On Tue, Jun 04, 2019 at 11:58:04PM +0000, Kris Kirby wrote:
56 kilobauds is because the Telebit Trailblazer existed; it used OFDM and 6 "baud" carriers.
We've never pushed the limits like wireline did. We should be able to cram 33.6 kbit/s through 0-4 kHz just like the phone modems did. It was possible to achieve 14.4 kbit/s bidirectionally with modems through a full-duplex telephone patch.
Hack apart the resistor bridge, TX to modulator, RX to demodulator, send the command to start data, and the command to answer. It'd have to be a full-duplex link, four radios involved, but that's 33.6 kbit/s bidirectional with 175 ms of modem delay or so. Good luck finding schematics today however.
Back in the day, I asked one of the Telebit engineers about doing something like that, and he told me two interesting points to consider: At least one model of the Trailblazer was jumperable to make it a four-wire circuit compatable device - in other words, separate receive and transmit. But he said he didn't think it would work very well because the signal-to-noise ratio and phase stability of the typical radio circuit were vastly inferior to telephone circuits. He did admit it would be an interesting experiment. As far as I know, no one ever tried it. - Brian