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| Title: | Welcome to the CD Notes Conference |
| Notice: | Welcome to COOKIE |
| Moderator: | COOKIE::ROLLOW |
|
| Created: | Mon Feb 17 1986 |
| Last Modified: | Fri Mar 03 1989 |
| Last Successful Update: | Fri Jun 06 1997 |
| Number of topics: | 1517 |
| Total number of notes: | 13349 |
21.0. "Any takers?" by TSC::TOPSTEAM () Tue Jun 26 1984 22:11
I have yet to see a clear explanation, including mathematical analysis,
of what happens when you take a set of sounds, listen to them with microphones,
run them through steep low pass filters, sample them at rate X, mix them down
with a digital mixer onto a digital master (presumably using the same clock),
transfer them to a CD master at clock rate Y (possibly differing from X),
convert them back to analog signals, pass them through steep low pass filters
and into your amplifier (speakers are a whole 'nother thing). If anyone has
seen such an explanation, could you please direct me to it?
NOTE: Do not direct me to explanations of what happens when you take a signal,
pass it through an infinitely steep low pass filter cutting off at .5X, sample
it at rate X, convert it back to analog signals and pass it through a similar
low pass filter. What happens in this case is simple and unambiguous and
impossible except as a mental exercise. What is not simple is analyzing what
possible distortions can be generated by the real world CD process, which of
these distortions are audible and to what extent.
I realize that this request will probably not pep up this notes file
any, as those interested in asking this question seem to be a tiny minority.
Robt. P.
| T.R | Title | User | Personal
Name | Date | Lines |
|---|
| 21.1 | | MUN02::ORA | | Tue Jul 03 1984 14:54 | 6 |
| I think there has been some discussion on these subjects in the earlier
notes in this file...
There is a huge amount of literature on this subject (long before the CD
era). It has been extensively studied e.g. in connection with digital
telephone systems etc. I don't feel like rewriting one of the books here...
|
| 21.2 | | TECATE::TOPSTEAM | | Sat Jul 28 1984 03:19 | 22 |
|
I'd like to say I told you so, but I can't. I do feel somewhat vindicated,
though. The last time I raised this issue, I was referred to a discussion of
the matter in Audio magazine. The July issue of Audio magazine presents a
discussion of nonlinear phase shift in CD recordings. Although my earlier
primitive hypothesis was wide of the mark, I am happy to find that not only am
I not the only one who hears harsh strings in CD recordings, but that some
people have a technical understanding of why this is and are working on answers.
Also encouraging are A) the fact that nonlinear phase shift can be
corrected in the recording (I've always had a dark suspicion that preprocessing
of the recording was necessary and sufficient) B) dbx (certainly a reputable
company in recording studios) is working on something called delta modulation
as an alternative recording technique (they must feel there is some commercially
viable motivation for this) C) (on a separate but related issue) there is
mention in the same month's Audio that someone is working on a signal processing
technique to remove noise from analog masters without altering musical content
(I think NASA (at JPL Labs) has shown this to be a feasible thing to do with
video signals, there is no reason it couldn't be extended to audio, with some
(great amount of) care) so that digital pressings of memorable old recordings
can be produced in an honorable way.
Robt. P.
|