Modems and external dialers.

[Fred Cisin]

On Wed, 5 Jun 2019, Grant Taylor via cctalk wrote:
> So the phone played an active role in modem communications.  At least in so 
> far as it converted the purely audio from the modem to telephony used by the 
> PSTN.

"MODEM" is short for "MODulator-DEModulator"
It is explicitly a device that took data and "MODULATED" it into audio 
tones for the phone, and took tones from the phone and "DEMODULATED" them 
into data.

Bell 103 used one frequency for 1 and another for 0, and a different pair 
for the other side, to at least in theory permit full-duplex.
1270Hz/1070Hz and 2225Hz/2025Hz

A couple of my students tried to make an almost entirely software based 
modem that just counted the waves.

If you calculate how many waves of the tone you get at 300 per second, you 
can see why the speed could not be increased [MUCH].
"it is impossible to go faster than that"

Later systems to try to speed things up went to PHASE-shift keying, 
"it is impossible to go faster than that"
quadrature, etc.
"it is impossible to go faster than that"
even compression  ("56K" V.90,etc.)
each of which had its own theoretical speed limits, and 
"it is impossible to go faster than that"