Analog Shift Registers
2002-02-01 by Tkacs, Ken
You can say that an ASR is sort of like an analog delay, and then quickly add "not really," but in fact, that's *exactly* what it is. The difference between the two is simply a matter of magnitude--the number of "stages" and the speed at which they are clocked. An ASR is typically a very small, slow, subset of an analog delay, and so it tends to be used for control voltages rather than audio. A single Sample & Hold module, as anyone who owns an MOTM-100 or -101 knows, "samples" voltages varying at its input, and when a clocking pulse triggers the module, whatever voltage was present that instant at it's input gets reproduced at its output, and HELD at that level (even though the input voltage may continue to vary) until the next clock pulse instructs the module to hold a new sample. An analog shift register is comprised of a few S&H stages in series, typically three (at least that's what the Serge had, and some custom Moogs I've read about). So what happens is that the same clock pulse is applied to all three stages, and each time it is, the voltage is "handed down the line." Stage 1 gives its voltage to stage 2 and grabs a new one, while simultaneously, stage 2 hands off its sampled voltage to stage 3 and now holds the "old" stage 1 voltage, and so on. All of this happens [theoretically] instantaneously. It helps to picture a line of three firemen, with a well on one side and a fire at the other. Rather than each fireman running back and forth from the well to the fire, they line up and "hand off" each bucket of water down the line in a rhythm. This used to be called a "bucket brigade," and this name is actually what was given to analog delay chips in the 70's! They used to be called "CCD [charge-coupled device] 'bucket brigade'" chips! The most famous (maybe because they were cheap and Radio Shack carried them?) were the Reticon SAD-512 and SAD-1024 chips. Those numbers, believe it or not, referred to the number of "sample & hold" stages in the chip! The '512 had 512 S&H stages! Since these were miniaturized, and marketed for audio delays, the capacitors in these chips were of course tiny and didn't hold the charge very long at all (they had high "droop rates")... they wouldn't hold a steady voltage for a long time the way an MOTM-101 doeas. But that wasn't necessary because the chips were being clocked at very high rates, in order that the fluctuating voltages that were passed down the line could still be perceived as audio. Anyway, I'm droning on, but I just wanted to point out to those that may not know how analog delay units work that an analog shift register is, in principle, exactly the same thing, but with less stages and optimized for control voltages. By understanding the two together it might help understand the ASR and start the gears turning about how you could put one to use. The "canonic" use to control three VCOs may now become clear with the bucket brigade analogy. (Picture each fireman singing a held note whose pitch is equivalent to the amount of water in the bucket in his hand at that instant... or is that just getting confusing again?). Anyway, hope this helps. Mr. T