signals without any kind of recording filter.
Many guys say 1 is the case, and one guy says 2 may be the case.
and the guy insisting No.2 asked me to add another question, which is below
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I'm very confused now. Please let me know what kind of 'FILTER' you talked about. Is that a kind of anti-aliasing filter used in the recording stage or a low-path filter included in the DAC machine?
You said, "I think that if we were to go to 500 kHz sampling, 24 bit, no 'FILTER' would be necessary because typical studio microphones don't respond beyond 20 kHz, and some even roll off at 15 kHz."
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Please help end our long-lasting, sometimes boring or harsh argue in a positive way. Thank you .
Sincerely.
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