Modems and external dialers.

[Grant Taylor]

On 6/5/19 12:01 PM, Fred Cisin via cctalk wrote:
> "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.

Yes.  That's generally what a modem is.  Data on one side and something 
else on the other side.  The other side could be audio like the handset 
of a phone, or it could be a phone line, or it could be coax for a cable 
modem.

Most of the modems that I've seen were data on one side and phone lines 
on the other side.  Conversely the modem that Ethan was talking about 
had data on one side and the audio portion on the other side.  The phone 
was required as an integral piece to convert the audio from the modem to 
the phone line required by the PSTN.  The modem would not work without 
the phone.  Where as most modems, the pone isn't involved and / or can 
be completely absent.

> 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"

Intriguing.



-- 
Grant. . . .
unix || die