Model 23

Analog Shift Register

While the Analog Shift Register was first introduced as a commercial product by Serge Tcherepnin in the mid 1970's, it was an adaptation of an original prototype conceived by Barry Schrader and built by Fukushi Kawakami in 1972 as an expansion module for the Buchla 200s at the California Institute of the Arts (go here for a photo, here for Kawakami's original block diagram and below for a sound sample of the original module). Simply put, an ASR is a multi-output sample and hold configured as a multi-tap analog memory in which control voltages are passed down a line of outputs canonically - each output receiving the voltage level previously stored at the preceding tap (see figure 1 below).

A simple analogy could be a game in which a single line of people, each holding a different type of fruit, are told to pass their fruit to the person standing at their right side when they hear a hand clap. Replace the people with output taps, the fruit with stored voltages and the hand clap with an incoming trigger and you've got the idea. Apply this concept to musical phrases (a canon) and one can imagine the possibilities.

The first of the mighty ELF series modules, the Model 23 consists of a single four-output analog shift register which unlike it's predecessors incorporates modern technology to assure that all taps are DC accurate within 1mV via the Analog Devices SMP04 precision quad S+H chip driven by the Atmel 2015 microcontroller. Laser-trimmed internal capacitors within the SMP04 allow a keyboard to be played through the M23 without noticable variations of output voltage as each note is passed down. Along with a voltage output, each tap is fitted with an LED display to indicate it's stored voltage level.

Expandable architecture

Multiple M23s can be linked together to produce either expanded outputs (two M23's = eight taps, 3 M23's = 12 taps, etc.) or may be run independently from one another. AN internal flat cable which connects between multiple modules allows patch-free linking.

Functional Conventions

The Model 23's SMP04 high performance quad Sample & Hold chip assures accurate distribution of voltages sequenced across the output taps within 1mv. This introduces exceptional improvement on other ASR's currently available in this regard. However, in accordance with the specs of the SMP04, if a tap is held for more than 5 or so seconds the Model 23 may exhibit a slight rise in output voltages across all taps at a rate of approx. of 2mv per second.

Figure 1 - Analog Shift Register Operation

Notes/Sound samples

As the Model 23 Analog Shift Register nears design completion, I've posted some sound samples to give you an idea why it's got us sort of excited. Before I do, let me go into a few details on the things about ASR's which I've personally found aggravating

A) Outputs do not track the input voltage, making driving them with a keyboard problematic

B) Output taps not tracking one another so that a voltages vary as they're passed down the line of outputs

C) Incoming clock causing a droop in held voltages at it's falling edge.

We're happy to say that the Model 23 does none of these things. As the following sound samples indicate, four separate VCOs driven by the (now) four outputs of the ASR track to the quantized input of an envelope generator: click here

Same patch, but slower, giving you a clear indication of the
uniformity of the voltages as they go down the line of outputs. Click here

Same patch, at even a slower pace. Click here

Having a little melodic fun, now with the four VCOs gated through a Model 13. Click here

Screwing around with the update speed, this time with the M23 sampling quantized random voltages via the Model 24 give the 'classic ASR' effect. Click here

Analog Shift Registers: Audio Propecia?

The 'classic' ASR sound is sonically pleasant, agreed. However there are more uses than creating that classic staircase effect.

Sample and Holds work much like analog to digital converts, but usually at a much slower speed. It might be better to think of them as bit-crushers. When driving them with a sub-audio LFO, the results are familiar. Click here.

But if you speed the sample rate (the speed of the gate driving the ASR) into the audio bandwidth, much like digital audio the human ear will not allow us to disearn the individual steps, although there is an audible thickening/fattening to the sound (same patch as above, fast clock speed) click

WIth a little clock speed noodling the fattening will be such that the ASR will make an otherwise dull event much more interesting (in this instance, the clock speed was slowed slightly from the previous sample above). click

The original Analog Shift Register

Although first used in Part 4 of his 1973 work Bestiary, the following is an exserpt from first movement of Barry Schrader's Trinity composed in 1976. This passage features heavy use of the original Analogue Shift Register controlling the Buchla 200. Click here for download (2.1 meg).

Click here for more information or to purchase Schrader's Lost Atlantis on Innova Records (which includes the piece Trinity).