A to Synth: Bitcrusher tested

Saturday, November 21, 2020

Bitcrusher tested

I got the bitcrusher from JLCPCB working today. Last week I discovered that some of the tiny 0402 resistors had lifted from their pads, so the input filter did not let the signal through. Last night my wife soldered them (I love her and how skilled she is, those parts are soooo small!), and today I was finally able to test everything.

The left pad of the two 0402 resistors have lifted. Not sure if this happened in production or when the pins were soldered.

I am still surprised at how recognisable the output is at 1 bit 27kHz. It has a lot of noise, but the music is still very audible. I am still not entirely convinced that it does not leak from the input, but when I reduce to 0 bits everything goes silent so I guess it is actually possible,

I had a lot of trouble getting the sample rate high enough, currently the max is at 27kHz with a 1:1 prescaler on timer 1 and a timer start value of 0xFEFF. Changing to 0xF0FF drops sample rate to 2kHz, so it's really sensitive. I need to work more on this but I am fairly confident that 44kHz should be possible.

I still want to confirm that the filters work as expected and have a 20kHz cutoff, but if that works as expected I think I'm ready to mass produce the filter. Perhaps just doing a quick input SPI check first.

Here is my test setup btw, for future reference.

I've also tested the filter response for the input filter. It works fairly well although there are some slightly strange things going on.

First of all, at 10-15kHz, the output signal (what the ADC sees) is amplified slightly, before it drops towards 20kHz. At 20kHz it is close to what I see in the simulation, so that's great. At 25kHhz it is even lower than what was simulated. There is also some distortion on the sine wave, not much but visible. I'm not too worried though, as this is not a hifi system. The tests sounded good enough to me. I'm more worried about the phase shift I see even at low frequencies, They may be very audible if the signal is mixed with the original. We'll just have to see (hear).