With over 100 PCBs already sold, the Syndrom is proving to be one of our most popular constructional projects ever. This month, David Ellis introduces some brief design modifications and a double triggering circuit.

This month, we kick off with a few (very minor) component changes to improve a) the triggerability of the Syndrom at slower than usual clock rates, and b) the range of the clock rate pot. These modifications go as follows:

C1 - change from .01µF to .0047µF.
C2 - change from .02µF to .033µF.
R3 - change from 2K2 to 1K.

Note also that a couple of the components shown in the PCB overlay in Part 2 weren't quite where nature (ie. the author) intended them. C4 managed to be in two places at once, pushing C7 out of the picture. In fact, what's shown as C4 next to pin 9 of IC3 should actually be C7, and its value should be 0.1µF rather than 0.01µF. Next, there's R3 (next to IC1) under the mistaken impression that it's resistive value is 4K7. This gets changed as per the above.

On the triggering side, we've discovered that the Syndrom triggers very happily direct from a piezo transducer. These objets de ceramique normally get made to beep for the purpose of gratifying Spectrum owners, but used the other way, ie. striking them as if you were hitting a micro running a seemingly un-debuggable program, they generate a voltage that can be used for a variety of (perhaps somewhat dubious) pleasures. Maplin Electronics are a convenient source for the piezo transducer (cat. no. QY13P, price 30p), and they also self a rubber disc which sticks onto the transducer (cat. no. QY16S, 5p), which helps when it comes to attaching the transducer to something that's going to be hit.

You'll need to use a bit of care when soldering a screened lead to the transducer - screen to the brass edge, live to the centre - but you'll find that plugging the other end into the Syndrom via a jack plug gives you an effective physical means of triggering the board. After you've bashed around with it for a bit and driven cat and wife out of house and home, it's time to experiment with attaching it to the underside of a practice pad. Remember that you can also use it to augment the sound of a normal drum kit by means of some Gaffa tape and judicious positioning - we found the combination of a bass drum and David's burp to be particularly effective. (What's all this 'we' nonsense? - Music Ed.)

One further point on the triggering side: re-triggering whilst the trigger pulse stays high can occur if your particular drum machine or sequencer is too generous with its pulse lengths. Fiddling around with the values of C1 and C2 will help, but an alternative is simply to disable the re-triggering feature of the Syndrom by short-circuiting C2.

Pitch Variations

One good thing about the Syndrom is that it gives you a very wide range of pitches. Admittedly, as you turn the pot way down low, you'll hear the inevitable aliasings creeping into the background, but provided you use a bit of imagination and commonsense, a single sound can be used in all manner of worthy ways.

The trouble, of course, is that triggering only yanks out the sound at the current setting of R2. So if you were after high, medium, and low toms (say), you'd be obliged to use three boards with individual sound EPROMs on each. That's a bit on the tough side, money-wise, though you should remember that having the three boards in parallel does mean that all three toms can be played together. The alternative to multiplying the hardware bill is to pursue the time-sharing principle by adding some circuitry that permits multiple pitch triggering from a single EPROM. In fact, this is the solidly economic basis behind most of the digital drum machines on the market.

The circuit in Figure 1 provides the means of adding a double trigger to the basic board. Again, it's all down to using gates to let the right pulses through into the digital farmyard. The gating may look on the complicated side, but in fact there are just two flip-flops (IC10a/b and IC10c/d), a couple of NOR gates (IC11b/c) to lock out the opposite flip-flop, and a further gate (IC11d) to deliver the necessary 'clear' state to the three counters on the Syndrom board. Finally, the clock pulses are combined at IC11a, and the appropriate Q output of the counters is differentiated to reset the flip-flops. This time, the trigger inputs are differentiated before being applied to their respective sections of IC11, so that repeated cycling of the sound is avoided.

The only major hassle in implementing this circuit is that there's no PCB available, which means that it's very much a question of getting out the old veroboard. The new circuit then has to be latched into that of the Syndrom.

First, the original 555 (IC1) should be removed and the clock output (from IC11a) from the double trigger circuit taken to pin 3 of the empty IC1 socket. Next, the 'clear' output of the double trigger goes to the original trigger input of the Syndrom, and a connection should then be made between pin 1 of IC2a and the 'reset' input of the new circuit. Finally, don't forget that the double trigger will also need its share of the Syndrom's +5V supply and ground.

New Sounds for Old

Various sounds have been added to the Syndrom library, and the new goodies to delight (or assault, depending on your percussive tendencies) your lugholes include:

Snare (4K) - lots of snare, plenty of bounce.
Bass Guitar (4K) - the slapped bass to end all thumb twangers.
Hi-hat (open) (4K) - a good metallic sound.
Hi-hat (closed) (4K) - good in conjunction with its open counterpart.
Low tom (4K) - nothing wimpy about this.
Rimshot (4K) - for those after a slice of military action.
Tambourine (2K) - eat your heart out, Sally Army.
Cowbell (2K)-snow-capped mountains, Gruyere cheese...

So, adding these to the original 21 gives the following list:

2716: Kick drum, Snare, Hi-hat (closed), High tom, Low tom, High bongo, Low bongo, Cabasa, Guiro, Tambourine, Cowbell, Handclap, Explosive finger click, Dog bark, David's aaah, Door slam, David's burp.

2732: Hi-hat (closed), Hi-hat (open), Crash cymbal, Snare, Rimshot, Low tom, Bass guitar, Orchestral thump, Brassy, Squawk 1, Squawk 2.

As before, these sound EPROMs are available from Silicon Sound, (Contact Details), and the prices remain at £6.75 for a 2K 2716 and £7.75 for a 4K 2732. Because of reader demand, Silicon Sound are now also offering a custom programming service, whereby EPROMs can be programmed with Syndrom users' own sounds. This costs an extra £5 on top of the price of the 2716 or 2732 EPROM. If you're interested in pursuing this, here are a few guidelines:

1 Record the sounds on a high quality cassette so that the tape is well-saturated. A modicum of compression may not come amiss.

2 Record the sounds several times in succession and make sure the sounds are really clean.

3 Keep the sounds as short as possible. We can sample longer sounds at a slower sampling rate, but do remember that there'll be a fairly drastic trade-off in quality in this instance. For the record, we usually sample at around 20kHz, which gives a fairly good compromise between quality and sample length.

Help!

Getting hold of four of the components used by the Syndrom is proving a major headache for some readers. The objects of this frustration are the 74LS163s and the DAC0800. Like most TTL chips, the 74LS163 has zoomed up in price over the past few months, and most places are quoting £1.20 or more. So, if you read in the Maplin catalogue that they're just 49p, take that with a pinch of salt! In fact, Maplin's current prices for the 74LS163 and DAC0801 are £1.22 and £4.45 respectively.

However, E&MM is now able to offer - for a limited period, we suspect - a package of the four chips (three 74LS163s and one 0800) at a special price of £5.50 including VAT and p&p. Send your orders in to the editorial address (cheques/POs payable to Music Maker Publications Ltd) and allow the usual 28 days for delivery. But be quick!

One extra component that's worth investing in if you've plans to change EPROMs regularly is a Zero Insertion Force socket for IC6. The problem (as you'll soon discover) is that 24-pin ICs don't take kindly to being levered out of tight sockets with a great deal of monotonous regularity, and the easiest way around bent and broken legs (the IC's, not yours) is to invest £4.42 in something like Maplin's 24-pin ZIF socket (order no. YX50E). The best way of using this is just to plug it into the 24-pin DIL socket that's already on the board. That way, the ZIF socket will be raised above the various jumpers and so on that wend their way across the PCB. On the other hand, if you're contemplating turning the Syndrom into a rack-mounted unit with multiple boards and a decent power supply, it's worth thinking about mounting the ZIF socket on the front panel, taking a ribbon cable from this to a 24-pin DIL header that plugs into the EPROM socket. The beauty of this approach is that changing a sound on a particular channel becomes a one-second operation.

A considerable number of readers have contacted us about using the Syndrom EPROMs in other digital drum machines. And since the Linn and MXR replacement drum chips cost in excess of £30 each, that's hardly surprising. The problem is that their manufacturers code the sounds into the ROMs in some wonderfully devious way, which means that they're both mutually incompatible and Syndrom-incompatible. Shame. Still, we're looking into this, and we may have an answer for Linn and MXR users in the near future.

Syndrom Demo Cassette

A demo cassette containing examples of sounds being generated by the Syndrom is still available from E&MM at the new price of £1.50 including VAT and p&p. Orders should be sent to the editorial address (cheques/POs payable to Music Maker Publications Ltd) and you should allow 28 days for delivery.

The Syndrom itself is, of course, still available in kit form or as a ready-built item, prices being £24.95 and £29.95 respectively. PCBs are also available from E&MM, price £4.95.