The Syndrom (Part 4)
Walking the Dog...
Audio and CV trigger circuits for E&MM's ever-popular digital percussion module. And there's talk of a Syndrom II on the way...
Circuits that enable E&MM's digital percussion unit to be triggered from any audio source and pitch-controlled from a synthesiser keyboard.
So, Part 4 of the Syndrom saga... isn't it amazing how time flies when you're having fun? No? Oh well, we promise that this will be the last instalment of the present incarnation of Syndrom.
In fact, this month sees the most important additions to the basic Syndrom circuit:
1 Audio triggering, including Simmons drum pads or microphones.
2 CV/trigger control from any one volt-per-octave synth.
As you'll no doubt be aware, the basic Syndrom will trigger from only a limited range of sources, ie. a +5V pulse or a piezo transducer. This trigger interface circuit, on the other hand, allows the Syndrom to be triggered from any audio source, including Simmons and other drum pads, microphones, or what have you. In fact, the dynamic range over which it'll trigger is 30dB or greater, and the sound can be of any length without re-triggering occurring.
The trigger interface circuit is shown in Figure 1. The input signal is DC-blocked by C1 and the level adjusted by VR1, before it's fed to the non-inverting input of the 1458 dual op-amp (IC1a). The gain setting switch enables gains of 50 or 500 to be selected. The output from IC1a is rectified by D1 and D2 and the peak amplitude is followed by C2, the time constant being set by the 470K resistor.
The level on C2 is compared at IC1b with the voltage on the inverting input, which is set at a half the value of the positive rail (9 or 12V, depending on whether you're using batteries or a PSU like E&MM's RackPack). If the value exceeds this voltage, the output of IC1b will jump from the negative rail to the positive limit of the chip: this is then differentiated by C3/D3 to produce a positive spike, which is divided by the subsequent 150K resistor and the input resistor of the Syndrom to an amplitude of +5V if the rails are +/-12V, or slightly less in the case of +/-9V. This is where the Syndrom takes over, and the first gate, a Schmitt NAND, turns the spike into a TTL pulse to operate the Syndrom.
Using the circuit is straightforward. First, the gain should be set by SW1. Drum pads, microphones, and so on will require the high gain setting (SW1 closed), while high level inputs such as line levels or another Syndrom will require SW1 to be open. Then, connect the sound source to the input and operate it repeatedly, adjusting VR1 until the Syndrom attached to the output triggers. When the circuit was first tested, a second Syndrom was used for the sound source, and this was triggered from a pulse generator. We found that the trigger interface was capable of distinguishing between sound envelopes even when they were repeated at a rate of about 10 times a second, so it should cope pretty well with just about anything you care to chuck into its input.
As anyone who's familiar with the Sequential Circuits Drumtraks will attest, a lot of mileage can be had out of a single drum sample if its pitch can be varied. But more than that, certain samples - slapped basses and orchestral thumps, for instance - are crying out to be used with a more rigorous control of pitching than can be achieved by just twiddling a pot in a more or less random fashion. This, then, is the rationale for the CV/trigger interface - a means of enabling the Syndrom to be controlled by any one volt-per-octave synth.
Although the 555 timer chip used in the basic Syndrom circuit functions as a voltage-controlled oscillator - twiddling the pot varies the controlling voltage, which alters the output clock frequency and therefore the rate at which samples are yanked out of the EPROM - these chips don't belong to any particular school of linearity, so attempts to make them function in a musically-meaningful fashion are doomed to failure. The way around this impasse is to bypass the onboard clock and substitute an external VCO that will work as nature really intended.
The circuit (Figure 2) is based around the CEM3340 VCO, a chip well suited for controlling the rate of sampling from an EPROM because of its superb specification and upper frequency limit of greater than 100kHz. Generally speaking, the circuit uses values recommended by the manufacturer, but the timing capacitor (between pins 11 and 12) has been reduced to 220pF to provide a higher frequency range. A full explanation of the IC's innards can be found in the data sheets supplied with the device. All components should be of 1 or 2% high-stability type, though it would take a mighty discerning ear to spot pitching discrepancies.
In order to get the VCO ticking to the Syndrom, the output of the VCO (pin 10) should be connected to pins 2 and 6 of the 555 (IC1 on the Syndrom board), ie. the trigger and threshold inputs. As pin 10 of the VCO is a triangular output, the 555 will regard this as if it were the rising and falling voltage on its own timing capacitor, and will therefore convert this to a TTL-compatible waveform to drive the counters. The existing timing components on the Syndrom board - R3, R2, and C1 - must, of course, be removed for this to work.
Triggering of the Syndrom in this situation will largely depend on what the synthesiser cares to pass on to the outside world. For instance, if the trigger rises to about 5V when a key is depressed, the combination of the 0.01uF capacitor and an IN4148 diode - a simple differentiator - will suffice as the conditioning circuit needed to be inserted prior to the normal Syndrom trigger input (Figure 3). On the other hand, if this voltage is exceeded, it's good policy to insert a zener diode (a 4.7V BZY88 type) in place of the IN4148, though with the same orientation. Finally, if the trigger is negative-rather than the more usual positive-going, a simple one-transistor inverter (Figure 4) should precede the differentiator.
Aside from the complication of the CEM3340 requiring +/-15V supplies, the operation of this add-on is again pretty straightforward. Once the VC and trigger from the synth have been connected to the relevant parts of the circuit, the only critical step is the adjustment of the two presets that determine the scale (RV1) and adjust the frequency (RV2). First, set RV1 to midway in its travel, and then adjust RV2 whilst playing the bottom key of the attached synth until the Syndrom falls in line with tine pitch. Next, play octaves up and down the keyboard adjusting RV1 as you go, until you're satisfied that the Syndrom is falling in line with the one volt-per-octave standard.
Well, four instalments on, the Syndrom has expanded considerably from its humble beginnings. Lots of expandability, lots of circuits, and lots of headaches for anyone trying to cobble the whole lot together. Frankly, it's all very well adding on just a single circuit to the basic unit, but once you start talking about adding on multiple and audio triggering and CV control, life's getting a bit too complicated for comfort. We sympathise. Which begs the question of 'where do we go from here?' Well, we're seriously thinking about a 'Syndrom II', a PCB-based design that'll incorporate all that's been added to the first Syndrom so far plus a lot more. But because of chip prices and so on, this needs careful thought before we jump in at the deep end. So, as they say, watch this space for further developments...
Part 1 of the Syndrom featured in E&MM April 84, the PCB overlay and parts list followed in E&MM June, while additions to the original design appeared in E&MM August. The Syndrom is still available in kit form or as a ready-built unit, prices being £24.95 and £29.95 respectively. PCBs are also available from the E&MM Mail Order Department at £4.95. All prices include VAT and postage and packing.
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