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Periodic Sample/Hold Patterns
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EdisonRex
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Joined: Mar 07, 2007
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Location: London, UK
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PostPosted: Sat Dec 08, 2007 2:36 pm    Post subject: Periodic Sample/Hold Patterns
Subject description: Some Saturday Fun
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Today was an amazingly rainy and windy day here. Unfortunately, I had to go out in it. One of the errands was to Maplin's to pick up a new battery for my Gen 3 iPod, $25 instead of a new iPod. Yay. More to the point, whilst at Maplin's (this is the equivalent to the US Radio Shack, for those curious) I was picking up kits for my son (who is now building kits unsupervised, with a QC check at the end of the day). So I noticed a little kit "Audio Function Generator Kit" for about £13, heck, that doesn't look like a major kit, so I popped it in with my shopping.

Got it home, built it in under an hour. It's a Maplin-branded kit supplied by Middlesex University Teaching Resources, http://www.mutr.co.uk and they have some fun little kits which I recommend people new to kit building get their soldering chops up with. It was curiously missing the power connector, and supplied an extra 20k resistor and was short a 330R and a 10K. It's based on the ATTiny2313 microcontroller. Which is only part of the story, yeah, Edison built a kit, so what. So here's why this isn't posted in DIY, and here's the point of the post.

What I wanted to do with it, once I figured out it was rather stable, is to use it as a sample-and-hold source. Old hands here (should) know about regular pattern generation using sampling well (!) below the Nyquist frequency. Wink Razz Folks who are new to using a sample and hold, or have only used one with noise as the sample fodder, might find this interesting.

So instead of noise into a sample and hold, let's send a fairly regular and predictable source into a sample and hold, and let's see what the difference is. For my purposes, I use a sawtooth, as it gives us the greatest range of predictable source voltage. If you kind of know the ratio between your sample rate and the frequency of the input waveform, you can, with some theory, predict a S&H pattern. In practice, it isn't fully predictable, but Picture 7 should show why.

So as you can see in Picture 7, the S&H output voltage is a step function, because our sample frequency is close, but not exactly equal to, the source. If it was equal, the sample would be uninteresting (it would look like Picture 8...)

Anyway, it's fairly simple to explore, if you have a stable LFO, a stable clock for your S&H, and a fairly quick and accurate S&H. I don't need to belabour the technique, as it should be somewhat obvious from the pictures and the samples (oh yeah, here's what it sounds like). The sound sample has no extra delay at the beginning, but I pop some delay in the middle to add a shuffle to it, something to think about too. This is not like an arpeggiator. Trying to predict a S&H sequence would require a way to lock step the function generator and the S&H clock, and even then, visualising it is somewhat more like divining than really just doing it. Anyway, think about it, and enjoy your art.

Speaking of art, the patch for this was basically the M5, with this little function generator I built from a kit today as the sawtooth source.
Basic patch was the output of the function generator runs to the S&H 2 input, S&H output runs the 3 VCOs via a multiple. The LFO2 trigger out is firing the EGs so I get something out of the filters/output VCAs. To add a slight percussive edge, there is some white noise, high pass filtered, and added in, as well as the negative (inverted) output (DC) of the EG1, which is fed in the audio mixer to provide a bit of kick. The most astute of you, if you listen to headphones, you should check out the VC pan. It's not just about the technology, it's about the technique too.


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