Most people are familiar with the electronic 'rhythm boxes' which have been available for over a decade and are now included in even the cheapest home organs. The most significant limitation of these is that only a small number of preset rhythms is available. However, with low-cost microprocessor control several sophisticated programmable drum synthesisers have come onto the market (see Warren Cann's review of the Linn drum computer, September E&MM).

The controller presented in this article allows the user to program rhythms in 'real time' or step-by-step, to edit them, string them together and to store rhythms on tape if required. The unit generates standard +15V trigger pulses for 14 instruments plus an accent output (to accentuate selected beats) and a special 'hi-hat open' output. LEDs show the status of each trigger and the position of the downbeat while four 7-segment displays show beat number, rhythm number and other variables.

Hardware Features

A block diagram of the drum controller is shown in Figure 1. The heart of the system is the MPC board, which is described in 'Using Microprocessors' in this issue. All the logic devices operate from the standard +5V supply. Also, +15V supplies are used for trigger outputs and for driving analogue sound generators (if these are not to be used the -15V supply may be omitted).

The front panel layout is shown in the photograph. Note that the instruments specified are arbitrary and apply to the sound generator board to be described.

Operation

In order to illustrate the operation of the system a programming and editing sequence will be described. Let us assume that the memory of the device is empty. This will be the case at power-up unless battery back-up of the RAM is employed. This option will be considered in Part 2.

First we have to allocate a rhythm number. This is done by pressing and holding RHYTHM SELECT then + or -. The rhythm number shown on the left-hand display will be incremented or decremented. When the desired rhythm number has been selected, + or - is released.

In auto program mode the system is programmed in real time in sync with a tempo output. The timing of a key depression will be resolved to the nearest half-note, quarter-note, eighth-note, etc. or triplets of these. To select the desired resolution, RESOLUTION and + or - are pressed. If, for example, 16th note triplets are required, stop when the display shows 16.

Now we are ready to program the rhythm. Press PROGRAM AUTO. The beat number will increment from 1 to 96 then reset to 1 and repeat continuously at a rate determined by the tempo control. At the same time a trigger is sent to the 'block' output every quarter-note so that the player can synchronise to the system clock. If you do not want the 'block' to be the sync pulse (you may want to program the 'block' first) press BLOCK then, for example, HIGH BONGO and that becomes the sync output. Whenever the beat number resets to 1 the downbeat lamp flashes.

To set the rhythm length press END at the moment when you want the beat number to reset to 1. If you do this at about the end of beat 16 you will have a 16-beat sequence. Now you can program any combination of instruments one or more at a time as the rhythm cycles. If you find you do not like the snare drum rhythm, for instance, you can press CLEAR and SNARE DRUM and all the snare drum beats will be removed. If your drumming is a little inaccurate it will be corrected to the pre-selected resolution. This applies to all the instruments and additional triggers as well as to the ACCENT key.

Whereas most triggers give a short +15V pulse output, the 'hi-hat open' output remains high for the whole beat length. When a trigger channel is active its associated lamp is turned on. When there are enough instruments programmed to make the sync output unnecessary, pressing the sync instrument button twice will cancel sync and allow that instrument to be programmed normally.

Programming is terminated by pressing RUN/STOP.

The rhythm may now be edited. Step through the rhythm one beat at a time by pressing BEAT SELECT and + or -. The LEDs will light to show which triggers are selected and they will be turned alternately on and off when the trigger keys are pressed, modifying the rhythm in memory. Of course, a whole rhythm can be built from scratch using this editing mode and it is an alternative and possibly preferable way of setting the bar length.

Two other rhythm editing functions are provided. The CLEAR key may be pressed with BEAT SELECT or RHYTHM SELECT to clear a beat or rhythm. The other editing function is rhythm spreading and condensing. To spread, press SPREAD/CONDENSE and +. As you press + the spread factor in the right-hand display will increment. To make the system foolproof you must release + and press END to execute the spread function. If the spread factor was 2 the number of beats will be doubled. If 3, it will be tripled, thus allowing triplets to be inserted.

The condense function is initiated as above but you should press - instead of +. If the condensing factor is 2, every other beat is removed (even-numbered beats). If it is 3, two out of three beats are removed, thus eradicating triplets. This function is useful in making the most of memory space.

Sequences

A sequence consists of several rhythms strung together to make a complete tune or song backing. To program a sequence first press SEQUENCE SELECT and + or - to select a free sequence number. Then press INSERT and + to set the insert mode. If we wish to start with three repeats of rhythm 2 we press RHYTHM SELECT and + or - alone until three repeats are indicated on the right-hand display. When both the number of repeats and the rhythm number are as desired, press ENTER to fix the data in the sequence memory. When the sequence is complete press END.

To remove data from the sequence press INSERT/DELETE and -. Then step to the entry to be deleted and press ENTER to remove it. You may also press INSERT and + at any point in the sequence to insert rhythms after the currently displayed entry without destroying those which follow.

As an alternative to programming in a number of repeats of a rhythm you may program the repeat number to zero. This will cause the rhythm to be repeated indefinitely until the operator presses the CHANGE key. The sequence will then step onto the next entry at the end of the rhythm. When in the continuous repeat mode the decimal points of the display flash to show that operator intervention is required. This mode is particularly useful for 'jamming' where the length of a piece will vary.

In Part 2 we will give the circuit diagrams and board layout of the controller as well as suggested sound generation and processing circuits to use with the controller.