From: kolstad@cae.wisc.edu (Joel Kolstad)
Subject: Re: 8x oversampling CD player
Organization: U of Wisconsin-Madison College of Engineering
Keywords: oversampling, CD, digitized audio
Lines: 40

In article <hcbC5un9L.DD0@netcom.com> hcb@netcom.com (H. C. Bowman) writes:
>
>Hello--
>
>I just bought a new portable CD player for the office, and I notice that
>it proudly proclaims "8 TIMES OVERSAMPLING" on the box.  Now while I think
>I understand what oversampling is (the rate of discrete "samples"
>exceeds the highest frequency component of interest by some factor),
>I don't understand this "8 TIMES" business...  It seems to me that when
>I bought my first CD player (was it REALLY 10 years ago?!), the specs
>said "4 TIMES" ...  Could someone please tell me whether I'm getting
>senile?  If I'm not, then what good does it do for the player to take
>samples at a higher rate?  If I really wanted better fidelity, wouldn't
>I have to have the same higher rate of sampling during the recording
>process?  Furthermore, am I wrong in interpreting the sampling rate
>(from the player's point of view) as being the same thing as the data
>rate for the bit stream coming off the optical medium?  Does this mean
>that the data rate (related to the rotational speed of the disk) has 
>changed since 1983?

[Note: I just tried to figure this stuff out about a month ago myself, from
various people on the net, so I could be wrong.]

The data is only ever read once (barring mistracks and such, of course),
and eventually gets turned into 44.1 KHz, 16 bit, two channel data.
Oversampling takes two discrete data points, and interpolates n-1 points
between them for n times oversampling.  When I asked, people said that the
interpolation was not simply linear interpolation, but significantly more
complicated.

Anyway, then, the purpose of oversampling is to move the "effective"
sampling rate up to n times 44.1 KHz, in order to use higher frequency
antialiasing filters.  For the same quality filter, higher oversampling
lets you build cheaper filters, whereas for the same price filter, higher
oversamplings lets you build better filters.  So, assuming the quality of
all other components in a CD player remained the same, oversampling should
allow a manufacturer to produce _slightly_ better sound due to anti-alias
filtering.

					---Joel Kolstad
