Peak voltage sample & hold

[StephenGiles]

I need to present a sampled peak voltage, proportional to the strength of say a guitar note played, to a VCF sweep generator. At present, the start and stop points of the sweep generator are controlled by potentiometers, such that the start of the sweep will always be to the same point (boring!)when energised. I want to replace the fixed voltage at the top end of the start control pot with a peak voltage proportional to the strength of the note played in order that (1) a dynamic feel can be incorporated into the sweep generator, and (2) the start point of the VCF will also be dynamically controlled. The sweep down to the point set by the stop control will be independent of the normal guitar note decay, and theoretically ripple free.

Can anyone perhaps point me towards a sample & hold with a suitable trigger pulse generator?

[StephenGiles]

Sample circuit with explanation for the sweep generator section. The sample & hold bit comes from a circuit I found on my hard disc, but I can't understand U3

http://www.4shared.com/file/89784300/b7d68e64/Space_Filter_S__H.html

With the system at rest, no current flows through the 10k resistor from IC1b output into IC2 pin 1. This results in the transconductance amplifier portion of IC2 being effectively disconnected from it’s darlington emitter follower output stage, hence the voltage at IC2 pin 7 follows pin 3, which is at the voltage set by the Stop Frequency control, buffered by IC1c.

When a note is played, IC2 is energised by the current into it’s pin 1 and becomes a voltage follower, forcing pin 8 to equal the pin 3 voltage, set by the Start Frequency control (see below). The pin 1 current soon ceases to flow, however, and the voltage at pin 7 decays through the RC network to the Stop Frequency voltage at a rate set by the Sweep Rate contol.

Now, we want the Start Frequency control to set a voltage proportional to the strength of the note played, in other words an Adaptive Start Frequency. The basic Space Drum sweep generator used in the circuit has a fixed Start Frequency however, which is determined by the setting of the control pot.

A possible way of achieving this is to pass part of the voltage output of buffer/amplifier IC1b to the pulse generator consisting of U2b, U3a (1/2 13600??) and associated components. (I am a little mystified about R9 hanging from U3!!) The output of IC1b is also passed to FET Q1. With the system at rest, Q1 is off. A pulse generated each time a note is played will momentarily turn on Q1, allowing the instantaneous peak voltage voltage taken from IC1b to be sampled by the other half of U2 and put on the tag of the Start Frequency control which was formerly connected to the positive supply.

So each note played both energises the sweep generator and controls it’s Start Frequency according to the strength of the note. The resulting voltage can be used to control a suitable VCF, so once again, we get a Start Frequency following the strength of the note played, but a Sweep determined by a separately controlled CR network.

[frijitz]

From the pinout it looks like U3 is an OTA. R9 would be connected to pin 6, supplying Iabc. The RC network at U3's input forms a differentiator. It's not clear what D2 does, though. Its right side is floating because of the large input impedance of the opamp. Also Q1's gate is floating when D3 is reverse biased. Where did this circuit come from?



Ian

[StephenGiles]

It sems to come from the KBD 2e - Keyboard Converter by EFM here

http://experimentalistsanonymous.com/diy/Schematics/Full%20Synths%20Drum%20Synths%20and%20Misc%20Synth/EFM%20Keyboard%20Interface.pdf

[frijitz]

That explains it.



Ian

[StephenGiles]

Can you see what I am trying to do Ian?

[StephenGiles]

Might it be possible to produce a positive trigger pulse where it's voltage is proportional to the strength of the note struck, which could be fed to the top of the Start Frequency pot?

[slacker]

Stephen the Electrax guitar synth, which I think you've posted about before has a decent trigger generator. You could then hook that up to Ian Fritz's sample and hold from his Threshold/Gate/Trigger/Sample/Hold, the schematic is here, that would let you use your spare OTA. I've done this using just the LM13700 and CA3140 parts, actually I used half a TL072, and it's a great sample and hold.
To mesh the two together I used a simple transistor pulse generator to make a thin pulse to drive the LM13700.
I haven't used it to try and sample the guitar signal but I think it should work, if I get time next week I'll try it.

[StephenGiles]

[StephenGiles]

Any more thoughts on this, I haven't had much time for it during the past week.