[sdiy] LFSR digital noise source

rsdio at audiobanshee.com rsdio at audiobanshee.com
Mon Nov 11 01:44:41 CET 2019

One advantage of a CPU-based noise source is that it can be combined with a DAC for CV modulation.

Digital noise chips typically use the single-bit output for audio. Thus, it’s like a square wave with random period but basically constant amplitude. The conversion to analog can vary the amplitude if there is a PWM effect (via capacitance).

With a CPU and a long enough register, it’s possible to gather 8 or more bits into a word to feed a DAC. Just be sure to shift in 8 new bits for each 8-bit conversion, otherwise the values aren’t entirely independent of each other. You can easily extend this to 16-bit or even a 24-bit DAC, although using a 24-bit DAC for CV is quite dubious. LFSR hardware should be able to run at 8x, 16x, or 24x the sample rate with no problems.

Of course, with counter logic and a stand-alone DAC chip, it’s possible to do this in pure hardware. So, I guess the CPU doesn’t really have a unique advantage here.


On Nov 10, 2019, at 4:32 PM, bbob <fluxmonk at gmail.com> wrote:
> sometimes for the sound directly (crunchy/brittle), but more often slowing the clock and then using it as a modulation source
> On Sun, Nov 10, 2019 at 7:11 PM ColinMuirDorward <colindorward at gmail.com> wrote:
>> Hope this isn't too off topic, but I've often wondered when/why you'd choose a digital noise source over an analog one. It is simply for the acoustic flavour?

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