Path: utzoo!attcan!uunet!husc6!bbn!gatech!gitpyr!byron
From: byron@pyr.gatech.EDU (Byron A Jeff)
Newsgroups: sci.electronics
Subject: Re: CD Player Questions
Message-ID: <5835@pyr.gatech.EDU>
Date: 25 May 88 18:14:24 GMT
References: <3385@ncrcae.Columbia.NCR.COM<
Reply-To: byron@pyr.UUCP (Byron A Jeff)
Distribution: na
Organization: Georgia Institute of Technology
Lines: 46

In article <3385@ncrcae.Columbia.NCR.COM< wte@ncrcae.Columbia.NCR.COM (Bill Eason) writes:
<(Maybe rec.audio would be better for this, but I'm an engineer who has had 
<his fill of audio salesman talk.  Read on...)
<
<I have been shopping for a CD player at a distance until yesterday, and
<the salesman I had couldn't answer my questions to my satisfaction.  First,
<in terms of CDs, what is x-times oversampling?  My salesman explained that
<the player scans the disk four times rather than just one so that it will
<get "all the information on the disk.  This gives you more harmonics
<than you would get without oversampling."  My understanding is that
<the sound on CDs is already sampled at 16 bits, 44.x kHz.  This should be
<plenty to cover the audible range.  Any harmonics over this wouldn't make 
<a whole lot of difference, would they?  Surely the CDs aren't digitized at
<88.x or 176.x kHz, are they?  If not, then does oversampling mean simply 
<that the player scans the same bits of the 44.x kHz more times, possibly
<as an error correction/detection scheme?  Would a non-oversampling player
<miss so many bits to make a difference?  Help!
<
<Bill Eason   (803) 791-6917    ...!ucbvax!sdcsvax!ncr-sd!ncrcae!util5!wte
<NCR Corporation		  ...!rutgers!mcnc!ece-csc!ncrcae!util5!wte
<E & M Columbia   3325 Platt Springs Rd.   West Columbia, SC  29169

I finally got an explanation that I understood from Hal Chamberlain's
book on Musical applications for Microprocessors. 

CD players have a low pass anti-aliasing filter that attenuates
any frequencies above the nyquist frequency after converting the digital
into an analog signal. These filters must have a flat frequency response
and a very sharp cutoff. They are rather expensive to make.

What oversampling does is basically resamples the signal at 2 to 4 times
the original frequency and runs this through a digital filter that attenuates
most of the aliasing frequencies. The analog filter after the D/A can
have a much gentler cutoff because the digital filter has attenuated most
of the frequencies just above the nyquist frequency.

That's the gist of it. The CD laser mechanism runs at 44.1 kHz. The
conversion from 44.x to 88.x or 176.x is done after error correction.

BAJ

-- 
Byron A. Jeff
ICS Programming Lab, Georgia Tech, Atlanta GA 30332
Internet:	byron@pyr.gatech.edu
uucp:	...!{decvax,hplabs,ihnp4,linus,rutgers,seismo}!gatech!pyr!byron