5 Jun
2019
5 Jun
'19
11:01 a.m.
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"