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Drum Machine Supplement

How It Works

Drum Machine

Article from International Musician & Recording World, October 1985


All drum machines, whether they're forty quid preset jobs or the latest digital giants, work in more or less the same way. That's not to say that different designs don't use higher and lower levels of technology, or apply them with more imagination, but there is a lowest common denominator in drum machine design which we can look at in order to get some idea of how they think.

Let's look at the problem. First; we want to be able to electronically generate the sound of a drum. Different drums make different sounds — a bass drum is vastly different from a snare drum — so from the start we have to think in terms of different circuitry to produce even vaguely life-like sounds.

In electronics there are three major methods of sound production. One is to use an OSCILLATOR, a circuit which produces a regularly varying electrical current. When this current is eventually presented to a speaker we'll get some sort of sound, as long as the oscillations are within the range of human hearing, which for argument's sake we could take as 50 cycles per second (50Hz) to 15,000 cycles per second (15kHz). Oddly enough, a pulse of 200Hz takes us most of the way towards a simple bass drum sound, while higher pitches give a simple tom-tom effect.

The second sound production method is to use a WHITE NOISE generator. White Noise is a random mixture of all oscillator frequencies, and comes out sounding like the sea — or, more to the point, like the hiss you get between stations on a radio.

White noise is just the thing you want to start adding more metallic effects — such as the snare of a snare drum — to your percussion sounds. There are several methods of generating white noise, the simplest being to use a ZENER DIODE which emits vast quantities of white noise whether you want it to or not. However, there are much more sophisticated methods of generating white noise, which takes us into the third method of sound production.

A simpler analog affair
(Click image for higher resolution version)


Digital techniques are applied to sound production in all the up-market SAMPLED drum machines. However, digital technology was being used before sampling became popular; for instance, you could use a very long chain of numbers read out from a digital memory to create white noise. Seems a long way to go about it, but it works well. It's also possible to produce a digital RING MODULATOR, although the simplest form of this circuit is a ring of diodes; however it's designed, the circuit's effect is to produce overtones and harmonics reminiscent of metallic cymbal sounds. Just the thing for turning your white noise splash into an acceptable hi-hat, ride or crash cymbal.

All the techniques of oscillator tuning, white noise production and ring modulation are abandoned on sampling machines such as the Drumtraks, Drumulator or RX-15, whether they work by digital recording or by PULSE CODE MODULATION. This trend was begun by Roger Linn on the Linn LM-1, after he's realised that all the attempts to electronically imitate drum sounds couldn't possibly compete with a digital recording of the real thing. Accordingly he developed the system of storing drum sound data on EPROM's and reconverting it into sound as and when desired.

But so far we've only discussed the production of drum sounds, not the way in they're strung together into patterns. Each sound can be set off by a trigger pulse within the drum machine, so we need to be able to generate trigger pulses in various patterns to play a drum riff.

This is the job of a central SHIFT REGISTER, which in turn is controlled by a CLOCK OSCILLATOR. The shift register holds lists of pulses relating to the different drum patterns you want to play, and the clock oscillator steps it along one position at a time. The speed of the clock oscillator is controlled by a big red knob marked TEMPO (yours may not be big and red, but the principle's the same).

There are many different sorts of shift register, from one holding a handful of preset patterns to a completely programmable one based on RAM chips. On the more powerful drum machines you can write not only patterns but also chains of patterns, or SONGS. All that's happening here is that another shift register takes command and steps through a list of patterns, while the lower register continues to step through the lists of drum sounds within each pattern.

A digital MIDI programmable machine
(Click image for higher resolution version)


After the advent of the Linn LM-1 it seemed that digital recording would be the way to go, but Yamaha, Korg, Roland and others preferred to take up the simpler PCM system. The technology of PCM is cheaper than that of recording sounds on EPROM, but there are some limitations, and above a certain recording time EPROM begins to look attractive again to the designer. For the user, the major difference is that EPROM chips can be erased, re-programmed and changed at will to give you new sounds, whereas PCM technology and the simpler ANALOGUE designs generally give you permanent sounds which can't be changed much.

Once we've established a set of drum sound generators, whether they're simple combinations of oscillators, white noise generators and ring modulators, or more complex system of reading out digitally encoded information, then arranged for the sounds to be triggered in the desired patterns, the rest is simply ergonomics. What does the front panel look like, what input and output sockets should we provide for the user, should we use rotary controls or digital up-down controls, should we give any control over the drum tuning or quality or simply have fixed sounds? It's at this point that the marketing men begin to work with the designers ("we don't want to release anything that supercedes our last design or our next design too thoroughly, do we?") and the prospective purchaser needs to consult a buyers' guide like this one.


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Ten Landmarks

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Machines up to £200


Publisher: International Musician & Recording World - Cover Publications Ltd, Northern & Shell Ltd.

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International Musician - Oct 1985

Donated by: Mike Gorman, Neill Jongman

Scanned by: Mike Gorman

Drum Machine Supplement

Previous article in this issue:

> Ten Landmarks

Next article in this issue:

> Machines up to £200


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