[sdiy] Prophet 5 noise sources

[Tom Wiltshire]

On 12 Mar 2011, at 22:06, Neil Johnson wrote:

> Jay,
> 
>> Were both clocked at the same rate?
>> 
>> I could see having a noise source for modulation being slower than one for
>> audio?
> 
> The MN5837 has an internal clock source, so you have no control over it whatsoever (other than perhaps varying the supply voltage, which *might* change the internal clock frequency).
> 
> You apply power and ground, and out comes the SCHUFFSCHUFFSCHUFFSCHUFF..
> 
> Neil

My understanding from the datasheet was that changing the supply voltage does change the clock frequency, pretty directly. Going back over it I don't actually see where I got that idea from. Anyway, the gist of it is that it covers the full range from "bad" to "dreadful", with clock frequencies from 24 to 56KHz. The cycle time is given as 1.1 to 2.4 seconds - No, it is, honestly. Seriously. You can check if you don't believe me;

http://pdf1.alldatasheet.com/datasheet-pdf/view/9284/NSC/MM5837.html

I did think it was significant that the Prophet rolls off the high end for noise modulation though. Significant enough that I went and tried it with a noise source both with and without a 100K/100n filter, feeding the result into my Pro-One (close enough to a Rev 3 Prophet voice). To be honest, I couldn't really hear much difference. I was expecting the filtered version to be "smoother" or "tamer" or some such, but if it is, I can't really hear it. Not that my hearing is anything to write home about, but then, neither is most people's. So I'd like to hear the rationale behind the decision to use pink noise for the wheel mod, rather than the white source directly.
It might have something to do with DC levels, since my PIC noise source kicks out a random signal from 0-5V, which means there's an average 2.5V DC level hiding in there. I suspect the MN5837 does similar. However, you'd be looking at highpass filtering to remove that, not lowpass filtering, so I still don't know what they had in mind.

T.