9 Feb
2025
9 Feb
'25
11:35 a.m.
On Feb 9, 2025, at 1:08 PM, Steve Lewis via cctalk
<cctalk(a)classiccmp.org> 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