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 Forum index » DIY Hardware and Software
Signal Splitting
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SineHacker



Joined: Mar 09, 2010
Posts: 99
Location: United Kingdom

PostPosted: Wed Apr 17, 2013 1:25 pm    Post subject:  Signal Splitting Reply with quote  Mark this post and the followings unread

I have a nice project on the go but I have been stuck for a couple of days on a problem that I am sure has a simple solution:

I am using a 4017 to blink a sequence of four LED's, each LED is tied in a DIY optocoupler - the idea is to open and close a gate with this somehow.

I have a single audio signal and I want it to be split in sequence across 4 different delays, but I am having trouble with the splitting itself.

My initial idea was to use each optocoupler LDR to pull the audio signal to ground when in a light state (in order to close the gate before each delay) - and when an LDR is in a dark state it would allow signal to pass - the gates were in parallel and I soon found the problem with this is that if any of the LDR's pull to ground it would kill the signal across all the gates. I tried using a resistor in series before each gate which kinda worked but this is of course lowering the signal - and either way the signal is being split passively 4 times which is making it really weak. I would boost it but it's already competing with noise from the CMOS and delay chips.

In case you were wondering, I tried the process with just a single path and it does work very well (and made me decide my next project will be a tremelo/delay pedal!!), it's not perfect as a small amount of signal does get through when the gate is closed, but there is plenty of difference between the volume of the gate being open and closed, and no noise issues other than the expected.

So I need to somehow split the signal into 4 isolated paths before the gate stage.

A friend suggested that I try a transistor buffer for each path and use the LDR's to gate the +Voltage. I tried this tonight and it worked but also sent loud pops as the transistors lost/gained power.

So that's my story, please help!

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prgdeltablues



Joined: Sep 25, 2006
Posts: 182
Location: UK
Audio files: 11

PostPosted: Thu Apr 18, 2013 1:37 am    Post subject: Reply with quote  Mark this post and the followings unread

I'd suggest you need to buffer each path (transistor or opamp wired as a voltage follower), and *then* apply a gated VCA to each - either using a simple transistor VCA (search the forum for a recent thread on that) or an OTA based VCA. Gating the buffers sounds less than ideal. The single transistor VCA does put out a dc offset, so you need to decouple it, and is liable to some bleedthrough, which may come out as a pop, but is simpler in design.

Peter
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SineHacker



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Posts: 99
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PostPosted: Thu Apr 18, 2013 2:00 am    Post subject: Reply with quote  Mark this post and the followings unread

Hey Peter, ok cool, I kinda tried using a buffer first for each path but I don't think my circuit was particularly good - I just pulled a transistor voltage follower example from the interweb so I will try that later.

On a single path I found that using the LDR to replace the input impedance resistor on a basic buffer (jfet common drain amplifier I think) was very effective for gating the signal on and off - but this still meant that the gate on each path was in parrallel so it wouldn't work - if the voltage follower circuit means that I can isolate each path before doing this then it will hopefully be perfect!

One question, what does OTA stand for?

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SineHacker



Joined: Mar 09, 2010
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PostPosted: Thu Apr 18, 2013 2:09 am    Post subject: Reply with quote  Mark this post and the followings unread

Ok I got it, Operational Transconductance Amplifier

I can swot up now Cool

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prgdeltablues



Joined: Sep 25, 2006
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PostPosted: Thu Apr 18, 2013 6:12 am    Post subject: Reply with quote  Mark this post and the followings unread

There's a sticky post 'collection of circuits by Nicholas', with a nice simple VCA using an OTA, or you could look on the MFOS pages.

Peter
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SineHacker



Joined: Mar 09, 2010
Posts: 99
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PostPosted: Thu Apr 18, 2013 7:22 am    Post subject: Reply with quote  Mark this post and the followings unread

Cool, I hadn't found that one but I did find an older thread, I also found this:

http://www.geofex.com/Article_Folders/VCA%20Applications.pdf

This project is growing exponentially at the moment and is already spread across my workbench on several breadboards which is making it impossible to work on anything else! So I am planning to build something quickly to solve the splitting/gating problem on a separate board to the rest of the project that can be easily upgraded later - having read that above document I really need some space to do some mad science!!

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