Signal Differentiator

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dougcl
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Signal Differentiator

Post by dougcl » Thu Dec 18, 2008 3:34 pm

Hi folks after thinking about how to detect changes in a pitch cv for the purpose of triggering an ADSR, I realized that taking the derivative of the signal would work in many cases, and that it can be realized using a S&H, LFO and voltage inverter:

Image

I think this could also be used to derive a quadrature signal from the input since the derivative of sine is cosine. The frequency of the LFO determines the accuracy of the circuit. Higher freq-> more accuracy, although the S&H can only go so fast before things start falling apart. I only had a few minutes to test this, but it looked interesting, if not perhaps complicated. It might make a nice module.

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plord
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Post by plord » Thu Dec 18, 2008 5:26 pm

Hm, would this behave differently than a CGS Slope Detector?

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dougcl
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Post by dougcl » Thu Dec 18, 2008 6:29 pm

The CGS Slope detector looks pretty neat:
The slope detector is an event-driven gate/trigger generating device. It monitors a control voltage, and responds according to what that voltage is doing. If the voltage is rising, the slope detector gives a "gate" output (approx. 5V) on its "rising" output. Likewise, if the voltage is falling, the slope detector gives an output on its "falling" output. The duration of this gate signal depends on the incoming CV and the setting of the sensitivity pot. When the CV is remaining constant, the "steady" output activates.

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Post by Luka » Thu Dec 18, 2008 6:33 pm

im surpized how much all my maths knowledge comes into practice in modular world. woo maths :S :)

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neandrewthal
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Post by neandrewthal » Fri Dec 19, 2008 1:56 am

So every positive going clock pulse you get a positive sample of the waveform's current voltage and every down going pulse you get an inverted sample?

If it were to function like a slope detector I guess your clock and your waveform would have to be related, like one so many degrees out of phase with the other.

I can't wait to build 2 slope detectors and 2 of these :hyper:
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dougcl
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Post by dougcl » Fri Dec 19, 2008 9:52 am

neandrewthal wrote:So every positive going clock pulse you get a positive sample of the waveform's current voltage and every down going pulse you get an inverted sample?
The mix out is always showing a difference between two consecutive samples.

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Kwote
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Post by Kwote » Fri Dec 19, 2008 12:34 pm

neandrewthal wrote:and 2 of these :hyper:
i thought about that one but i think i'm gonna go for the Blacet Window Comp instead.
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neandrewthal
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Post by neandrewthal » Fri Dec 19, 2008 2:09 pm

Kwote wrote:
neandrewthal wrote:and 2 of these :hyper:
i thought about that one but i think i'm gonna go for the Blacet Window Comp instead.
I want that one too, but I'm cheap, so I chose the one I could build on a scrap of protoboard for a couple bucks rather than a kit.
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Kwote
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Post by Kwote » Fri Dec 19, 2008 2:22 pm

neandrewthal wrote:
Kwote wrote:
neandrewthal wrote:and 2 of these :hyper:
i thought about that one but i think i'm gonna go for the Blacet Window Comp instead.
I want that one too, but I'm cheap, so I chose the one I could build on a scrap of protoboard for a couple bucks rather than a kit.
i'm broke enough most times to go for the cheaper route but when someone like blacet provides a kit that fits my interest i'd rather just toss the extra money their direction because the ease of build will always outweigh other DIY projects. not to mention the Blacet WC kit doesn't require any soldering of wires. just plug and play :hyper:
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neandrewthal
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Post by neandrewthal » Fri Dec 19, 2008 2:37 pm

dougcl wrote:
neandrewthal wrote:So every positive going clock pulse you get a positive sample of the waveform's current voltage and every down going pulse you get an inverted sample?
The mix out is always showing a difference between two consecutive samples.
Oh, clever. I get it now. How fast do you run the lfo? It seems the faster you run it, the higher the resolution you will have, but less sensitivity.
Initiate the sequence, create catastrophe.

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dkcg
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Re: Signal Differentiator

Post by dkcg » Fri Dec 19, 2008 3:11 pm

dougcl wrote:Hi folks after thinking about how to detect changes in a pitch cv for the purpose of triggering an ADSR, I realized that taking the derivative of the signal would work in many cases, and that it can be realized using a S&H, LFO and voltage inverter:

Image

I think this could also be used to derive a quadrature signal from the input since the derivative of sine is cosine. The frequency of the LFO determines the accuracy of the circuit. Higher freq-> more accuracy, although the S&H can only go so fast before things start falling apart. I only had a few minutes to test this, but it looked interesting, if not perhaps complicated. It might make a nice module.
Awesome doug! Now this is what modular is all about! :D

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dougcl
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Post by dougcl » Fri Dec 19, 2008 3:49 pm

Hi guys, there is actually a problem using the S&H to do this. While this works to get a change between consecutive samples, the sign changes each time (pos/neg). I'm back to my original thought of using channel 1 and 2 of an ASR (which I need to get) instead of the S&H. Still the alternating signs will be ok for signal change detection.

The other consideration is that the derivative is actually dy/dt. In this circuit, dt is always effectively 1. You need therefore to apply a gain after this circuit that is increased as the clock is increased. I am back to needing an amp in modular format, but I digress.

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