So your 2048x oversampling DAC basically has a lookup table of 65536 2048-bit patterns, one for each possible PCM input, and just shifts out the right one at 90Mhz? And the cleverness is in designing the 2048-bit patterns?

No, actually, that wouldn't work either---you'd still be limited to 2048 voltage levels. The patterns are not simple functions of the input level at all. Instead, these devices use nonlinear feedback within themselves to figure out when to switch back and forth.

The technology is called "sigma/delta modulation". The confusing part is that, in a sense, you're doing analog to digital conversion---because you're turning a higher-bit signal into a 1-bit signal. Here you can find a few diagrams that might prove helpful, but this introduction is very incomplete. Here is an on-line paper called "A Fundamental Introduction to the Compact Disc Player" which discusses many aspects of digital audio, including MASH and sigma-delta modulation. I'm sure a search of "sigma delta modulation" on various search engines would dig up more.


Michael Grant
12GB Green
080000266