[sdiy] Prophet 5 noise sources

[Jay Schwichtenberg]

Tom,

Just a thought.

Were both clocked at the same rate?

I could see having a noise source for modulation being slower than one for
audio?

Jay S.

-----Original Message-----
From: synth-diy-bounces at dropmix.xs4all.nl
[mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of Tom Wiltshire
Sent: Saturday, March 12, 2011 10:32 AM
To: synthdiy diy
Subject: [sdiy] Prophet 5 noise sources

Hi All,

Today I've been exchanging emails with a chap who's put one of my noise
generator PICs in a Rev 2 prophet. This has thrown up an interesting
difference between Rev 2 and Rev 3 Prophets - beyond the obvious fact one's
full of SSMs whilst the other's full of CEMs.

The Rev 3 Prophet uses two separate MN5837 noise generators. There's one for
the white noise which is fed to the voices, and a entirely separate MN5837
chip for the wheel modulation noise source, which is filtered to make pink
noise. Well, pinkish; 6dB rolloff from 100K and 10nF.

The Rev 2 Prophet uses the same source for both purposes, feeding the
unfiltered noise to a SSM2020 VCA, and also filtering it for pink noise
before feeding it to the wheel mod section.

So there you go! Between Rev 2 and Rev 3, Sequential decided it'd be a good
idea to have independent noise sources for modulation and audio. God only
knows why!

T.


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