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From: ingoldsb@ctycal.COM (Terry Ingoldsby)
Newsgroups: comp.dsp
Subject: Re: how oversampling works (* LONG *)
Summary: What distortion does this introduce?
Message-ID: <472@ctycal.UUCP>
Date: 25 Sep 89 19:41:22 GMT
References:  <1737@draken.nada.kth.se>
Distribution: rec.audio
Organization: The City of Calgary, Ab
Lines: 42

In article <1737@draken.nada.kth.se>, d88-jwa@nada.kth.se (Jon W{tte) writes:
...
> Oversampling's very simple. To get good sound quality, you have to
> filter out everything from and above half the sampling frequency.
...
> This cases the use of VERY steep filters ( > 100 dB / octave) which
> causes BAD phase distortion ( ~ 180 degrees :-( )
> 
> The solution is to "fake" a higher sample pitch, causing the CD to
> interpolate the samples between the actual samples. The waveform
> now looks like this: (4-fold oversampling)
...
> And requres much less steep filters (typically 30 dB/octave) which means
> no phase shifting. Good CD players use more bits in the output stage
> than is recorded on the CD (i.e. 18 or 20 instead of 16) to get better
> resolution in the oversampling if the variation between each sample is
> small (i.e. low output) so that they don't have to interpolate 47 -> 48
> as 47 47 47 48 48 but could use 47 47.25 47.5 47.75 48 and thus get a
> smoother, cleaner output.

Something I've always wondered is what distortion this introduces.  If I
weren't so lazy I'd test it out in the frequency domain (if I don't get
any satisfactory answers maybe I will).  I understand that this is
practically the best way of doing the filtering, but what I want to
know is the difference between the output using oversampling (with
30 dB/octave filters) vs no oversampling and perfect (non-existent)
filters that have no phase shift.

I note that Philips recently went the other direction and used a
single bit output with a *very* high sampling frequency.  They claim
that although there is lots of noise, it all occurs at very high
frequencies (>50 KHz) where it can be filtered out easily.  ie.
filters that have appreciable phase shift at 100 KHz don't have
much at 20 KHz.

Comments?

Hooray for comp.dsp.  (Finally, someone who understands me :^)
-- 
  Terry Ingoldsby                       ctycal!ingoldsb@calgary.UUCP
  Land Information Systems                           or
  The City of Calgary         ...{alberta,ubc-cs,utai}!calgary!ctycal!ingoldsb