Advanced Music Synthesis
Gate, Trigger and Clock Pulses
Trigger, gate and clock pulses
In order to produce a note or a sound on a synthesiser, the keyboard needs to provide two things; a control voltage for pitch and tone and a gate/trigger pulse to 'fire' the envelope generators - without either or both of these you'll be back in the days when Stockhausen messed around with Wien-bridge oscillators. The control voltage we have already looked at but for those of you who have not been following these workshops (tut, tut, shame on you) I'll recap briefly. The Voltage Controlled Oscillators (VCOs) require a control voltage applied to their input in order to produce a change in pitch. When derived from the keyboard each semitone represents 1/12th of a volt (1 volt giving an octave change) which gives us the diatonic scale we are so familiar with here in the West. The 1 volt per octave system has been standardised by most manufacturers so that synthesisers of different makes can be interfaced but sadly this is not the case with the gate and trigger pulses, as we will soon discover.
The GATE pulse initiates the Attack and Decay portions of the envelope and sustains the note at the level set by the Sustain control for as long as you hold the note. Upon release of the note the Release portion comes into play and the sound will die away at a rate determined by the Release control. On a monophonic synthesiser, if you hold a note whilst playing another note the gate pulse won't be released and so the Release portion won't occur - neither will the Attack and Decay portions of the 'new' note. All you will get is a change of pitch at the volume and filter level set by the Sustain control. This is known as single triggering (Figure 1) and can be found on older Moog instruments as well as other synths. Single triggering can be a most expressive way of playing as it allows you to bring in the Attack and Decay with your playing techniques so that legato notes can be played. Be careful, however, if you set the Sustain control(s) to minimum because without releasing your finger off the keyboard before playing the next note, you can end up with no 'new' note because the Attack and Decay portion won't be initiated.
The TRIGGER pulse, on the other hand, is a one-shot pulse that does not sustain and is responsible for triggering the Attack and Decay portions of the sound regardless of whether or not another note is being held. Although both the Gate and Trigger pulse are used to 'fire' the EGs the end result can be quite different because regardless of how you play the keyboard, the Attack and Decay portions will always be present, making it difficult to play legato lines. It does solve the problem of having a sound with no sustain because each new note will have its own envelope articulation and you're not left with a silence if you inadvertently leave your finger on another note. This mode of operation is known as multiple triggering (Figure 2) and is more useful for plucky staccato lines where each note has the initial 'bite' of the envelope.
If all that sounds confusing, don't worry. It seems that triggering and gating are some of the most difficult concepts for some synthesists to understand but with experimentation and practice it should all become clearer. Most synthesisers nowadays have the option for single or multiple triggering as it is quite easy to turn a multiple system into a single system simply by switching out the trigger pulse and leaving the envelope shaping in the hands of the gate pulse only. You should therefore be able to try out various envelope shapes and compare the effect in the two different triggering modes.
The most common form of pulse is the positive 'V' trigger and gate type. This is a pulse which rests at 0 volts when no note is being played. As soon as a note is played the voltage rises to a positive value (between 4 and 15 volts, depending on the manufacturer). As soon as the note is released, the voltage drops to 0 volts again and waits for the next depression of a key. This system is used by Roland, ARP, Sequential Circuits and Oberheim and is considered the standard gate/trigger pulse system.
Another type is the 'S' trigger. This is found only on older Moog instruments. When no note is played the voltage rests at 12 volts, and as soon as a note is played the voltage drops to 0 volts. The term 'S' trigger is not entirely accurate as it is only a gate pulse and not a trigger pulse in the strict sense of the word, so older Moogs were just single triggering instruments.
The other type of gate/trigger pulse available to synthesists is the negative 'V' trigger. This type of pulse is found on Korg and older Yamaha instruments and instead of rising to a positive voltage level from 0 volts (as with the positive 'V') it falls to a negative voltage (usually around - 5 volts).
What all this means is that without a suitable interface it is impossible to use, say, a Minimoog with a Roland SH101, because the gates are totally incompatible. Although manufacturers standardised the 1 volt per octave system for pitch, they didn't alter their gating systems.
Those then are the keyboard-initiated pulses. Because envelope generators require a hard-edged pulse to initiate an envelope cycle, it is possible to substitute the manually provided pulse with a stream of automatically generated pulses. The low frequency oscillator (LFO) can generate a square wave and this can be used to trigger the EGs. The 'auto-repeat' function on some synths is simply a switch that routes the square wave output from the LFO into the gate input of the EGs. In this mode the LFO is operating as a Clock Pulse Generator.
Many other devices require a clock pulse to initiate an event of some form and many of these have their own clocks built-in so that they can be used as free-standing devices. It's a simple matter to take the internal clock to an output so that it can be used to control some other device, and most manufacturers have seen fit to provide a clock output and also the facility to override the internal clock so that it can be triggered off an external one. Electronic music has a wide range of devices that generate or require a clock pulse and the table (Figure 3) lists all these devices. This table does not take into account the fact that some units may be incompatible and some form of interface (such as the E&MM Universal Interface) may be required to link devices and feed it to two or more other items so that it becomes the master clock setting the speed of everything else. The possibilities are virtually endless and limited only by your imagination and the hardware at your disposal.
So far we have only looked at 'one-event-per-pulse' triggers but there are other types of triggers and gates such as FSK, SMPTE and, of course, MIDI codes. These use a stream of pulses for one musical or rhythmic event. These codes are more versatile and more precise than the other types of trigger but they can be a bit more complex to work with. They also require some expensive hardware to get them operational but once this is done the results can be, quite simply, stunning.
That just about concludes triggers and gate pulses. I've not mentioned the use of a click-track to trigger a synthesiser as this can also be a bit involved but don't worry, this will be covered fairly soon.
Over the next few months, I'll be looking at specific types of sounds such as strings, bass sounds, percussion, and so on, as well as interfacing various items of equipment.
Feature by Steve Howell
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