Subtractive synthesis is often referred to as analogue synthesis
because most analogue synthesizers (ie non-digital) use this method
of generating sounds. In it's most basic form, subtractive synthesis
is a very simple process as follows:
OSCILLATOR ---------> FILTER ---------> AMPLIFIER
* An Oscillator is used to generate a suitably bright sound (usually
a SQUARE WAVE). This is routed through a Filter.
* A Filter is used to cut-off or cut-down the brightness to something
more suitable. This resultant sound is routed to an Amplifier.
* An Amplifier is used to control the loudness of the sound over
a period of time so as to emulate a natural instrument.
In essence, this is subtractive synthesis. You start with a sound,
subtract out the unwanted bits and control its loudness over time.