E&MM's series of Electric Drummer articles have introduced and incorporated the latest state-of-the-art technology into electronic drum design — and by following the guidelines and ideas in the series, experienced constructors will be able to make use of and adapt this knowledge to produce one of the most up-to-date and versatile drum machines available.

In this, the final part of the series, we give a concise example of programming rhythms, details of the front panel layout and a circuit for battery back-up.

Constructors may find it helpful to refer to past articles (issues November '81, and January, February and April '82) and an understanding of the use of microprocessors is also essential. E&MM's 'Using Microprocessors' series is recommended, in particular the articles leading up to the Electric Drummer's MFC board in the November '81 edition.

Many people have asked how the Electric Drummer can be used to help the electro-musician. To illustrate this, there follows a description of a typical programming session.

Powering Up

When the Electric Drummer is powered up its four-digit display shows the number of free beats available. This will be half the number of bytes of RAM less some work space for the processor. For the full 4K allowed for on the MPC board it is more than 1,900 beats.

Selecting the Rhythm

Press the RHYTHM SELECT key. The display shows 0.0 00. The decimal point in the rhythm field indicates that a rhythm number is required. Using the keys 1-9 and 10 (which = 0) enter a number between 1 and 24. If you enter a number outside the permitted range, EE is displayed to indicate an error. Otherwise if, say, rhythm 12 was selected, the display would show 12 01 and the status of the first beat of rhythm 12 would show on the LEDs. As this is a new rhythm however, all the LEDs will be off.

Editing

First let us program in a rhythm in editing mode. Let us say that we want to program in the following rhythm:

Initially the display shows beat no. 1 and all the LEDs are off. Pressing any of the trigger keys (1-15 and Accent) will turn the LED on indicating that when the rhythm is played, that instrument will sound on beat 1.

Pressing the same key again will turn the instrument off. So, for beat 1 we press Low TomTom and Accent. Then we move on to beat 3 by pressing '+' twice.

Note that at any point we can go back and re-edit beat 1 by pressing the key. We can also jump to any beat by pressing BEAT SELECT followed by the 2-digit beat number.

When we have programmed beat 8 we press END to tell the Electric Drummer that this is the last beat in the rhythm. If '+' is pressed again beat 1 will be displayed.

Trying it Out

To listen to the rhythm simply press RUN/STOP. You may adjust the tempo with the control on the front panel. Each time beat 1 comes around the DOWNBEAT lamp will flash. The beat counter and LEDs continuously show the beats as they are played. The rhythm plays repeatedly until RUN/STOP is pressed again. As it is playing, you may 'ad lib' using the trigger keys to try out additions to the rhythm.

A Change of Resolution

Suppose we now decide that we wish to add a snare drum beat between beats 6 and 7. Pressing SPREAD/CONDENSE then '+2' will double the number of beats - see Table 1. Note that so that we can edit the rhythm the beat numbers have been changed. However, the pointers in the Electric Drummer continue to point to the same beat. Thus, if we were examining beat 6, after the SPREAD function we will be looking at beat 11. We can now step to beat 12 and add the snare. Should we later wish to return to the original rhythm, we can condense by pressing SPREAD/CONDENSE then '-2'. Similarly, we could have added 2 beats instead of one by pressing SPREAD '+3'.

Table 1.

Realtime Programming

Many drummers prefer to program 'live' without bothering to write the rhythm down. This can be done using the Auto Program keys. We will program the same rhythm as we did in the first example before Spreading.

The rhythm is in 4/4 time and is resolved to the nearest eighth-note. To set the resolution, hold down the RESOLUTION key whilst pressing '+' until the display shows 08. (You can also resolve to a 16th, 32nd, 64th or triplets of these.) To set the bar length, go to beat 8 and press END. Now we are ready to program. Press AUTO PROGRAM. The rhythm now cycles continuously as it does when playing, but the block sound generator taps out a constant metronome. If you wanted, say, the hi-hat as metronome, you could press BLOCK then HI-HAT. After adjusting the metronome to a comfortable speed you can tap out the rhythm using the trigger keys. As you press them you will hear them added to the rhythm. It is not necessary to play accurately as the Electric Drummer will automatically resolve to the nearest eighth-note.

If you make a mistake on programming an instrument, you can simply remove the instrument from the rhythm by pressing CLEAR and the trigger key. At any time you can stop programming by pressing 1/STOP then edit beat by beat as before.

When you have programmed enough instruments, to remove the metronome, do so by pressing CLEAR then BLOCK.

Sequences

If we want to play the backing for a whole song we can set up a sequence of rhythms required for that song. Suppose we have programmed in rhythms 7, 8 and 9 and that they occur in the following sequence: 7, 7, 7, 8, 7, 7, 7, 9. First press SEQUENCE SELECT then 1 for sequence 1. The Electric Drummer is now waiting for you to enter rhythm numbers followed by the number of times they are repeated. This rhythm is programmed by entering:

07 03 ENTER 08 01 ENTER
07 03 ENTER 09 01 ENTER END

A further option is invoked by entering a repeat number 00. This causes the rhythm to play until the CHANGE key is pressed. The sequence then continues at the end of the rhythm. This is useful for 'jamming' sessions where the number of repeats is unknown.

We can edit sequences using INSERT/DELETE. Following the key by '+' adds an entry to the sequence and following it by deletes an entry.

The Triggers

Most of the triggers simply go to the trigger inputs of the sound generator board and therefore trigger the instruments programmed. ACCENT adds an accent to all the instruments. HI-HAT OPEN/CLOSED operates differently from the other triggers. It functions like the footpedal on a Hi-hat, so the Hi-hat may sound open (like a crash cymbal), closed (a short tap) or it may be closed at any time after striking.

Triggers 11 to 15 can be used to trigger other drum sounds such as the Syntom or to control synthesisers. Particularly interesting effects can be obtained by passing the Electric Drummer audio output through filters, flangers, etc. which are themselves controlled from the trigger outputs. In addition, the downbeat output can be used to synchronise other equipment.

Tape Storage

The contents of the Electric Drummer's memory may be saved on cassette using the TAPE STORE and LOAD keys.

Memory

When powered up, the Electric Drummer determines the amount of memory available. You should install the full 4K of RAM on the MPC board for maximum storage capacity. If more RAM is made available the software will use this too. The current issue of the MPC board does not support battery backup but more experienced constructors may wish to add the modification shown in Figure 3.

Since the publication of the Electric Drummer circuit in January some changes have been found to be desirable, as shown in Figure 1.

When the 74C917 is used to drive displays directly, the contrast is very poor. This can be corrected by the addition of four PNP transistors as shown in Figure 1a. The cassette interface requires a very high input level to operate correctly. This may be improved by changing the 4k7 resistor to 27k (Figure 1b).

The prototype was constructed in a large Verocase as can be seen from the photographs. This has ample space for all the PCBs and power supplies. The 7805KG voltage regulator may be mounted on a heatsink inside the case; the heatsink's resistance should be 4.5° C/Watt or less. Figure 2 shows the front panel layout with drilling centres.

The amount of RAM installed is optional. During the initialisation, the program determines the highest available RAM address. The EPROMs for the electric drummer are available from Cactus Consultants, (Contact Details) at a cost of £10.50 each including documentation. If demand is sufficient a complete kit of parts may be made available, including a single circuit board on which all lamps, switches and sockets can be mounted.

The transistor outlines on the Maplin sound generator board (GA60Q) refer to devices with T092 leadouts; the BC179 transistors specified in part 3 of the series have a none-too-convenient TO18 leadout configuration, and are therefore best substituted for BC212LS for ease of assembly.

The output/mix amp shown in the sound generator circuit diagram is doubled up for stereo operation on the Maplin PCB. Therefore, the parts list should be amended, two of each of the following being required: R70, 71, 672, C31, C32 and RV10. The latter could be a dual gang pot. The pads marked 'R110' are not used. Also note that the mix resistors (R61 to R69) can be assigned via two mix buses to one or other of the IC11s (viz: IC11L and IC11R); alternatively, a pair of identical mix resistors may be used to pan the output of one or more of the percussion generators to both outputs.

The unmarked pads on the Maplin sound generator PCB around IC12 are provided to allow alternative programming of the chip: refer to page 236 in the Maplin catalogue or consult the manufacturer's data. As this option will involve a degree of experimentation, we recommend that you mount R74-79 on Veropins to avoid fouling the PCB, at least until final values have been established.

On the Maplin MPC board, IC14 is 74LS124. Also, note that in the MPC text (Nov '81), the reference to IC 17 should read IC 14.

Sound Generator

Many long hours have been spent by the electronics team at E&MM improving this basic percussion board. From the outset, it was not intended to be the ultimate in drum sounds - an impossible task without considering complex cymbal circuits and sampled sounds (as in the Linn instruments). Nevertheless, it does provide the features necessary for creating a good range of drum kit sounds. These include Woodblock, Bass Drum, Low tom-tom, Hi tom-tom, Low Bongo, High Bongo, Side Drum, Cymbal and Hi-Hat. The latter can be set by a TTL pulse on/off for open/closed effect. All the sounds have a common 'Accent' control.

Since we have had many requests from readers to make provision for this board to operate from any micro with output port(s), the input trigger stages have been suitably modified to accommodate positive or negative (+15V max.) pulses. The Electric Drummer control circuits send negative pulses to the board, but for 'external' micro users it is more practical to deal with positive pulses i.e. a '1' on a port line. This means that instruments can be specified by 11 port lines (thus two 8-bit output ports are required for full operation). The unit has worked perfectly well with the ZX81 interface published in E&MM for the EDP instruments, and also with standard ports on other micros, including the Sharp MZ-80K.

A program for an external micro simply outputs the correct codes to the port lines for each event. So a possible wiring plan of these lines could be: Port 1, output code 1=accent, 2=hi-hat open, 4=hi-hat, 8=Bass Drum, 16=Side Drum, 32=Cymbal, 64=Hi tom-tom, 128=Low tom-tom. Port 2, output code 1=High Bongo, 2=Low Bongo, 4=Woodblock. This allocation puts the main drum kit on one port only for users with single port micros.

A typical rhythm sequence might start off: Accent/Bass Drum, rest, Hi-hat closed, rest, accent/Side Drum/Hi-hat closed, rest, Hi-hat open etc. = 9, 0, 4, 0, 21, 0, 6 etc. The tempo can be set by the program using a 'FOR NEXT' LOOP containing a further 'FOR NEXT' that sets the delay period between events. Alternatively a single line of a port can be used to input a clock tempo pulse as an interrupt from an external LFO (e.g. Synclock, Universal Trigger set to 'Micro'). The latter method is preferable as it allows the clock pulse to be used elsewhere for synchronising sequences and modulation effects in your studio.

The sounds are fed to two output preamp stages for stereo operation. Situated at the far right of the Generator Board is a group of resistors. These resistor values are chosen by the user to set the position of each instrument in the stereo field. They are R63, 61, 62, 67, 66, 65, 64, 68 and 69 in the parts list. It is very easy for you to decide the layout of your drum kit for yourself. Simply use preferred resistor values that add to 100k approximately for R64-69, and to 47k for R61-63 inclusive. A typical layout might be (from left to right): Hi-Hat, Side Drum, High-Bongo, Woodblock, Bass Drum (centre), Low bongo, Hi tom-tom, Low tom-tom and Cymbal. The two resistor values for each of these positions would be:

L1 not used/100k
L2 22k/82k
L3 33k/68k
L4 33k/56k
C 47k/47k
R4 56k/33k
R3 33k/22k
R2 33k/15k
R1 47k/not used

An added bonus is the use of the SN76477N device, which has veropins located at relevant control pins on the IC for the constructor to experiment with. In the circuit described, only the white noise source is in use and the possibility of creating whistles, pitched sounds, helicopter, sea, trains, planes and other effects is available by making the appropriate pin links. (The Maplin catalogue gives full information on this device.) Final sound balance is done via the presets on the board, although musicians may like to use pots on the control panel instead. It is useful too to add micro buttons at a later stage for manual triggering of each instrument (as well as automatic control) for that improvising touch! We've also tried the Kraftwerk idea of percussion plates that replace the micro buttons and complete the trigger circuit with two metal probes (see photo).

The Electric Drummer project presents the opportunity for musicians to use today's technology at low cost for maximum control in the studio or on stage. Many improvements to the design are possible, with further EPROM programs and improved percussion circuits. Meanwhile, the sounds of the Electric Drummer, like most drum synthesisers, may be considered unique to the instrument and it only remains for the musician to create his own unique rhythmic track to complement it.