Some dgital synthesizers now exist in the form of 'softsynth' software that synthesizes sound using conventional PC hardware.
They are generally speaking more flexible than an [analog synthesizer]?, though they require substantial hardware to get the same response time as an [analog synthesizer]?.
Relevant links.
There are also a Mpeg-standard for synthesizing sounds, Mpeg4-SA. http://sound.media.mit.edu/mpeg4/
An Mpeg4-SA community page: