Actually, if somebody wants to start developing PC based voice encryption, there's a pretty significant installed base of machines that can handle it already. By the end of 1992, there were about 3 million machines with sound cards, by the end of 93 it's projected to reach 6 million. Anyone that has a Soundblaster or Soundblaster compatible has both a DAC output and a microphone input. On a machine with a 9600 or 14,400 kilobaud modem, sufficient real-time compression of voice to fit within the modem bandwidth is a quite reasonable objective. I know of at least three people in the computer game industry that have been working on it, and at least one of them already has functional code. I'm sure there's a pretty fair number of Macintoshes out there that have all the hardware to support real-time encrypted voice communications also, though I don't follow the numbers in the Mac market these days...
The biggest problem is CPU power. The compression schemes that work best are very computationally expensive. Add to that the fact that you need to do simultaneous encryption and compression, and if you want full duplex make that simultaneous encryption, decryption, compression and decompression. You also have to send it over the modem, and probably frame it too. I'm currently implementing one scheme (LPC) on a DSP chip. Hopefully my end product will be <$50. I plan put its own ADC/DAC chip on board (to save computer<->DSP bandwidth). Possibly some high end CPU's like 486 and 040 could handle the load, but wouldnt leave much cpu for anything else.
Dr. Cat / no .sig, why bore people?