[sdiy] Single chip digital delay

[Antti Huovilainen]

On Fri, 27 Jan 2006, Seb Francis wrote:

> Do you mean by CODEC, a combined ADC and DAC?  I hadn't even thought about

Yes. Cheaper and less trouble than separate ADC & DAC. There is some 
quality cost but it's unlikely you could get true 100 dB SNR without prior 
mixed signal experience anyway.

> The problem with using a dsPIC is I have only a Picstart Plus which can't 
> program them.  But I think a PIC18FXXX would be enough for my simple delay

The main issue here is interfacing. You REALLY want constant reliable 
timing. dsPIC's codec interface would handle that automatically.
If the adc & dac have internal oversampling, processing power is a 
non-issue.

> Both have SPI interface so it would be easy to interface them with a PIC, but 
> I'm a bit confused about the amount of oversampling needed for the DAC. 
> These DACs support up to 16x or 8x oversampling respectively.  Does this mean 
> they can effectively be driven up to 16x 48kHz?  There doesn't seem to be a

That depends on whether it has internal oversampling or not. Preferably 
you'd want both ADC & DAC to have internal oversampling. Manual 
oversampling (2-4x) is also possible, but you pretty much need dsPIC or 
other dsp for that.

> the maths based on the SPI max clock rate it looks like they mean 16x 48kHz. 
> Where am I going to get all this data from?  The max data I will have from 
> the ADC will be at 96kHz, and even then I'd prefer only to store half of it 
> (48kHz).  How is this oversampling data generated?

You downsample the input and upsample the output. In practise this means 
zero-stuffing and FIR filtering = lots of multiplications = DSP.
Or you can just get a codec that does it automatically.

Antti

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