[sdiy] Differential 12dB/oct filter

[Eric Brombaugh]

On 04/27/2016 08:24 AM, Tom Wiltshire wrote:
> Well, I don't know about "excessively", but I'd certainly like less noise. I'm playing with using the 33FJ64GP802 for short delays. I've got audio input into the 12-bit ADC (at 32KHz sampling rate), and audio output from the 16-bit DAC at 64KHz, so it's currently a one-chip-wonder!
> 12-bit audio isn't too bad (after all, 12-bit/32KHz was an expensive studio rack mount delay processor in the middle 1980s!!) and the 16-bit output gives me headroom for processing.

I've done something that's similar, but with STM32F373:

http://ebrombaugh.studionebula.com/synth/f373_mod/index.html

That uses two of the 16-bit SDADCs for input and two of the 12-bit DACs 
for output. It sounds pretty good, but suffers from some quantization at 
low output levels of course. The F373 is pretty cool in that its analog 
section is one of the best I've come across in embedded MCUs - very low 
noise. Downside is that there's only 32kB of on-chip SRAM, so you can't 
do huge delay effects with it. It does run most kinds of DSP pretty well 
though - I've got a few different effects running on it, selectable 
through the OLED / button interface.

> It's funny, I haven't had such noise problems with it when doing oscillator designs, so I think perhaps some of my noise is coming from the ADC or the signal going into it. I should probably do everything I can to ensure that the input is smooth and clean before worrying about the output.

The 12-bit ADCs on the dsPIC are pretty noisy. They quote 12-bit in the 
spec sheet but I typically see about 2lsb of noise on them. I'm not 
surprised that you can hear it.

The 16-bit DACs are no great shakes either. There's a considerable 
amount of dithering and limit cycling on the outputs - I can recall 
driving a steady DC output signal and seeing noticeable dithering on the 
scope trace at the oversample rate. Good reconstruction filtering is a must.

Eric