the great voice-over-data protocols and products introduced by Intel, Rockwell, ZyXEL and others at Comdex which will make Voice-PGP so much easier
As I understand these voice-over-data products, the voice goes over analog, added to the modem signal. The modem signal is interpreted, and then reconstructed and subtracted from the incoming signal, leaving voice. Very clever, but insufficient for secure phones.
I can't quite see how this would work unless the voice was run at a very low level relative to the data. In order to subtract the modulated version data coming from the other end you have to know exactly what it is, and in order to do that you need sufficient signal to noise of data over everthing else to reliably demodulate it or you need some means of reliably predicting it. Now I recognize that some of the time there is little or no entropy in the information in one direction (it is completely predictable - such as flags during LAPM idle intervals) and it is possible that one could contruct a syllabic gizmo that would turn off the entropy in the data when talkspurts happened by doing flow control and stopping information transmission during periods that speech was loud. But all of this seems a bit much, and certainly would be subject to lots of kinds of degradation depending on the speech content and any nolinearity in the channel. In general the modulations used in modems require at least 12-15 db of SNR for decent BERs - this would imply that if the line was not timeshared with voice in talkspurts that the peak level of the voice would have to be about 20 db below the modem tones. I guess that this would still result in intelligible speech even though it would effectively be transmitted by only a couple of bits per 8 khz sample. And yes I guess that by using forward error correction on the data at a variable coding rate one could allow the voice to creep up on the data a bit more by using a heavier duty error correction during talk spurts. I had assumed that these products digitized voice and multiplexed it with the data stream however. Certainly there are technologies to do this including variable rate vocoding (ala Qualcomm) that would allow almost the full bandwidth of the line to be used for data. [ And yes I'll bring up premail tommorow so I can sign things if I can get around to it without interrupting serious work work - but I'm left wondering who in the hell would want to forge posts from me ? ] Dave Emery N1PRE