[sdiy] Mixed-signal problems (dsPIC digital delay project)

Richie Burnett rburnett at richieburnett.co.uk
Thu Jan 26 18:13:37 CET 2017


"The spikes are causing an oscillation at around 2.7MHz. This comes out as basic white noise on the audio."

What makes you think that the 64kHz pulses or 2.7MHz ringing are causing white noise at the audio output? You can't hear 64khz or 2.7MHz after all?

It's not unusual to see all sorts of RF clock noise if you probe around Dsp systems, from things like bit clocks, word clocks, MCLK, etc. These have to have fast edges to do their job, and as a consequence they couple on to practically everything nearby to some degree.

I'd suspect a noisy audio DAC in the dspic. I've used the dspic series for lots of commercial projects and have found that you can achieve genuine 12-bit performance from the ADC even at full sample rate, but you have to work at it. Four layer board with internal power planes, separate analogue and digital planes, separate low noise LDO regulators for analogue and digital power, careful layout and decoupling, etc, but decent ADC performance is achievable, if 12-bits is good enough for your application. 

The on board audio DAC on the other hand is noisy. Very noisy. Far more noisy than it has any right to be. Even if you use all of the good practices of segregating power planes, seperate regulators, ferrite beads etc. It's just noisy, presumably in its delta sigma modulation. It doesn't do a great job of pushing its quantisation noise out of the audio spectrum, which is a shame because it has got convenience going for it.

So if you want decent audio performance I think you would be better biting the bullet and using an external 24-bit CODEC chip.

Best regards, 

-Richie,

Sent from my Xperia SP on O2




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