4 Jun
2019
4 Jun
'19
8:23 p.m.
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