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Path: utzoo!watmath!clyde!burl!ulysses!allegra!bellcore!petrus!karn
From: karn@petrus.UUCP
Newsgroups: net.audio
Subject: Re: CD Reflections-Question, Mr. Simpkins...
Message-ID: <264@petrus.UUCP>
Date: Fri, 18-Jan-85 18:14:09 EST
Article-I.D.: petrus.264
Posted: Fri Jan 18 18:14:09 1985
Date-Received: Sat, 19-Jan-85 01:58:16 EST
References: <15100001@hpfcmp.UUCP> <3411@mit-eddie.UUCP> <1420@hplabs.UUCP>, <77@vice.UUCP> <849@hound.UUCP>
Organization: Bell Communications Research, Inc
Lines: 41

> ... Assuming there  something to the argument
> that brick-wall filters mess up the sound audibly so that oversampling
> is beneficial, what about the brick-wall filter the signal is put
> through in the recording (encoding) process? a) I assume there is one.

Yes, there is. Its purpose is to filter out all frequencies above half
the sampling rate. This is a much more critical function than the low
pass filter on the player, since allowing a high frequency through the
filter would produce unwanted aliased signals in the audio band. There
would be no way to remove these components once they had reached the digital
domain.

> b) I assume it's brick-wall. c) Doesn't it mess up the phase in the same
> way that similar filters are alleged to in the decoding process? d) Is
> there anyway to undo those effects? e) is there any alternative? e.g.,
> can oversampling or something be used at the encoding end with similar
> alleged benefits (still retaining the 44.1 rate on the disc)?

Ideally, it would also be brick wall. But brick wall filters cannot be built
so some practical approximation must be used. I don't know exactly what
is in the Soundstream recorders, but you can make very good analog filters
if you are willing to pay enough. In particular, if you make a Bessel
(maximally linear phase) filter you need LOTS of sections to get a quick
cutoff slope. Perhaps in commercial recorders the extra money available
makes it feasible to build them this way. I suspect that they simply
use Butterworth filters, compromising between the Bessel and the
elliptical (Cauer) filters which are common in CD players. Digital low
pass filtering similar to that used in the Phillips players could be used
here, but is considerably less practical since it requires faster A/D
converters which are considerably more expensive than faster D/A converters.

If you knew exactly what kind of filter was used in the recording process,
yes, all you would have to do is construct a filter with a compensating
phase response to make the concatenation of the two "linear phase". In
modem design, this is known as "equalizing the channel". But this
is easier said than done (it requires a lot of analog allpass sections or,
more easily, transversal digital filtering), and it all hardly matters
anyway. Just sit back and enjoy the music and quit worrying about all the
red herrings.  Only my oscilloscope can tell the difference.

Phil