[sdiy] Suitable DACs for Demultiplexing

[Antti Huovilainen]

On Fri, 23 Oct 2009, Ingo Debus wrote:

> You mean it remains a sine no matter how low the multiplexing rate is? Are 
> you sure? I think this would require an adaptive filter of some sorts (cutoff 
> goes down with input frequency). Not very desirable for audio...
>
> Or am I missing the point?

Yes. You're assuming a sigma-delta audio dac behaves like an ordinary dac 
and will nicely settle to some value. This is unlikely. First there's the 
rougly 64 tap upsampling filter. That is enough to lower your effective 
control signal samplerate from 48 kHz to 750 Hz. It is then followed by 
a 1 bit comparator with feedback loop, which will of course never settle 
but will instead output a complex waveform thats _spectrum_ will be 
equivalent to the input signal between 0 and 20 kHz.

There is simply no reason to optimize offset or gain (or their tempco and 
stability!) to closer than about 7 bit accuracy in an audio dac. Do you 
really want a slow and complicated 7 bit dac when you can buy a fast and 
decent 12 bit one dirt cheap?

Antti

"No boom today. Boom tomorrow. There's always a boom tomorrow"
   -- Lt. Cmdr. Ivanova