Generalize the browser physical layer from binary MARK/SPACE to N-level
amplitude symbols, so each baud tick carries log2(N) bits (N=2 = old binary,
N=4 = 2 bits, N=16 = 4 bits). Each byte is framed START(level 0/MIN) ..
STOP(level N-1/MAX) with the carrier idling high, so the receiver locks on the
high->low edge and self-calibrates its low/high amplitude reference every byte
(tolerant of channel gain drift). Symbols are sampled over the middle of each
window to avoid averaging across boundaries. Tunable via ?levels=N (2|4|16),
default 4. Misdecodes fail safe: a bad symbol breaks the frame CRC and the
frame is dropped rather than shown as garbage.
Roundtrip-simulated (clean + noisy channel): N=2 robust at all bauds, N=4
reliable to ~50 baud, N=16 to ~50 baud on a clean channel.
Russell Ballestrini
ad78c77bfc