Sound Synthesis, Part 2

PART 2: Lost in a jungle of synth acronyms? This month, Paul White unravels the mysteries of ADSRs, VCAs and VCFs. This is the last article in a two‑part series.

Last month, while examining VCOs (Voltage Controlled Oscillators), I explained that the character of a sound is largely determined by its harmonic content and its envelope, the envelope being the way in which the level of a sound changes with time. Percussive sounds start suddenly and then die away, whereas a bowed sound might start quite slowly and then sustain at a more or less fixed level.

The envelopes of real sounds can be very complex, but in the early days of synthesis it was felt that an adequate approximation could be achieved using a basic, four‑stage ADSR (Attack, Decay, Sustain, Release) envelope generator controlling a VCA (Voltage Controlled Amplifier), as shown in Figure 1. The envelope shaper is triggered by the keyboard Gate signal and generates a control voltage which has the shape of the desired envelope.

The Attack portion of the envelope is simply the time taken for the amplitude to reach maximum, so for percussive sounds the attack time should be as short as possible. Once the sound has reached its maximum level, it starts to decay at a rate set by the Decay time setting. The sound then continues to decay until it reaches a level known as the Sustain level (another user‑variable parameter), and it remains at this level as long as the key is held down. Once the key is released, the sound resumes its decay, this time at a new rate determined by the Release setting. If a new key is depressed before the envelope generator has completed its Release phase, the old Release is abandoned and a new envelope initiated. This type of envelope generator, and more complex variations of it, are regularly found in digital synthesizers. Figure 2 shows the four stages of a simple ADSR envelope; it is important to note that while Attack, Decay and Release control the rate at which the envelope settings change, the Sustain parameter is a level, not a rate.

Modern synths often have envelope shapers with far more than four stages, the reason being that many sounds cannot be accurately synthesized using a simple four‑stage envelope. For example, a brass ensemble sound may contain several staggered attack transients which correspond to several performers starting to play at slightly different times. Using a multi‑stage envelope shaper, an envelope could be created with several closely‑spaced spikes at the start of the note, as shown in Figure 3, a feat quite impossible using a single ADSR generator. Of course, it would be possible to combine the envelopes of two or more ADSR envelope shapers to give a similar result, but most of the simpler analogue synthesizers don't permit this degree of flexibility. Only the larger modular systems provide sufficiently versatile routing to explore creative analogue synthesis in any depth, though there are affordable exceptions such as the Oberheim Matrix 1000, which boasts a very comprehensive modulation system.

Advanced Envelope Control

We nearly always associate envelope generators with level (amplitude) control, but they have a far wider range of creative uses, the most common alternative being the control of filter frequency. Filters will be covered in detail later in this article, but for the moment, imagine the filter to be just a rather fancy voltage‑controlled wah‑wah circuit. If we control the filter's frequency from an envelope generator, we can create a huge range of filter sweep sounds that range from a slow, lazy sweep to a percussive techno twang. We can also invert the output of the envelope generator so that the filter sweeps from low to high instead of high to low. Even fairly modest analogue synths tend to have a switch for inverting the filter sweep direction, though on the very basic models you may find that the same envelope generator is used to control both the amplitude envelope and the filter sweep, thus restricting the range of sounds that can be produced. Even so, the filter invert switch helps squeeze the maximum sonic variation out of what is available.

An envelope may also be used to modify the VCO's pitch. This has an obvious application in creating sci‑fi oscillator sweeps but, used subtly, it may also be used to put the finishing touches to a patch simulating a natural instrument. For example, some wind instruments tend to go slightly sharp when they are first blown, and this characteristic can be imitated by setting up an envelope with a fairly fast attack and decay, and then feeding just a small amount of this signal into the VCO's control input along with the usual keyboard control signal. As the envelope rises at the start of a note, the pitch will be forced slightly sharp, then it will return to normal as the envelope finishes its decay phase. Figure 5 shows this patch in more detail.

A more drastic level of envelope pitch control can be useful when setting up oscillator sync effects. In this example, two VCOs are used, one set to track the keyboard pitch normally and the other with a generous helping of added envelope control to produce a long pitch bend of several semitones. When the second oscillator is sync'd to the first, the envelope shape will control the way in which the harmonic structure changes whenever a key is pressed. A long envelope will produce a harmonic sweep effect whereas a very short envelope could be used to create a harmonically rich attack to the sound.

It isn't possible to consider all the elements of an analogue synth in isolation, because the key to analogue creativity is the way in which the various modules interact.

As you can see, it isn't possible to consider all the elements of an analogue synth in isolation, because the key to analogue creativity is the way in which the various modules interact when used in combination. Indeed, this is the secret behind the versatility of an instrument that, on paper at least, appears to comprise relatively few different elements, each with very limited parameters. The last of the important elements is the filter.

Filtering

In a conventionally patched analogue synth (if there is such a thing as a conventional patch), there is a stage of filtering between the sound source (oscillators) and the envelope shaper. Similar results could be obtained by putting the filter after the envelope shaper, but it is common practice to place the envelope shaper at the end of the signal chain so that any noise generated by modules earlier in the signal chain is faded out as the envelope decays.

The reason we need filters is twofold: firstly, simply mixing the basic sine, square (pulse) and triangle waveforms doesn't provide us with a broad enough range of timbres and, secondly, real sounds very often change their timbre as the sound develops. For example, a picked string produces a sound that is initially rich in high‑frequency harmonics. As the sound decays, however, these harmonics die away faster than the fundamental. To simulate this effect using a filter, we need a filter that can be made to vary its characteristics in a controllable way. As with the other aspects of the synthesizer, voltage control provides the answer. Just like an oscillator, a VCF (Voltage Controlled Filter) has a frequency to which it is tuned, so it's very convenient to adopt the same 1 volt per octave control law as used by oscillators.

Like oscillators, VCFs may be controlled from a keyboard voltage, an envelope generator, an LFO and so on, to produce a filter characteristic that varies rather than remaining static. Cheaper synths tended to share the same envelope generator for filter and amplitude (controlled via the Voltage Controlled Amplifier or VCA) while more sophisticated models had separate envelope generators for each function. Obviously there's more flexibility in being able to set up one envelope for the filter and a different one for amplitude.

Filter Types

Most synthesizer circuits are built using a circuit configuration that can operate as a high‑pass, band‑pass or low‑pass filter, though some instruments don't make all three options available. If an instrument has only one filter type it's invariably low‑pass, which means that it allows only frequencies below the frequency at which the filter is set to pass through. The slope, or sharpness, of the filter is usually 12dB per octave or, in some cases, 24dB per octave, depending on the make of synth. The two types produce a subtly different sound, which is one reason why two synths offering apparently identical facilities might sound quite different.

A filter as described, with no other controls, would sound much like any other form of EQ, enabling the user only to soften bright sounds, but the addition of a Q or Resonance control greatly increases the creative potential of a filter. Essentially, increasing the Q or Resonance of a filter makes it emphasise harmonics at the filter's cutoff frequency, rather like a wah‑wah pedal. If the filter frequency is then varied, the familiar filter sweep sound is produced. The graph in Figure 6 shows the effect of varying the Resonance of a low‑pass filter. By juggling the starting frequency of the filter with the rate, depth and direction of sweep, a wide range of dynamically changing timbres can be created. It is worth noting that some filter designs actually oscillate when set to their highest Q setting, at which point they produce a sine‑wave output. Because the filter can be voltage controlled, there's no reason not to use it as another oscillator played from the keyboard, but you may find the pitch isn't particularly stable on some models.

Filter Applications

Just because a filter has variable frequency and Q parameters, it doesn't mean that every patch you set up has to be based on a fierce filter sweep. For example:

• To produce a mellow string pad, you might choose a couple of slightly detuned, gently modulated sawtooth oscillators as your source and then use the filter as a simple top‑cut control to reduce the high frequency content of the sound. In this mode, the filter isn't being controlled from the envelope at all — it is set up as a purely static filter. Increasing the Q slightly would produce a sharper, more tightly focussed sound that would cut through a mix well, but if you increased the Q too far, the sound would become obviously resonant and the illusion of a string sound would break down.
• Adding a little keyboard control voltage into the filter's control input makes the filter frequency dependent on the note you're playing. This can be useful if you're trying to simulate an instrument that sounds brighter on the higher notes and more mellow on the lower notes.
• Brass sounds tend to make use of filter sweeps, where the filter opens fairly rapidly (but not percussively so) as soon as a note is pressed, but then closes down again over a quarter of a second or so. The result is a bright attack followed by a more mellow sustain period. The attack time of the filter envelope needs to be adjusted by ear so that it matches the time it takes for a typical brass instrument to 'speak'. Convincing brass patches are easiest to achieve using a synth that provides separate envelope generators for both filter and VCA, but quite passable imitations can be achieved using a single envelope generator.
• Most classic synth bass sounds are set up in a similar way to the brass patch but require the oscillators to be set to a lower octave. Fashionable techno bass sounds tend to make use of fast attack times and fairly high Q settings, and if the direction of filter sweep can be inverted, this produces interesting alternatives. A fast release time on both the filter and VCA envelopes helps create a tight, well‑defined bass sound, whereas longer release times are better suited to more atmospheric music, such as New Age.
• Classic filter sweep sounds tend to use a Q setting that is so high the filter is almost (but not quite) oscillating; either a long attack or release is used, to really make a meal out of the sweep effect. In all cases involving envelope control, the manual tuning control on the filter sets the starting state of the filter and the envelope level sets the range of the sweep.