Some dgital synthesizers now exist in the form of 'softsynth' software that synthesizes sound using conventional PC hardware. Others typically use specialized DSP 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: