I guess that figure is set by some ancient standard for transmitting bits semi reliable on AM, which doesn't employ the same encoding tricks that modems did. PO

I guess that figure is set by some ancient standard for transmitting bits semi reliable on AM, which doesn't employ the same encoding tricks that modems did. POTS lines had a max baud rate of 2400, and the higher speeds were accomplished by encoding more bits pr baud. Parts of the modem noises was to establish which encoding scheme to uses, based on capabilities of the two devices and the line quality. AM broadscasting can't really make such handshakes, so my guess is that 1200 bps was some lowest common denominator. edit: wasn't too clear that I meant to say that the 1200 bps must be from an old standard