adventures in DIY music

Tuesday, 6 June 2017

Roland CR-8000 CompuRhythm

This drum machine arrived in 1981, the same year as the TR-606, and a year after the TR-808. They all share similar analogue voice circuits. The CompuRhythm's programmability is less intuitive than the other two, and the mainstay of it's rhythms are rock, latin, and cabaret dance presets. What's different about the CR is how these presets can be varied instantly via a couple of unique features. One is the "Arranger" buttons along the top row, that select a preset motif of a single instrument that can be superimposed on the pattern currently playing. For example, a cymbal on every 8th note, a handclap on the 2 and 4. More than one can be selected at once. The motifs for the hi hats are interactive - start the 16th note hi hat pattern then add the open hats on the off-beat eights for instant disco! The other feature is the "Register" button. This flips between two states: an "A" and a "B" pattern of your choosing, along with whatever "Arranger" options are selected. This button can be "played" in real time, or triggered via a jack input, allowing you to "cut" a mix of the two play states. Since the machine also sports a trigger output you can, by plugging a cable from the trigger out to the Register input jack, make the CR-8000 actually auto-arrange a new composite rhythm. These deceptively simple tricks, along with the fill-in implementation, make the CompuRhythm a lot of fun, and embody the "semi-automatic" ethos.

The CompuRhythm 8000 is superior to the 5000 in that:- it possesses the handclap sound (the most complex in terms of circuitry), it is externally sync'able via Din Sync with tempo shown by a segmental LED display, and, it has eight user-programmable 2-bar patterns and four programmable fill-ins. The 5000 is a preset only machine without external clocking options (although you can use the "Restart" input jack creatively to get around this limitation). The 8000's programmability is unfortunately not extended to three of it's voices - the Rimshot, the Clave, and the High Conga. The only time you can hear these is as part of a preset, which is one reason why I have wanted to look at external voice trigger options.


There are 14 voices: Bass Drum, Snare, High Tom, Low Tom, Cowbell, Rimshot, Clave, Hi, Mid and Low Congas, Open and Closed Hi Hats, Cymbal, and Hand Clap. Examination of the service manuals for the CR and its TR cousins from the same era, reveals they all share similar circuitry to create these voices. In particular, the cymbal and hi hat sounds are based on a filtered mix of square wave oscillators, giving a nice metallic sheen, as opposed to Roland's rhythm machines from the previous decade such as the TR-77 which used simple white noise filtering for these sounds. The membrane drum sounds (BD, SD, toms, congas) use one or two "bridged T-network" damped oscillators as shown in the TR-808 service manual Figure 11. Additionally, the tom and snare oscillators output have pink and white noise, respectively, mixed in.

Above image copyright Roland Corp
Figure 12 shows the single-transistor VCA that is used to process that mix of six square wave oscillators for the metal instruments.
The rimshot voice in the CR-8000 is outstanding - a wonderfully woody, cutting sound. Looking at it closer, one finds it is created differently here than in other drum machines, where it usually a T-network voice. Here it takes the output of the lowest two of those six oscillators (I measured these around 574 and 378 Hz) through a "swing type" VCA, as above. What does this do? An engineer on the synth-DIY list explained it to me (thanks RB!). Basically it is a very cheap and dirty ring modulator, that creates a lot of complex non-harmonic frequencies from just a couple of oscillators.
Check out the circuit for the rimshot voice below. The VCA in question is Q36. To the right of this is a resonant high pass filter that cuts all the lows out and emphasises around 1kHz.

Above image copyright Roland Corp
You can see the "lower" two square wave oscillators on the upper left, the sum of their outputs entering the base of Q36. Note the rimshot trigger has an input to these via Q37. This is to "reset" the oscillators at the start of the impulse, so that there will be a consistent attack to the sound. You can see on this scope shot, the upper trace is the square wave mix measured before C149, the lower is the rimshot audio at the combined audio output:

Again, compared to the TR-77 and it's ilk, these circuits are more sophisticated, and "realistic". The eighties instruments feature nice subtleties in the bass and tom circuits whereby resonant frequency changes with amplitude, in imitation of physical vibrating membranes (part of the reason a 909 kick drum can seem so powerful).

Voice triggering

"Accent" is Roland's term for emphasising the volume of certain steps in a rhythm pattern. This emphasis was needed because the early machines (TR-77, 33, 66 etc) had no "velocity" or volume variation in a voice, so their patterns were criticised for being mechanical and lifeless. The addition of just a single level of volume change between steps within a pattern makes a huge improvement in this regard.
There are different ways of implementing "Accent". In the TR-808, each individual voice's trigger pulse is increased on the accented steps. This results in not just volume increase, but also some tonal change in some voices, especially noticeable in the snare and toms. Accent in the CR-8000 is achieved further down the audio chain by using a trigger pulse identical to those used for the voices to cause a VCA (Roland BA662 OTA) to bump up the volume of the voices mix buss, the amount being controlled by a front-panel potentiometer. This VCA affects the mix of bass drum, snare, toms, rimshot, hi hats, cymbal and congas, but is placed before the addition of the handclap, claves, and cowbell to the master mix, and so these later voices are always left un-accented.  Implemented this way, the CR-8000 accent should not introduce any tonal change to the voices. It's not until you get external control of the voice triggers that you discover some of these voices really do respond in tone and volume to different pulse amplitudes.

Above image copyright Roland Corp

The voices and Accent are triggered by outputs of the CPU. These triggers are a negative-going pulse from a baseline of 5 volts, to zero volts. How long the pulse lasts depends on the tempo set, the variation is from about 4 milliseconds up to 70 millisecond -  pretty wide. This pulse duration seems to have no effect on the volume or tone of the voices, with one exception that I'll demonstrate.

To gain external control I disconnected the internal trigger buss and connected an Elby midiSDS-16 (midi-to-trigger unit), after configuring it to send 5 volt negative pulses of around 4- 5 ms duration, with midi velocity determining pulse amplitude.

Turning the Accent pot to maximum gives the widest amplitude response to midi velocity. In the following audio demonstration you can hear voices that are all triggered with midi velocity of 100. In order, you hear the snare, then kick, rimshot, hi conga, low tom, cymbal then closed hi hat. For each, the first eight hits are un-accented, then an accent pulse going from maximum to minimum is triggered simultaneously with the subsequent 16 hits. What you hear is mostly volume change, very little tonal change. The little there is might be some distortion at the total mix amp circuit on the loud hits.

The cymbal and the open hi hat are the two voices that have a little "tail" to them, and you can hear the accent pulse interact with this, in possibly useful ways.
In this demo, hear the initial cymbal hit, followed by accent pulses punching into the decaying tail. It plays twice. Following this, hear how you can get a kind of "pedal" hat sound, by putting the accent pulse just at the tail of the open hat. These applications suggest that the accent pulse is the one trigger point where it might be useful to have control over the duration of the pulse, not just the amplitude.

So much for the effect of Accent. What is the effect of changing the individual voices pulse amplitude? Using a midi file such as that shown below,

the following demo has each voice triggered with a decreasing velocity, but note: NO accents are triggered. In order, you hear hi conga, mid conga, low conga, cowbell, cymbal, mid tom, open hat, low tom, closed hat, snare, bass drum, clap, clave, and finally rimshot. At low velocities some instruments cut out early and won't play all the notes. Most have some volume change with velocity, and some such as the toms and snare have interesting tonal changes, such as the noise components dropping out quickly at lower levels. Tonal changes in the rimshot and congas are much more subtle, and are pretty much absent in the cowbell and claps.

And finally, here is a mess of drum hits to show the dynamic range the combination of accent and velocity can get you with these voices under external control.

dyno demo wav

Text, images and audio copyright © Adam Inglis 2017 except where indicated. 

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