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Modular Concepts: Analog Shift Registers

Dynamic Voice Allocation, Arabesque Patches, and More

Troy Jansen · 08/16/24

Some Eurorack Modules are intuitive—their functionality is apparent, and their controls are usually self explanatory to someone even remotely familiar with synthesizers. Things like oscillators, filters, and effects are standard fare in Eurorack cases. However, some things like utility modules are not always intuitive to understand, despite being a vital part of any system. Utilities can seem like boring additions at first, but they are the spice of life that really brings a modular system together, and makes it stand out as a unique instrument.

The analog shift register (or ASR) is perhaps one of the more cryptic modular synth utilities, yet it can provide tons of versatility in any Eurorack system. The ASR can be used for generative or polyphonic sequencing, evolving CV modulation, and much more. Continue on as we discuss the functions, history, and applications of the analog shift register, as well as compare some recent Eurorack ASRs.

What is an Analog Shift Register?

An analog shift register works by taking a single CV input signal, sampling it, and shifting the sampled signal to different outputs. The timing of these shifts is set by an incoming gate signal. This is similar to both a sample and hold module and a bucket brigade style delay. The input is sampled and held at an output until it receives a gate. Then, it will send the signal from the first output to the second one, essentially delaying it by one step, or “bucket.” This process continues, moving older information down the chain and replacing it with newly sampled information.

At their core, most ASRs have a CV input for the signal to be sampled, a gate or trigger input to shift the signal, and a number of outputs for the shifted signals. Three or four outputs is typical for most ASRs, though there is no hard rule about how many outputs an ASR can have—some may have more. Each of these outputs is known as a stage of the ASR.

Some modules may have feedback capabilities, forcing the last output stage to cycle back to the first. This causes the ASR to loop, which can be pleasing in a musical context.

A common use of the ASR is to allocate a single pitch sequence to a number of different voices, triggering each one at different times. This technique is sometimes referred to as an Arabesque pattern. The term "arabesque" originally referred to art and architecture with repeating visual patterns that combined many different elements. The use of this term for an ASR-controlled polyphonic modular synthesizer patch dates back at least to the 1970s, with the September-October 1976 issue of Synapse magazine—in which the mysterious Arpad Benares (the nom de plume of a notable electronic musical instrument designer) contributing an article entitled Constructing An Arabesque Generator…the Analog Shift Register.

[A lovely visual pattern from Arpad Benares's analog shift register article—art by Gary Chang aka "geechang."]

The article describes a way to create polyphonic textures from a single CV/gate source using a Serge Modular Music Systems Analog Shift Register module (which is, to our knowledge, the first such device ever produced commercially). This patch combines different elements via multiple voices or sound sources, each repeating and dancing around, following one another's contours—all thanks to the sequencing-like behavior of the analog shift register. Arabesque patterns are also a good way to create polyphony in modular, which can often be difficult to sequence. The ASR sends pitch information to different voices, each with its own envelope and VCA. With longer envelopes, each voice will ring out over one another, creating polyphonic sounds.

This patch can also make chords or pads—just trigger the voices simultaneously instead of separately. For an alternate application, remove the envelopes and VCAs to let the oscillators drone together as the chord changes. Quantising the CV source going into the ASR also helps to make sure the chords are not discordant.

History of the ASR in Modular Synthesizers

The concept of an analog shift register goes back much further than its use in early electronic music. Shift registers have been used in computing for decades, and still see continuous use in non-musical applications since then. In fact, the concept of an analog shift register is a sort of "play" on the more standard and common concept of the digital shift register—which we'll discuss more extensively in a bit.

For now, we can say that the exact details of who first implemented an ASR in a modular syntesizerh are unclear, but many attribute it to Yamaha engineer Fukushi Kawakami—sometimes referred to as Mr. Fortune. Kawakami created a set of modules conceptualized by composer and educator Barry Schrader, intended to augment the Buchla 200 system Schrader used for his compositions at the California Institute of the Arts (CalArts). Kawakami and Schrader's modules have since come to be known as the Fortune Modules, and they included an analog shift register. This early ASR featured one input and three outputs for CV, as well as a shift trigger input.

[A diagram documenting the functionality of the Fortune ASR module.]

Another early, noteable ASR was created by Serge Tcherepnin for his eponymous modular system around the same time—no huge surprise, since the Serge Modular Music System came to life initially when Tcherepnin, too, was in residence at CalArts in the early 1970s. Tcherepnin was inspired to create modular systems like Buchla's, but more accessible for young composers. While the Fortune modules were one-off designs made specifically for Schrader, one can still find schematics and even build their own Serge ASR with information that is readily available online today.

Analog vs. Digital

Though we refer to modules of this type as shift registers, the "analog shift register" is a sort of invented concept in the world of analog synthesizers. Digital shift registers are a more standard concept in electronics and computing—and indeed, some Eurorack modules are essentially digital shift registers in synthesizer form, as well. So…what's the difference between an analog shift register and a digital shift register?

Analog shift registers work with analog control voltage at the input, while digital shift registers are intended to work with binary/digital information. The analog shift register samples and holds the incoming voltage, and then rotates it to the different outputs. Digital shift registers determine whether the input is high or low at the clock pulse, and then sends a high or low gate to the first output in time. Digital shift registers also shift information to each stage via the clock pulse, but the information being output is a gate signal rather than CV. (Note that some digital shift registers such as the omiindustriies Cascading Register output pseudorandom CV as well—as digital shift registers are a common part of pseudorandom number generators. This is, however, a topic for another day.)

So, digital shift registers still move information to different outputs sequentially, but the information is gate signals rather than CV signals as with an analog shift register. To make matters somewhat more confusing, it's possible to create a so-called "analog shift register" with either digital or analog technology—so, the term "analog" and "digital" in this context doesn't necessarily refer to the underlying technology, but simply to whether the device is designed to sample and delay control voltages (analog shift register) or gate signals (digital shift register).

Implementation in Eurorack

There are thousands of Eurorack modules out there, which means that there are several options even for obscure types of modules like the ASR. Here are some top picks of ASRs from popular manufacturers, and a look at their specs. Each one of these modules can implement the ASR concept in different ways, allowing for creative and complex patching.

Shifty from Intellijel is a digital ASR that packs some innovative functionality in a relatively small package. Shifty has four CV outputs, as well as separate gate outputs for each stage. The gate outputs are useful for controlling voices without external gate sequencing. The order of the outputs can be changed from ascending to descending, or random.

Shifty can function as a typical ASR or as a voice allocator. The latter mode updates only one output at a time to control single separate voices in succession. This mode also lets you select how many voices it will cycle through, from two to four. The module also has a manual gate button and a reset input for increased playability and patch programming.

The Verbos Electronics Random Sampling is much more than just an ASR—it’s a fully fledged random voltage and noise generator. However, the inclusion of an ASR circuit brings lots of potential when used in conjunction with the rest of the features of the module. Random Sampling has several random CV outputs, two of which are quantized to a controllable set of values. It also has three different types of noise sources.

These various sources can all be used to feed the four stage ASR for plenty of unpredictable sequencing and modulation. The ASR on random sampling is unlike most others in that it features normalled gate and CV inputs for each stage. This means each stage can be used independently, so you can have 4 sample and hold circuits as well as connected ASR stages.

The Joranalogue Step 8 is a large and fully featured general-purpose module capable of ASR-like techniques. Like many offerings from Joranalogue, Step 8 can perform multiple functions. These include sequential switching, sequencing, waveshaping, counting, and of course, an analog shift register.

The ASR on Step 8 features eight stages, each with its own output attenuator. The stages can be switched from sample and hold to track and hold, allowing for more variation in CV processing. Step 8 also has trigger inputs for pausing, reversing, and resetting the stages. The module boasts an all-analog design with high levels of precision, minimizing drops in voltage.

An honorable ASR mention goes to the multitool module Ornament and Crime. o_C has dozens of different modes that cover a hearty roster of utilities. The ASR mode on this module is called Copiermaschine. This is a four-stage ASR with some extra digital control options. There is a freeze button that prevents the CV from changing, which lets the ASR act as a CV recorder. Copiermaschine is a great option for trying out the functionality of an ASR without committing to buying a whole new module (especially if you already have an Ornament and Crime).

Ornament and Crime is an open-source design produced by many manufacturers, in several slightly different form factors—so it's relatively easy to track down and try out for yourself.

Shifting Onward

The analog shift register is a versatile module with an interesting history, going back to the very roots of modular synthesis. Adding one to your modular case can benefit modulation, sequencing, routing, and much more.

Each of the Eurorack ASRs covered here have unique functionality and expand on the core concept in innovative ways. Whether you need a smaller ASR for simple sequencing or a more advanced one for complex patches, there is certainly a module out there that will fit the bill.