[sdiy] Simplest random source

[Magnus Danielson]

From: Seb Francis <seb at is-uk.com>
Subject: Re: [sdiy] Simplest random source
Date: Sat, 16 Nov 2002 01:29:05 +0000

> I'm not convinced sofware is the way to go.  It would be no problem to do it in software, but I have a couple of reasons for wanting to use analog:
> 
> - CPU cycles are a premium - this PIC is doing a *lot* and I don't want to waste more than 10 cycles or so getting a random number.
> 
> - I have a hunch that a simple random algorithm (such as what could be achieved in 10 cycles) would sound a whole lot different (not so nice) to the human brain than for example taking a sample via A->D of some white noise.
> 
> I've seen really simple noise generating circuits using just a diode or transistor plus a couple of opamps .. I guess I was hoping there might be a way to get analog noise using just a couple of transistors or something - the quality of the sound doesn't really matter, as long as it's random.

Ah, right. Well, all the diode/transistor source stuff I've seen use up the
opamps for 1) Gain 2) Shaping/filtering noise. Now, if you don't want a pink
noise or a random walk, getting white noise is very easy. If you look at the
ASM-1 Noise Source schematics, you have a transistor as noise source (100k
resistor from +15V into a reversed BE of an NPN transistor) and two op-amps
acting as pure gain-stages.The first one has a gain of about 450 times and the
second up to about 210 times, thus giving a gain close to 100 000 times.
Naturally, it is classic transistor tricks to do this, but you need a gain of
that level. If you chooses to use transistors or op-amps I don't think really
cares, but I think a TL082 and a handfull of resistors isn't such a bad thing.

In the ASM-1 case one has separated the gain-stages and the filters, since one
wants the white noise output. However, if you really only need other forms,
then you can integrate filtering and gain-stages and the filter sections from
the ASM-1 case should give you a pretty good idea of how you could do that.
If you only want the Random Noise output, you could compress things into an
2-amp solution fairly easy.

Cheers,
Magnus - keeping the noise level up around the signal