Oversampling just means outputting the DAC value multiple times to average across multiple sets (though how this would make a difference at the sample/hold stage in such a short time period, I don't know).

That's not quite how oversampling works. In order to do 4x oversampling, 3 zeros are inserted between between each consecutive pair of samples. Then, the result is low-pass filtered to smooth it all out. If your filter is designed right, the original samples are usually untouched, and the 3 intermediate zeros are replaced with a combination of several of the nearest samples. (That's a conceptual description of it; in practice, the calculations are combined together and reduced so that the three "zeros" are never actually generated.)

Now the nice thing is that this low-pass filtering is a necessary part of the D/A process, in order to prevent aliasing distortion. Doing some of that filtering digitally (i.e., before the D/A) enables you to use a less aggressive analog filter stage to finish the job. Doing all the filtering in analog (i.e., after the D/A) often introduces magnitude and phase distortion that you just don't want. The digital filter response can be tailored much more exactly than an analog filter can.

I believe that "noise shaping" is related to the digital filtering process. Due to the way the digital filtering is performed, the 3 intermediate samples are likely calculated out to 24-32 bits. Now they have to be chopped down to 16 bits in order to send them out of the D/A, so that is going to introduce some noise into the signal. "Noise shaping" is a way to control the frequency content of that noise. You really can't change how much noise that you're introducing; however, you can shape it so that most of the energy sits in frequency bands that you can't hear very well anyway. (Or for that matter, since the D/A converters are running at 176.4kHz, you can shape much of the noise to sit in frequencies that you can't hear at all.) In a sense that fits in real well with the MP3 concept...


Michael Grant
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Edited by mcgrant on 6/9/00 00:49 AM.