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Noise as modulation source
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griessenboeck



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PostPosted: Mon Mar 26, 2018 12:07 am    Post subject: Noise as modulation source Reply with quote  Mark this post and the followings unread

Hi, I am working on a standalone monophonic synth at the moment. Does it make any sense to use white noise as a modulation source for VCO Pitch or VCF Cutoff Frequency? I think the SH-101 does something like this, but i am not sure if it modulates with noise alone or if i need some additional circuit. I am not talking about S/H.....i just want to get a litte dirt in the sound. Thank you.
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Grumble



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PostPosted: Mon Mar 26, 2018 2:23 am    Post subject: Reply with quote  Mark this post and the followings unread

If you don't want to use a s/h then a filter would be appropriate to get a usable control voltage, because if you use just noise as a control voltage you end up with, well..., just noise Rolling Eyes
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griessenboeck



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PostPosted: Mon Mar 26, 2018 3:07 am    Post subject: Reply with quote  Mark this post and the followings unread

Thanks for the answer. Perhaps you can help me a bit more....in the circuit of the SH-101 there is a simple noise source which can be routed to the VCO and the VCF. I can only see a little circuit between noise an the modulation switch. Is this some kind of highpass filter or is this just to bring down the level?


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Grumble



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PostPosted: Mon Mar 26, 2018 5:19 am    Post subject: Reply with quote  Mark this post and the followings unread

Well it's hard to see, and I use a lot of my imagination right now and assume a lot because a lot of the diagram has been removed, but I think the resistors form a kind of voltage divider, and I think so looking at the two parallel resistors of 100k and 2.2M which obviously are designed to form a specific value this way.
There is also a capacitor just before the 1k resistor, and it forms a filter with the output resistance of the noise source.

HERE you can figure out at which frequency this filter is designed.

But.... it would help if I could have a look at the complete schematics.

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griessenboeck



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PostPosted: Mon Mar 26, 2018 6:03 am    Post subject: Reply with quote  Mark this post and the followings unread

Here is a link to the service manual with the circuit.

http://www.synfo.nl/servicemanuals/Roland/SH-101_SERVICE_NOTES.pdf

It is on page 7, J7


Edit: OK, the Filter has a cut-off frequency fc = 15915.49430919[Hz]
So, first a voltage divider to bring the level down followed by a filter to limit the frequency.
This helps me already a lot! Thank you.
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Grumble



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PostPosted: Tue Mar 27, 2018 2:37 am    Post subject: Reply with quote  Mark this post and the followings unread

griessenboeck wrote:
Filter has a cut-off frequency fc = 15915.49430919[Hz]

Are you sure? How much is the capacitor C60? ( .01c? )
Also looking at the schematics my first assumption was wrong: signal is coming from the right, and the resistors R173 and R174 are NOT parallel but R173 is connected to some obscure voltage source Cool and R174 is connected to ground.

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griessenboeck



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PostPosted: Tue Mar 27, 2018 5:12 am    Post subject: Reply with quote  Mark this post and the followings unread

C60 (0.01) and R172(1k) are forming a lowpass filter i think. The frequency seems to be fc = 15915.49430919[Hz]
C61(10uF) and R174(2.2M) looks like high-pass. Perhaps only there to block DC voltage?
R173(100k) is bit of a mystery....i think it is connected to 5V. But i don't know why.

Anyway, in my project the noise modulation is almost working like i want it to do. I attenuated the noise to a useable level and can now modulate the vco with it. It has a nice whistle character. Unfortunately it also shifts the pitch of the vco up a few notes......so i guess there must be a dc-voltage somewhere in there. I'll have to find out how to block it.
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stolenfat



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PostPosted: Thu Apr 12, 2018 12:24 am    Post subject: Reply with quote  Mark this post and the followings unread

I'm not sure if this is helpful, but could a simple 'fuzz' circuit get you the grit your interested in? I don't have a euro set up so im not sure how op amps with a dual supply would react but i think it at least may be something worth considering. Just an idea, maybe it will spark the brain.
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JovianPyx



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PostPosted: Thu Apr 12, 2018 3:15 pm    Post subject: Reply with quote  Mark this post and the followings unread

If you modulate a VCO with frequency and carefully set the level, I think you can get a tonal noise sound. Surely as Grumble stated, if the modulation level is high, you'll get pretty much just noise. However, I think that if the modulation level is low enough, you should be able to get what I call tonal noise - a sound I first heard from a Moog synth played by Wendy Carlos (Switched on Bach). The noise modulation of the pitch CV will cause the frequency of the VCO to "spread out", the more modulation, the wider the spread.

HTH

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