There's also the less obvious modules. The ones you learn about along the way. Not absolutely essential, but useful nonetheless: S&H, Ring Modulator, Clock dividers, Slew Limiters. These are creative tools that will stretch your sound pallet, and are well documented. Any basic book about synthesis will be mentioning them.
Then, there is what we'd call more esoteric modules. The ones for which you must take a pause and either mentally figure how they could be actually used or, take a plunge and experiment ... and waste a lot of time having fun with unexpected results. Here you'll find special designs like MOTM's Cloud Generator, Livewire's Dual Cyclotron, Plan B's Heisenberg Generator. You'll also find more generic ones like Random Control Voltage Generators, Trigger Delay, and Analog Shift Registers.
I'd like to spend some time on the latter one: Analog Shift Registers aka ASR.
My first contact with Analog Shift Registers was in an interview Electronic Musician did with artist Steve Roach. The interview was revolving around the integration of the latest modular technology in the making of the Possible Planet Album. Among the modules used, Steve puts a focus on the impact that Plan B's Analog Shift Register had on it's composition. It caught my attention.
Ok, so what is an ASR? An ASR module is a sophisticated S&H. It will, at every clock pulse, sample the CV value of the input and makes it available at the first output. So far, it's a basic S&H. The thing is, a ASR has many outputs, and what it does is that at every clock pulse, it shifts the value of output 1 to output 2, and likewise, value of output 2 gets to output 3. So at every clock pulse, the CV values gets' shifted to the next output with the exception of output 1 that samples a new value at every clock.
An article by Stan Levine in the 1976 November/December edition of Synapse, leads to think that the ASR was an original concept of Serge Tcherepnin: "His approach from the beginning has been a personal one; Serge designs for an individual, not a mass market. Serge's are modular systems including, VCOs, VCAs, VCFs, and some modules of his own invention, such as, positive and negative slews, the smooth and stepped generator and the analog shift register."
For sure, the ASR takes it's root in CalArts as Serge was there at that period. But so was Barry Schrader. Although it seems certain that Serge Modular Music System was the first company to offer an ASR in a commercial system, the concept first appeared in a serie of custom modules, known as the Fortune Modules built by Fukushi Kawakami for composer Barry Schrader in the early 70s. According to Barry Schrader, Fukushi asked him for ideas to complement the Buchla 200 aka Electric Music Box. This collaboration resulted in four modules, the ASR being one of those.
Now, what are the current ASR incarnations:
Serge - ASR
The Serge Modular Music System, or Serge synthesizer, was created by Serge Tcherepnin at CalArts. To the best of my knowledge, this is the first proposal of the ASR in a commercially available system. From Serge-fans.com, here's how this module is presented in the Serge Catalog:
"The ANALOG SHIFT REGISTER is a sequential sample and hold module for producing arabesque-like forms in musical space. Whenever pulsed, the previously held voltage is sent down the line to three consecutive outputs to produce the electrical equivalent of a canonic musical structure."
The circuit diagram of the Serge ASR can be found in the 1976 September/October issue of Synapse Magazine, p.30.
Plan B - Model 23
Peter Grenader of Plan B offers his own version of the circuit in the form of the Model 23 a 3 output ASR based on a Atmel 2051 chip. I say it's own version, because Peter took notice of some caveats inherent to the ASR concept and made a design that wouldn't exhibit these problems. Quoted from PLAN_B_analog_blog, these are:
B) Output taps not tracking one another so that a voltages vary as they are passed down the line of outputs.
C) Incoming clock causing a droop in held voltages at it's falling edge.
Peter also gives possible usage of the ASR and points to some historical audio usage of it in audio examples. I would add to that selection an except from Steve Roach's Possible Planet that can be heard directly on Steve's site.
Metalbox - Gate Comparator
I can't test this one, but according to the description, we're talking about an 8 stage ASR with what seems like a RCV. Wow! Seems very good. More expensive than Plan B, but more that just ASR functionnalities. If you're into Frac-rac, it deserves a thought.
CGS/Ken Stone - ASR
Ken Stone has a ASR circuit available that differs form Serge's way of doing it, for the DIY fans among you. Or, if you prefer having it already built, you can have a look at Cynthia.
Cynthia - Psycho Shift Register
This one is based on the Ken Stone circuit, but like the Metalbox offering, the Psycho Shift Register module does more than just ASR. Here's from the Cyndustries site:
"Ken's latest development is his own modern and improved design of an Arabesque Pattern Generator similar to the classic Serge Analog Shift Register schematic originally published in Synapse years ago. Thus it is incredibly useful for auto-compositional harmonies as it is, but what sets this version apart is that in this module it is combined with a certain popular little quad LFO called The Psycho LFO which is also from the lineage of the Australian Cat Girl Synth system.
(...) when the "Link" switch is thrown, one of the oscillators acts as a simple clock driving the Analog Shift Register, while the other three are free to create the jumble of control voltages that the register actually shifts!
(...) an exclusive control voltage "Modulation" input has a hand in adding more of your own animation to the Monster. The unique "Blend" and "Character" controls are useful for transforming the total output from something joyous to downright disturbing!"
Doepfer - A-152
I own a A152, and my opinion is that the claimed ASR function of the A152 will not get you the true ASR results you might expect. The A152 is a switch. A very powerful and sophisticated one, but it will not push the value of one output into the next.
Therefore, not a ASR as we defined.
Synthesizer.com - Q960
I doesn't own a Q960, so I can't make a claim check, but I must say I'm sceptical about Synthesizer.com's as an ASR. But I can't verify, so here's how they put it:
"The Q960 is a 9 stage shift register. Each stage is linked to the next like a chain. Stage #9 is linked back to stage #1. Each stage is either ON or OFF.
Each time a shift occurs, the state of each stage is shifted forward and the last stage is sent to stage #1. In other words: a circular rotation."
So there you have it.