Well, I've actually been doing some reading on sigma delta dac design to understand what the modulator step does.
In a generic 1bit SD design, the modulator accepts multibit data streams, and converts them to 1bit. There is generally *always* a filter after the modulator, to remove the high frequency noise that is generated by the conversion to 1bit.
There is a trade off when performing the conversion, that needs to be understood. As the word length is reduced, the sample rate increases. This is pretty obvious, and I don't know why I didn't jump to that conclusion earlier.
I've read a little bit about why oversampling is generally used in all SD designs. Basically the oversampling pushes the data up further away from the inherent noise floor. I haven't read how that helps any downstream processing, I haven't got that far.
I believe the PCM1794 chip is a multibit SD design. I haven't gotten to that either, to understand what exactly the modulator does in a multibit design? I think there are multiple 1 bit streams, but I'm not sure.
Oz,
All modern SD DACs use multibit modulators. There are different ways of arranging the bits. There are a few white papaers around will see if I
can a few up for you. The 1794 is advanced segment which approaches the bits a different way. All of these MB SD chips work at
very high frequencies, many MHz.
FWIW I think currently the Sabre has the most advanced moodulator - and the most linear DAC.
It's not as easy to work with though as the 1794.
The other thing WRT Sabre is that you can actually change many settings, even the number of bit's. There are many hidden settings.
The reason is this: the Sabre is basically an FPGA which is very programmable. You can disable oversampling, run I2S direct,
make your own filters etc etc.
I've been playing with spice to optimise a few OP stages that will work with Sabre that don't use opamps.
There are quite a few options, some that use tubes, some transformers.
cheers
Z