Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!psuvax1!brutus.cs.uiuc.edu!ginosko!uunet!mcsun!sunic!draken!d88-jwa
From: d88-jwa@nada.kth.se (Jon W{tte)
Newsgroups: comp.dsp
Subject: Re: how oversampling works (* LONG *)
Message-ID: <1788@draken.nada.kth.se>
Date: 26 Sep 89 21:41:42 GMT
References:  <1737@draken.nada.kth.se> <472@ctycal.UUCP>
Reply-To: d88-jwa@nada.kth.se (Jon W{tte)
Distribution: rec.audio
Organization: Royal Institute of Technology, Stockholm, Sweden
Lines: 43

>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

In article <472@ctycal.UUCP- ingoldsb@ctycal.COM (Terry Ingoldsby) writes:

- Something I've always wondered is what distortion this introduces.  If I

- 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.


Oversampling really introduces no new distortion (that's the real thing
about it...) but instead spreads (shifts) the spectra of the distortion
upwards proportional to the oversampling rate, and also softens it, also
proportional to the oversampling rate.

This is as compared to the STEEP 100 db/octave filters...

Compared to non-existant phase linear cut-completely-after-n-Hz filters,
of course there is the phase shift from the 30 db/octave filters (24 ?
48 ? depends, I think) and the possibility that, after all, a little of
the distortion from the "transients" between levels in the D/A leaks.

And, me too, says HOORAY for comp.dsp, although I am merely a young
student in computer science who happens to have stumbled over some
intuitive knowledge about sound from being into the new-wave era in
'82 :')

h+@nada.kth.se
-- 
All that glitters has a high refractive index.