Last month Steve Howell looked at the various ways of synthesising 'kit' drums. This month, as a logical follow-on from that, he takes a look at more electronic percussion effects, all of which can be played manually, sequenced or automatically triggered from a drum machine, clock, or LFO.

The basic constituent of all these electronic percussion sounds will be for the most part a noise generator, preferably with a white noise output. You'll need only the one generator but this will be fed into a number of processing devices for more complex stereo effects, but more of that later.

One of the most basic electronic effects, in terms of ease of setting up, can be found in Figure 1. Here the noise generator is heavily filtered by a Voltage Controlled Low Pass Filter (VCLPF) which is 'opened' and 'closed' randomly by the Sample and Hold (S/H). The envelope of the sound is shaped by a straightforward EG/VGA combination, but the important factor in this patch is that the S/H is being advanced by the same trigger source as the EG. This can of course be anything: a keyboard, drum machine, LFO or even a drum pad such as a Simmons, MPF or Remo practice pad with a piezo transducer mounted inside. If you do elect to use a pad, the trigger pulse will probably have to be boosted to an acceptable level by a simple voltage multiplier. I would imagine, however, that most of you will be using a drum machine to trigger the sound, and if so, it can be an effective alternative or counterpart to the hi-hat pattern, especially if used with a sequencer and - a small but important point this - set off-centre in a stereo image.

An extension of this 'random' technique can be seen in Figure 2. Here, the CV output of the S/H is routed to the trigger input of the EG and the S/H is again advanced by a drum machine or similar. The principle behind this patch is that your percussion sound will only trigger when the CV from the S/H is higher than the EG's required trigger input level, so that both the tone and the actual firing of the sound will vary at random. This technique is a little too unpredictable for most commercial dance music (though it could be quite effective in a 12" remix!) but could be quite useful in more 'Enoesque' pieces, for example, You could, of course, trigger the S/H of a click-track and multitrack the effect. With an exaggerated stereo image, the net result can be quite astonishing as each sound triggers randomly around the stereo image. The filter controls can be set as desired but a high resonance setting will yield the most dramatic results. Likewise, the ADSR controls on the EG can be set as desired, but it's worth remembering, with many percussion sounds, a slow attack can be just as effective as a fast, hard one.

Sequencing

S/H can, however, be replaced by a sequencer. This can be anything from a simple eight-note model to a spare channel of a Microcomposer. The CV output of the sequencer is fed into the VCF control input and the sequencer then programmed to give precise tonal changes. This can be a useful alternative to Sample & Hold as the noise can be tuned to bear a closer relationship to the rest of the piece.

If, however, you don't have access to a sequencer or S/H device (though I'd be very surprised if any of you modular synthesiser owners didn't have such things!) then a keyboard will suffice in their place, and one tip you might like to heed when recording is that should you wish to use an 8th or 16th note random noise pattern in the context of an uptempo track, simply record it at half-speed and play the keyboard randomly. This will give more or less the same effect when played back at normal speed.

It is also possible to sweep the VCF cutoff frequency with an LFO although the cyclic repetitiveness could prove somewhat boring. If this proves to be the case, you could use two or more LFOs to create some variation. By using different waveforms you should be able to create even more interesting effects but you won't be able to sync the sweep to the drum machine or sequencer, which could be a disadvantage.

Any of the above patches can be usefully employed in a rhythmic context but they will all be essentially mono sounds. They can, of course, be used with other percussion sounds in a stereo image. Many varied and novel effects can be created in stereo, however, by patching up as in Figure 3. Here, we use two VCFs swept by one controller, but one channel has its' CV inverted so that it goes in the 'opposite direction'. If you use just an LFO, one channel 'opens' as the other 'closes' and, if each channel is panned hard left and right, the sound will pan (obviously) from one side to the other dramatically. Using an S/H instead of the LFO will give the impression that the sound is actually 'bouncing' back and forth, as the CVs move abruptly in opposition to each other.

Clap Sounds

Moving now to a 'one-shot' percussion sound, one of the most sought after and popular sounds these days, especially in dance music, is the clap, and a synthesised clap is often used in preference to digitally sampled or real claps because of the variability of such sounds. To my ears, one of the best clap sounds around can be found on the Roland TR808 drum computer, but an identical sound is now also available in the shape of the Boss and Amdek Handclapper pedals. For those of you who don't have access to such devices, however, a modular synthesiser can be an effective substitute.

The sound of a clap is derived from passing a white noise source through a band pass filter which, in turn, is shaped by multiple envelope generators. The EGs have a delay so that the effect of multiple claps can be easily obtained. These factors can be set up in a number of ways on a modular synthesiser and patches are shown in Figures 4a and 4b.

In Figure 4a, the noise is fed into a VC Band Pass Filter (VCBPF) and the output from that is fed into a number of EG/VCA combinations, each of which have their own trigger delay. There is also another VCLPF which is 'opened' and 'closed ' very slightly by a random S/H. This portion allows for the randomness that exists in 'real' handclapping, but the S/H can be omitted if desired. The VCBPF should be set to give a 'middley' sort of sound with a hint of resonance, though it can be varied to suit. The ADSR controls on each EG should be set to give a fairly clipped sound and the decay times could be slightly different for each one to make the clap a little more interesting. The trigger delay times should be set to give a few milliseconds delay so that the effect of multiple claps can be created, and if the delay time is voltage controllable then a slow sweep with an LFO will help create more variation. The VCLPF is fed into EG/VCA 4 and the decay/release times set to around 750ms, depending on the effect you require.

Figure 4b shows an alternative patch which is considerably easier to set up and requires much less in the way of hardware (I doubt if there are many of you who have two trigger delays). Here, Channel 1 is fed into a digital delay and this provides the effect of multiple claps. The delay time should be set to a few milliseconds and could, if you so desired, be swept slightly by an LFO: be careful you don't end up flanging the sound though. If a DDL is not available, an analogue echo will suffice - in fact, the restricted bandwidth of an analogue delay can actually help the sound, precisely because of its' 'middley' sonic character.

If, however, you don't have a band pass filter in your system, then an HPF in series with an LPF will give you much the same effect or, alternatively, you could simply EQ the bass out of the sound if you only have a low pass filter. Using a graphic or parametric EQ will also help greatly in creating the 'cupped' effect of hand claps.

When recording the clap, it's best to add some degree of reverb, while you could also try putting it through a chorus or a harmoniser to 'spread' the sound. Multitracking the clap will also give a much thicker and more powerful sound, especially if you make small variations to the sound with each track and/or use varispeed.

Percussive Attack

Before we finish with percussion sounds, there is one technique which can be applied to any sound requiring a percussive attack if you have the appropriate facilities.

Many VCAs on modular systems give the user a choice of either linear or exponential response. Linear response is the more commonly used on synthesisers, but if an exponential response is used the rise and fall times of the VCA are effectively halved so, in other words, an attack time of 2ms will give an apparent attack time of 1 ms: referring to Figures 5a and 5b should explain why. In the linear response graph, the attack time is set so that it takes three seconds to reach 3 volts and so that the VCA takes three seconds to reach its' full level output. In Figure 5b, however, the attack time is still set at three seconds but the VCA response is such that it only takes two seconds to reach full output. The reverse happens on decay and release times.

As such, the exponential response is ideal for sounds that require more 'bite' if they're to be used to the full. If your synthesiser doesn't have this facility you can overcome this by using varispeed on your tape machine. By recording the sound slower than the correct playback speed, when the piece is played at the normal speed the ADR times will have sharpened-up considerably but you will have to adjust a tuned sound to compensate.

That just about concludes our look at percussion sounds for the moment, but before you all ask about congas, bongoes, tambourines, Mongolian clay cymbals, and Martian log drums, these (with the possible exception of the last two) will be covered in future issues. I think, though, that these patches should keep you rhythmically inspired for some time to come.