by Theo Verelst
Musical instruments, and in fact all sound sources, can be analized by plotting the amplitude of the sound against time.
That plot of evolution of that amplitude is in synthesizer terminology called the 'envelope' of a sound.
To create a synthetic sound, the making of such an envelope is one of the ingredients for a succesfull sound imitation, which since the first analog electronic synthesizers is done in ways which can also be simulated by computers.
amplitude ^ | /-- | / \ D | / \ S | / -------------//---- | A / \ | / \ R | / \ | / \ +--------------------------//------------> time
The above is a schematic representation of how the loudness of a sound can vary from the start to the end of the sound, using what is called a ADSR curve, which we will simulate in tcl.
ADSR stands for Attack, Decay, Sustain, and release, corresponding with the phases of a sound coming up, decaying, being sustained (for instance like the key on an organ bein held down), and finally dies away.
On traditional synthesizers, there are knobs for these 3 rates (ADR) and level (S), so that each phase of the sound can be made to take as long as seems fitting to the synthesizer programmer.
The envelope is not the waveshape of the actual sound, it's only an approximation of the amplitude of the sound, because a sound is always made of vibrating waves, in the range of 20 Herz to 20kHz, so that the amplitude is always an approximate of some kind, for instance the average root mean square power of the sound signal (in electrical or sound wave form), or the height at the top of the bottom of the wave (at the zero crossing of the derivative).
The amplitude gaph can be seen as multiplied with the waveform of the sound source at each point in time.
Am I the only one who finds pages like these pretty far off-topic for a Tcler's Wiki? -jcw