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Path: utzoo!watmath!clyde!saf
From: saf@clyde.UUCP (Steve Falco)
Newsgroups: net.audio
Subject: Re: CD Reflections - 44.1k?
Message-ID: <755@clyde.UUCP>
Date: Tue, 15-Jan-85 08:31:46 EST
Article-I.D.: clyde.755
Posted: Tue Jan 15 08:31:46 1985
Date-Received: Wed, 16-Jan-85 03:57:12 EST
References: <15100001@hpfcmp.UUCP> <3411@mit-eddie.UUCP> <1420@hplabs.UUCP>
Organization: AT&T Bell Laboratories, Whippany NJ
Lines: 32

> It seems to me that the disc itself was created with a given sampling rate.
> How can a player change this?
> 
> Bob

The disc has a sampling rate of 44.1k.  However, you can feed that into
a digital filter along with additional "made up" samples (a value of 0
is used).  The new sampling rate depends on how many zeros are added. 
If you add 3 zeros, you now have 4x the rate or 176.4k.  According to
the article in the Phillips Technical Journal (1982), the net effect is
to suppress the aliases of the original spectrum.  They achieve about
50db of suppression which makes the analog filter that much easier to
build.

Note that there are still aliased components at 24.1k (44.1k - 20k)
but they are much lower, hence easier to suppress.  This stuff looks
like out-of-band noise in some sense.  By the way, the output of the
digital filter is 28 bits, 16 from the original sample, plus 12 that are
added by the multiplier coeficients in the filter.  This gets averaged
down to 14 and then fed to the D/A.

Now for the part that bothers me:  The Nyquist theorem always seems to
be based on continuous signals.  I.e, the reason you can sample a 20k
signal at just slightly more than 40k is that EVENTUALLY you will get
some samples around the peak amplitude - you will also get some around
zero.  But music isn't like that - notes start and end.  In effect, you
are multiplying (or modulating) the sine wave by an envelope which also
contributes to the spectrum.  HOW MUCH ADDED SAMPLING DOES THIS REQUIRE?
I don't want to listen to a 20k sine wave, I want transients (anybody
remember TIM distortion?).  How about it digital types? - any guesses?

	Steve Falco   AT&T Bell Laboratories  Whippany, NJ