Cool Sample and Hold Functions

[Scott Stites]

While waiting for the thread placement of a neat sequencer idea Tom brought up, I figured I post some other stuff....

One of my favorite synth functions is the Sample and Hold. These cute little buggers can be used to add anything from a slight non-predictability to a particular modulation to being used as the master controller in a patch.

Thought I'd list some various S&H designs just for reference here:

Ray Wilson's Sample and Hold - this was the first sample and hold I ever tried, and ranks right up there in functionality. Seems to me I've been able to get it to do 'track and hold' as well, though I'd have to go through my notes. BTW, most S&H's are capable of being modded for the 'Track and Hold' function. A sample and hold uses a very narrow sample pulse (aperture). The trick is to turn that pulse into something much wider so the input to the S&H can pipe straight through while the aperture is open. Once the aperture is closed, the sample 'freezes'.

This is the one I worked with long ago:

http://www.musicfromouterspace.com/analogsynth/sandh.html

A newer one:

http://www.musicfromouterspace.com/analogsynth/NewAugustNewSampleHold.html

and of course, Ray's one-chip design here (never tried that one):

http://www.musicfromouterspace.com/analogsynth/singlechipsampleandhold/singlechipsampleandhold.html

A very simple, yet quite effective S&H is Rene Schmitz's YASH (Yet Another Sample And Hold):

http://www.uni-bonn.de/~uzs159/

This one is quite easy to convert to 'Track and Hold' function as well.


Then there's Ken Stone's Analog Shift Register, which I have yet to try. A specialized string of sample and hold's that pass the sample along a 'register', enabling arabesque type patterns with a string of voices:

http://www.cgs.synth.net/

There's a couple on the Buchla 266 Source of Uncertainty Module that really take the cake. You can find the schematic at Magnus Danielson's Buchla Schematics page:

http://rubidium.dyndns.org/~magnus/synths/companies/buchla/

Wha? You say there's only one S&H on that module? Actually, there are a couple (the other functions are handled by Shift Registers - and very, very cool functions at that, though I'll save those for some future thread).

First of all, the obvious sample and hold:

This S&H actually has *two* sample and hold sections in it. It accepts a single sample input and sample pulse and derives three sample outputs and two pulse outputs from the two inputs.


One output is like a 'normal' sample and hold. It puts out a sample for every pulse input. The other two sample outputs alternate each sample taken. In other words, the first sample comes out of one output, the next sample comes out the second output while the first sample is held on the first output, then the third sample comes out of the first output while the second sample is held on the second output, and on and on.

The pulse outputs follow the alternating outputs, so each alternating output can trigger for its own voice, for example.

This leaves a huge amount of variation for patching with it. One voice can play all the samples, two other voices can alternate between the samples. Or, one output can control a device, say a VCO modulating another VCO, the other output can control the carrier VCO, and the third output can control the modulation index. Which output you use for which function can drastically alter the flavor of the patch.

The aperture on this S&H is a bit wider than I've seen on other S&H's: with a fast moving sample source, a bit of a chiff can get through with each sample taken. This gives it (to me) an unmistakable 'Buchla' flavor.

Now to that 'second' sample and hold on the 266. It's none other than the Fluctuating Random Voltage (there are actually two of them on this module). This function is near and dear to my heart.

The core of the FRV is a sample and hold circuit, but it's specialized in a couple of ways. The first specialization is that it always has the same sample input - an internally generated 'noisy triangle'. In other words, a fast moving triangle LFO is modulated with a noise source. This gives a very special self-sameness, yet unpredictability to the output of the S&H.

The second specialization is directly related to the name of the function "Fluctuating Random Voltage". This is not a stepped output, it's a smooth output, but it is very different from the smooth output you would expect putting lag on a 'normal' S&H output.

A 'normal' lag/S&H will act as follows: the more you increase the lag, the less deviation of output CV you can expect. This is simply because the voltage doesn't have time to rise or fall to the present sample level before the next sample is taken. So, with a fast S&H rate, increasing the lag decreases the range of output. Or, conversely, with a high lag set, increasing the clock frequency has the same effect.

On the FRV the clock speed and lag time are directly related - you control the clock frequency and lag time with the same control - Probability Of Change. You increase the clock and you decrease the lag time. You decrease the clock and you increase the lag time. The end result is a smooth, randomly fluctuating voltage that can move to all excursions, but more slowly (and less often) or much more quickly (and much more likely to change). Cool function indeed.

Grant Richter created a different monster based on the same principle with the Wogglebug.

http://www.musicsynthesizer.com/WoggleBug/WoggleBug.html

Buchla also had a module that used 4 sample and holds, ostensibly to help establish polyphonic patches. I'll have to dig through my files to find that one. I imagine it's on Magnus' page, though, if you look for it hard enough.

Any other S&H's out there you all have grown to love?

Cheerio,
Scott

[ian-s]

Found this simple one yesterday.

http://members.tripod.com/urekarm/synth/sh09_shn.pdf

Includes noise generator.

I like the way that Buchla/Wogglebug mix the held output back into the input to control the '% deviation from last sample', very musically useful.

[Wild Zebra]

Hey thanks for all the info Scott. I've made the single chip. Works great and its a very easy project, low parts count. I've got the S&H with white noise on the way. So its nice to hear good things about it. I'll have to agree the S&H is a wonderful synth function.

gt2ian Dig the avatar. I've got a 700. Gotta love that traveller!!

[ian-s]

[Scott Stites]

Ooh, good call on the SH09 schemo from Marjan's site. Nice - no IC's, pretty tiny circuit. I notice the external CV input is AC coupled! Never thought of trying that. Obviously they intend everything to go in to be AC, but a signal that slows down to a crawl (or stops) would sound pretty cool when sampling.

Patching the output back into the input - the correlation function - forgot about that! I do that on occasion by patching the CV input through a mixer, and mixing the output of the S&H back in.

Another patch that is great for those 'Subotnickan' sounds is to patch the output of the S&H into the CV input of the clock (if your clock source can be controlled by CV - I use the pulse output of a VCLFO oftentimes). If you don't invert the S&H output, and you're controlling another VCO (audio range) with the output, the higher notes blip by faster than the lower notes. Inverting the CV gets the opposite response. If you're mixing a DC voltage with the CV that's controlling the clock, cranking the DC up and down is a pretty neat effect as well.

I used the Buchla S&H as the controlling device on this piece, using FM, AM, RM and the various outputs to control mod index, clock speed and so forth. There's a tiny bit of sequencer controlled the same way at the very end (a three or four note sequence IIRC):

http://mypeoplepc.com/members/scottnoanh/sitebuildercontent/sitebuilderfiles/lpgd84.mp3

Forgot to mention - the Buchla 266 SOU S&H only works with positive voltages (CV's on the Buchla are between 0 and +15V). I added a DC offset to the input of mine to allow bipolar signals. I've toyed with creating a complementary copy of the offset voltage and mixing that with the output, so that increasing the offset on the input doesn't put an offset on the output, but instead makes it sound like progressively more of the bottom half of the input wave is being sampled.

I like Ray's offset control on the output - that really is a useful function.

Cheers,
Scott

[blue hell]

I like the mp3 track, very noodle like, and yes Subotnick.

Scott, you could post the mp3 as an attachment here, it would show up on hte media page then as well (provide the right forum selection is made)

Very interesting stuff about the S&H as well, makes e want to patch .. pitty I don't have too much time.

[Scott Stites]

Thanks, BH!

Is there a size limit to attachments? This thing is a bit over 5 MB.

Cheers,
Scott

[blue hell]

The limit is 10 MB I think, maybe 20. But I guess Howard would expand that for music like this :-)