Xref: utzoo rec.audio:21714 sci.electronics:12221
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From: chuck@umiami.miami.edu
Newsgroups: rec.audio,sci.electronics
Subject: Re: My CD player is running slow! (Correction)
Message-ID: <6382.266ac585@umiami.miami.edu>
Date: 5 Jun 90 00:33:09 GMT
References: <1990Jun2.182334.21396@athena.mit.edu> <6378.26693284@umiami.miami.edu> <55024@microsoft.UUCP>
Organization: Univ. of Miami (IR)
Lines: 49

In article <55024@microsoft.UUCP>, gordonl@microsoft.UUCP (Gordon LETWIN) 
writes:
> In article <6378.26693284@umiami.miami.edu>, chuck@umiami.miami.edu writes:
>> 
>> There is no way in the world that the CD player would play "slow" or "flat"
>> because of the nature of the beast.  ...
>> ...Pitch-shifting in the 
>> digital domain is a fairly ugly computational process, usually handled by
>> high-speed DSP chips.  
> 
> This is wrong.  Pitch shifting is easy.  My first CD player was an
> inexpensive one and it had a pitch adjustment control on it.  It's my
> guess that you can pitch shift just by changing the rate at which you
> feed samples to the DtoA convertor.  CD players already have circuitry
> to adjust the rotation rate to keep the convertor FIFO happy, so I expect
> that if you just change your sample clocking rate the slight change of
> the disc rotation rate would occur automatically.
> 
> Regardless of how it was done, my old unit, which I paid $400 for maybe
> 5 years ago did it, so it's not a big deal.
> 
> 	Gordon Letwin

As many of you have pointed out, I was wrong in my statements included 
within.  Pitch shifting is *easy* when playing back CD's, cause all you do 
is change the sample rate.  However, what I said was true for the disc speed, 
cause if you slow down just the disc speed, you end up with "noise" (which 
someone else has also realized in this discussion).  As Gordon has pointed 
out in this article, you must change the actual sample rate of the entire 
system to make a pitch change.  I have the feeling I was thinking of sample 
rate conversion, not pitch shifting.  Anyway, I wasn't totally wrong... :-)

The point is this - most CD players today (correct me if I am wrong, you 
guys are good at that... :-) ) use oscillators that are usually pretty damn 
exact anyway, so there's no reason to want to change it.  I would rely on 
the pitch of a CD over the pitch of a tape deck any day, especially since 
the tape speed varies over the length of the tape, and the fact that the CD 
player is (supposed to be) accurate to 1/44100 of a second.

I never thought this would be such a big thing... :-)
-- 
===========================================================================
   Chuck Urwiler    University of Miami Music Engineering   Voice & Keys  
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 chuck@miavax.ir.miami.edu |
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