HOW TO: Understanding Synth Gates and Triggers

From S-Trig and V-Trig to modern modular and MIDI
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If you’ve worked with synthesizers, specifically the analog variety, you’ve undoubtedly used gate or trigger signals. Understanding these signals is a basic requirement for using a modular synthesizer, a virtual modular, interfacing gear, and a number of other instruments in a typical studio, even electronic drums.

BACK IN THE DAY

When some of the first analog synthesizers were being designed in the ’60s by the likes of Bob Moog and others, it was obvious from the start that several types of signals were needed to control the various aspects of the instrument. One of the major advancements during this time was voltage control, which allows the output of one circuit to control parameters of another in real time, effectively automating changes of a sound. Voltage control uses a variable voltage to control a variable parameter, like an expression pedal provides variable control over volume.

But another signal was needed—an On/Off signal to indicate a key had been pressed, to start a sequence, or to activate individual drum beats. In many early systems, this signal was called Trigger. The name implies a quick signal marking the start of an event, but in many cases this signal indicates a start by turning On, then an end by turning Off. This applies nicely to keyboard keys being pressed then released. Most modern synthesizers call this signal Gate.

Fig. 1. In a keyboard controller, a bus wire is shared by all keys and each key makes contact to ground to produce a single, shared On/Off Trigger signal. The first versions of Trigger signals were not voltages but a switch to ground; these were called Switch-Triggers, or S-Trig for short. The benefit of an S-Trig is that multiple devices can short the signal to ground in parallel without additional circuitry. In a keyboard controller, for example, a bus wire is shared by all keys and each key makes contact to ground to produce a single, shared On/Off Trigger signal (see Figure 1).

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Fig. 2. An S-Trig plug and vintage Moog STrig inputs S-Trig made sense for the early hardware and to the designers of the time, but the need to parallel multiple controllers was overestimated and the benefits of simpler circuitry inside the keyboard controller was a benefit to the manufacturer, but not the user (see Figure 2).

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The disadvantage of S-Trig is that a switch-to-ground signal is unlike a typical voltage signal in a voltage-controlled synthesizer, and it requires a conversion to and from S-Trig to a voltage version for many common operations. This voltage trigger is sometimes called V-Trig on older systems, or simply Trigger. Conversion requires special cables and/or modules adding to the complexity of patching. For example, instead of patching a sequencer clock directly to an envelope generator, the user must patch through a V-Trig to S-Trig converter module using two different types of cables—not very intuitive.

GATE ADOLESCENCE

Fig. 3. Korg Mono/Poly with Trig I/O (which offered polarity control) As time went on, many designers and manufacturers moved to a simpler voltage version of S-Trig for On/Off signals and referred to it as Gate. Gate, being a voltage, has the added benefit of being compatible with other signal sources in a typical analog synthesizer. For example, a square-wave LFO can produce a Gate signal to fire an envelope generator, and a sequencer’s clock output can fire an envelope generator, all without conversion. Through the first golden era of analog synthesizers, Gates prevailed, although some couldn’t agree on voltage levels, polarity, or connector style (see Figure 3).

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MID-LIFE

In addition to Gate, some modern systems recycled the term “Trigger” for another purpose. This type of Trigger is used by keyboard controllers in conjunction with a Gate signal to retrigger envelope generators when multiple keys are pressed. This Trigger pulse is typically 5 volts and very short—somewhere around 5ms.

MODERN MATURITY

Modern analog synthesizers, both modular and normalized, have largely settled on a standard for Gate, and this makes life simpler for everyone in this new second golden era. These Gates are typically 5-volt signals with positive polarity (5V = On, 0V = Off ). Technically, there is a specific voltage where the input circuitry trips from one condition to the other, and that could be 1 volt or 4 volts or somewhere in-between. The good thing is, virtually all modern Gate inputs, regardless of format, will trigger on most Gate outputs—and in most systems, Gate uses the same type of connector as audio and control voltages.

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COEXISTING WITH SWITCH-TRIGGERS

Fig. 4. A simple cable providing a passive circuit converts from from Gate (Voltage-Trigger) to Switch-Trigger Integrating vintage equipment and even modern gear that use Switch-Triggers is relatively easy, although cumbersome. In most cases, the conversion needs to go from Gate (Voltage-Trigger) to Switch-Trigger, and that’s easily done with a simple cable containing a passive circuit (see Figure 4). These cables are readily available and inexpensive, or they can be DIY’d if you have the tools and skill.

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Going from Switch-Trigger to Gate is a different matter, though, because producing a voltage requires a voltage source. The solution to that is typically a special interface module.

THE MIDI CONNECTION

Fig. 5. A Modern MIDI interface providing a 5-volt Gate output, along with pitch and velocity If your experience with interfacing gear is mostly with MIDI, you might be wondering where Gate signals fit into the grand scheme of things. MIDI transmits messages including Note On, Note Off, and a host of other information. MIDI Note On and Note Off messages are basically Gate and Pitch information in a digital form. To get Gate and Pitch voltages from MIDI requires a converter box or a module. The converter reads the MIDI messages and creates Gate and Pitch signals that are then used to control the oscillators, envelope generators, and other modules in an analog synthesizer (see Figure 5).

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In a nutshell, Gates are simply On and Off voltage signals typically carried on a 1/4" or 3.5mm phone plug, and sometimes a banana plug, depending on the system’s format style. Gates are used for carrying information about when a key is pressed or when a note starts and stops, but also commonly used to turn things On and Off, start sequencers, and fire envelope generators. Gates, like so many other concepts in analog synthesis, are simple ideas capable of producing powerful results.