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Understanding Electronics

Trigger Interfacing

Article from Electronics & Music Maker, July 1982

A regular column that explains the electronics of music.
All you need to know about Trigger Interfacing

There are many areas of technology where a lack of standardisation complicates what should really be quite simple tasks, and unfortunately electromusic is one of these areas. Taking a trigger signal from one item of electro-music equipment and using it to control another piece of equipment should simply entail connecting the two together using a lead fitted with the appropriate plugs.

In practice this may give perfect results, but there is a real possibility that the required triggering will be unreliable or fail altogether, or it may be produced reliably but delayed slightly.

Figure 1. A voltage pulse has amplitude, polarity, rise and fall times, and length.

Pulse Parameters

A voltage pulse has a number of parameters, and Figure 1 helps to clarify these. First there is the amplitude and polarity of the pulse: the amplitude is normally between 5 and 15 volts in practice. Most synthesisers, sequencers and associated systems work using positive pulses, but there are plenty that use negative ones. The two are not really compatible, and in a direct coupled circuit it is unlikely that a trigger pulse of the wrong polarity will have an effect.

There is also just a slight risk of damage to the piece of equipment which is driven with pulses of the wrong polarity (but not to the equipment providing the trigger pulses). But if capacitive coupling is used it is very possible that triggering will be obtained. However, if we take a positive pulse being fed to equipment which requires a negative trigger pulse, on the trailing edge of the waveform there is a negative going signal which may well provide a suitable trigger signal. Similarly, the trailing edge of a negative pulse might provide a suitable positive going trigger signal for an item of gear that needs positive trigger pulses.

Even if this method does give reliable triggering, it is likely that satisfactory results will not be obtained since triggering will be delayed by an amount equal to the trigger pulse length, and this may be sufficient to give unacceptable results 'off' the main rhythmic beats of the music. Fortunately, it is not difficult to produce a negative trigger pulse from a positive one, or vice versa: all that is required is a simple circuit which is appropriately called an 'inverter'.

The amplitude of a trigger pulse is not normally critical, and although a pulse amplitude of 15 volts may be specified it is quite likely that a very much lower voltage, perhaps as little as 5 volts, will be sufficient to give reliable triggering. An excessive trigger pulse potential is usually no problem either, and although there is a slight risk of damaging equipment by using a slightly excessive trigger pulse amplitude, in practice most equipment has protection circuitry in one form or another that enables a substantial overload to be sustained repeatedly without any problems occurring. If the pulse voltage does need to be boosted or attenuated this is again something that can be achieved using simple and inexpensive circuitry.

Some equipment is intended to have a direct coupled trigger signal from a logic circuit, and then the trigger signal should switch between some maximum acceptable voltage and a certain minimum acceptable potential. The figures depend upon the family of logic devices concerned, and in some cases on the supply voltage in use. As an example, the popular TTL logic devices (which have a nominal 5 volt supply potential) have a maximum acceptable voltage of 0.8 volts for logic 0 and a minimum acceptable voltage of 2 volts for logic 1. These are then the voltage parameters for most micro control ports.

While it may sometimes be necessary to build a simple interface circuit when driving logic circuitry from a source which is not specifically designed to operate with the particular logic family concerned, this is often unnecessary, and logic devices can often be driven successfully from a capacitively coupled source.

Pulse Length

The length of the trigger pulse is not usually critical, and it is usually only a minimum length that is specified. There is normally no need to specify a maximum acceptable pulse length simply because most equipment has built-in circuitry which effectively shortens the input pulse if necessary, and this effect may well be inherent in the circuit. The minimum pulse length is important merely because a very short pulse may not activate the equipment properly since the trigger circuitry obviously takes a small but finite time to operate. If necessary a simple pulse stretcher can be used to lengthen a trigger pulse and produce reliable results. This feature has been included in this month's Universal Trigger Interface.

Of course, the above is only true in the case of a genuine trigger pulse, and the situation is quite different for a gating pulse. The gating pulse to an ADSR envelope shaper for example, controls the duration of the signal up to the final release (R) part of signal. When controlling a circuit of this type electronically, it is obviously necessary to set the gate pulse duration at just the right figure to give the desired effect. In other respects though, the notes given here apply equally to gate and trigger pulses.

Rise And Fall Times

The rise time of a signal is merely the time it takes to go from its minimum level to its maximum one, and the fall time is the time taken during the transition back to the minimum amplitude. Some circuits require a trigger pulse having very short rise and fall times, and multiple triggering or some other form of malfunction can result if these requirements are not met. A circuit known as a Schmitt Trigger can be used to speed up the rise and fall times of a signal, but trigger pulses normally already have very fast leading and trailing edges and in practice it is unlikely that this would be necessary.

Korg MS-02 Synthesiser Interface.

An Interface Unit

While we are discussing interface units, it may be interesting to look at a commercially available model; this one is by Korg and called the MS-02. It may prove a viable alternative to readers requiring an interface module but not wishing to build our own project this month (Universal Trigger interface).

For musicians wishing to play other synthesisers from one 'main' synthesiser, the MS-02 will overcome most of the problems encountered. Normally, a suitable control voltage is needed to drive the other synthesisers' VCOs and a trigger pulse is also required to switch on the EGs driving the VCFs and VCAs. This is usually quite straightforward when using two or more synths of the same make because they would use the same type of control for their range of instruments.

Among presently available synthesisers, there are two main types of control voltage systems for use with VCOs and other voltage controlled devices and also two types of trigger systems for the EGs. One of these is used by Korg (on earlier models) and Yamaha and is termed the Hertz/Volt System, where the VCO oscillator frequency is proportional to the control voltage. The other is employed by most other synthesiser manufacturers and is the Octave/Volt System, in which the oscillator frequency changes one octave for every one volt change in the control voltage. Trigger control generally works from a 'pulse to ground' or between specified voltage levels, the latter being harder to match in practice.

Provided your synthesisers are equipped with the conventional input and output jacks for control voltage and trigger or gate signals, you can use this interface for accurate signal processing. Facilities allow conversion from Hz/V to Oct/V and vice versa. An adding amp boosts voltages for frequency matching when used with VCOs and will take voltages from a foot-pedal, joystick or DC battery plus potentiometer arrangement for pitch-bend and modulation effects. Two trigger matching sections are given with flashing LED indicators and an array of jack sockets internally linked into three groups allow several machines to be controlled from one voltage source.

Further details of this unit can be obtained from the UK distributors: Rose-Morris & Co. Ltd., (Contact Details).

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Publisher: Electronics & Music Maker - Music Maker Publications (UK), Future Publishing.

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Electronics & Music Maker - Jul 1982

Feature by Robert Penfold

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