> On Feb 9, 2025, at 1:08 PM, Steve Lewis via cctalk <[email protected]>
> wrote:
>
> I was about to ask if anyone ever built a "Parallel Modem" - but I searched
> around first, and lo and behold, Microcom did ! (v.fast / v.34 era, c.
> 1996)
I don't know what "parallel modem" would mean. Can you explain?
> ...Related but different question:
>
> Is there any "natural rate" (Hz) of a modem? Meaning is 1200/2400
> baud-equivalent modem an accelerated-by-enhanced-encoding version of 300
> bps? and 9600 likewise an accelerated-by-encoding version of 2400? is
> 300bps itself some kind of special accelerated-by-encoding? I see 1200
> baud was also still sub 3KHz (did any modem protocol go above 3KHz?).
>
> Or maybe I need to ask it this way: did 300 baud modems use a more 1:1
> translation of the data-word bits into Hz signal over the modem (giving a
> more "natural" translation rate?) But then beyond that speed, does a modem
> need to "cache" a few bytes and determine some encoding scheme to then give
> modems an apparent speed boost? (is that "kind-of" like USB's 8B/10B?
> (not in implementation, but in the general concept that a different
> encoding can result in improved data throughput, without actual faster
> movement of that data?)
For the most part the answer is "no".
The job of a modem is to carry a digital signal over a wire, at a given speed
and given level of data integrity, and with a given channel bandwidth.
When the channel bandwidth in Hz is well above the bit rate in bps, the job is
easy, an FSK modem can do the job. That's what the first modems looked like
(and perhaps even earlier devices used to deal with radio transmission for
Baudot teleprinters, commonly referred to as "tuning units").
When data rates go up and bandwidth doesn't, you need more complex modulation
schemes. Modulating a carrier produces sidebands, so roughly speaking your
baud rate can't exceed half the channel bandwidth. (I'm sure I'm handwaving a
lot here.) You can't do 9600 bps FSK in a voice channel, it won't fit. It
would fit just fine if you have a 40 kHz channel, so it's certainly possible to
do FSK over VHF radio at that speed if you're authorized that much bandwidth.
So for high speed on a telephone line the exercise becomes "more bits per baud"
-- not one bit per signal element as you get from FSK or PSK, but two (QPSK) or
4 (QAM16) or even 8 (QAM256). Note that those hairy modulation schemes require
a pretty clean channel; you're not likely to find them on shortwave radio
systems for that reason. Indeed, doing data transmission over radio is an
entirely separate art with a host of interesting and exotic methods. Some of
them can reliably send data using transmission weak enough you can't hear them
if you listen to the signal with an ordinary audio receiver.
paul
