“Bird Box” sequencer

One of my favorite digital logic constructs is the linear feedback shift register (LFSR). Traditionally they find use in cryptography for generating encryption keys, and in digital spread-spectrum communications. LFSRs are also used fairly commonly as white noise generators (MM5837N for example). In both of these applications, the patterns they produce are valued for their near-random properties. However, the patterns are not random at all, and repeat on predictably long cycles. How long are the cycles? That depends on several things, but in general, the maximum pattern length for an LFSR that is n-bits long, is (2^n-1).

From a Xilinx application note:

“A 63-bit LFSR counter has a repetition time of (2^63-1) clock periods. Running at 50 MHz, such a counter repeats after more than five thousand years (5,849 years to be more precise), which is long enough to be irrelevant for most practical purposes. “

Within these long cycles is much detail, full of suggestive self-similarity and evolving motifs. Appreciating this, some designers have made use of LFSR patterns for aesthetic uses. Most notably, the Triadex Muse, designed by computer pioneers at MIT in the early 1970’s, is an LFSR configured as an automatic music composing machine.

A review of the Triadex Muse from Studio Sound (September 1972):

“The Muse, says the manufacturers blurb, is a computer. It composes and plays music, instantly. Limitlessly. The Muse is the invention of two MIT professors that harness the most advanced computer technology for the purpose of putting notes together in interesting ways. In other words, of creating musical compositions. There are more than 14 trillion potential note combinations inside the Muse. That’s what makes it so intriguing. It’s almost impossible to exhaust its potential”

Wow! Sounds like fun. When I first learned about the Triadex, I studied its design and built a near-clone on breadboard. That circuitry evolved into the MuseWave (audio synth) and the Geometric Oscillator (graphics synth). Later on, I found myself itching to try out an LFSR for drum sequencing. And that is the story behind the “Bird Box”. It is loosely based on the Triadex Muse architecture, but stripped down to two feedback taps and one variable-modulus divider. I crammed that stuff into nine HC-TTL logic chips, gave it some jumpers for parameter settings, and added a set of four analog trigger outputs.

another Bird Box drumloop

The Bird Box’s maximum tap length is 32… so in theory, it could make a drum loop that repeats after about 17 years. I havent bothered cranking it up that high yet!

It is built inside an Otagiri white lacquer jewelry box. I added a word-balloon sticker to the top, so the bird is singing a song of binary 1’s and 0’s. The box has a wind-up music movement that plays “Sound of Music” when the lid is opened. There is a glass mirror on the underside of the lid. (to check your pupils!)

Currently I’m using it to trigger the TR-808 kick, snare, and hihats in the Big Box o’ Techno.

4/4 birdbox drum pattern