The science of synthesis part 7 - Voltage Controlled Amplifiers (VCA)

An amplifier, electronic amplifier or (informally) amp is an electronic component that can increase the power of a signal. An amplifier functions by taking power from a power supply and controlling the output to match the input signal shape but with a larger amplitude. In this sense, an amplifier modulates the output of the power supply based upon the properties of the input signal. An amplifier is effectively the opposite of an attenuator: while an amplifier provides gain, an attenuator provides loss.

Once the filters have sculpted a sound, the signal then moves into the final stage of synthesiser. The amplifier. When a key is pressed, rather than the volume rising immediately to its maximum and falling to zero when released, an 'envelope generator' is employed to emulate the nuances of real instruments.

The ADSR envelope

Few, if any, acoustic instruments start and stop immediately. It takes a fine amount of time for the sound to reach it's amplitude and then decay away to silence again; thus, the 'envelope generator' - a feature of all synthesisers - can be used to shape the volume with respect to time. This allows you to control whether a sound starts instantly the moment a key is pressed or builds up gradually and how the sound dies away (quickly or slowly) when the key is released.

These controls usually comprise four sections called attack, decay, sustain, and release (ADSR), each of which determines the shaping that occurs at certain points during the length of a note. An example of this is shown in Figure 1.20

• Attack: The attack control determines how the note starts from the point when the key is pressed and the period of time it takes for the sound to go from silence to full volume. If the period set is quite long, the sound will 'fade in', as if you are slowly turning up a volume knob. If the period set is short, the sound will start the instant a key is pressed. Most instruments utilise a very short attack time.

• Decay: Immediately after a note has begun it may initially decay in volume. For instance, a piano note starts with a very loud, percussive part but then drops quickly to a lower volume while the note sustains as the key is held down. The time the note takes to fade from the initial peak at the attack stage to the sustain level is known as the 'decay time'.

• Sustain: The sustain period occurs after the initial attack and decay periods and determines the volume of the note while the key is held down. This means that if the sustain level is set to maximum, any decay period will be ineffective because at the attack stage the volume is at maximum so there is no level to decay down to. Conversely, if the sustain level were set to zero, the sound peaks following the attack period and will fade to nothing even if you continue to hold down th key. In this instance, the decay time determines how quickly the sound decays down to silence.

• Release: The release period is the time it takes for the sound to fade from the sustain level to silence after the key has been released. If this is set to zero, the sound will stop the instant the key is released, while if a high value is set the note will continue to sound, fading away as the key is released.

Although ADSR envelopes are the most common, there are some subtle variations such as attack-release (AR), time-attack-delay-sustain-release (TADSR), and attack-delay-sustain-time-release. (ADSTR). Because there are no decay or sustain elements contained in most drum timbres, AR envelopes are often used on drum synthesisers. They can also appear on more economical synthesisers simply because the AR parameters are regarded as having the most significant effect on a sound, making them a basic requirement. Both TADSR and ADSTR envelopes are usually found on more expensive synthesisers. With the additional period, T (time), in TADSR for instance, it is possible to set the amount of time that passes before the attack stage is reached. (See figure below)

The TADSR Envelope

It's also important to note that not all envelopes offer linear transitions, meaning that the attack, decay and release stages will not necessarily consist entirely of a straight line as it is shown in the following figure On some synthesisers these stages may be concave or convos, while other synthesisers may allow you to state whether the envelope stages should be linear, concave or convex. The differences between the linear and the exponential envelopes are shown in Figure below.

Linear and exponential envelopes

Here concludes the seventh part of this post, if you want to know more about acoustic science please read. Rick Snoman's Dance Music Manual (Second Edition) Tools, Toys and Techniques.