I was told once that 1 baud is one sine wave per second. Past 2400 it is too hard to squeeze more cycles onto a telephone wire so they started using amplitude to get more bits per baud.
Almost.. It's one "symbol"/sec. Early modems just used frequency A for 1 and B for 0. 1 symbol = 1 bit. There's a speed limit there (phone is about 4kHz bandwidth), so they defined four frequencies for a modem to hum, those could represent 00, 01, 10, 11 and send 2 bits at once. And so on...
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