Making Music
  Computer Music Software and Hardware

Four more or less

With just these four phases, it's possible to mimic most natural sounds and acoustic instruments - enough to fool our ears, anyway!

However, not all sounds require all four phases.

A wood block, for example, simply has a fast Attack and fast Decay - the sound doesn't hang around long enough to enter a Sustain phase.

This is a good illustration of the difference between the Decay and Release phases. The Decay phase kicks in immediately after the Attack phase whereas you have to wait for the key to be released before the Release phase starts. The wood block envelope is totally independent of the length of time the key is held down.

An electronic organ sound has no Decay phase. It hits maximum volume - the Sustain level - as soon as you press a key and stays there until you release the key when it enters the Release phase.

The piano has a more complex envelope that must be constructed carefully. When you press a key the sound hits maximum volume almost immediately.

If you hold your finger on the key, the sound dies away slowly; if you release the key it dies away more quickly.

There is no Sustain phase as such. You need a fast Attack, a slow Decay, a faster Release and a zero Sustain level.

The importance of the envelope generator in determining how we perceive a sound should not be underestimated. It's every bit as important as the tone generator. To prove it, dial up a piano sound on your favourite synthesiser and then change the envelope to one you'd normally use for strings. Sounds odd. Likewise, apply a piano envelope to a string sound. It probably sounds a little like a harpsichord.

For most acoustic sounds, Attack, Decay and possibly Release times will be measures in milliseconds. However, in the world of synthesis the phases can be several seconds - or minutes! - long.

labmyC

A favourite sound among synthesists has long been the backwards cymbal sound and this has enjoyed a new lease of life in Dance music.

When a normal acoustic sound is produced, something is hit (a drum), stroked (a violin) or otherwise excited (the air inside a brass instrument) and the resulting sound is generated fairly quickly. In other words, it has a relatively fast Attack time. When such a sound ends, it always takes a little while for the sound to fade away completely so there is always a short Decay or Release phase. These sort of envelopes sound natural to us.

A backwards sound is unnatural because the sound builds up very slowly and then abruptly stops with no Decay or Release phase. No natural acoustic instrument has this sort of envelope. We can easily construct a backwards envelope by using a slow Attack time with Decay and Release times of zero.

Breaking points

ADSR envelopes were the norm with voltage controlled analogue synthesisers although there were also variations such as AD (as above) and ADR. As synthesis technology developed it didn't take long before more complex envelopes were designed.

One variation was to break some of the phases into two sections using what was called a Breakpoint so you might find a ADBDSR envelope. This enables more complex and realistic envelopes to be constructed.

A breakpoint is simply a point at which a change takes place and breakpoints are now commonly used in audio editing software where you can draw a volume or pan envelope onto the signal. You can click breakpoints, sometimes called nodes, onto the envelope line and drag the points to create virtually any envelope contour you wish.

<< Previous    1  [2]  3    Next >>