9 Feb
2025
9 Feb
'25
11:33 p.m.
On Mon, Feb 10, 2025, 12:14 AM Steve Lewis via cctalk <cctalk(a)classiccmp.org>
wrote:
Peter Corlett:
Yes and no. Winmodem lacked the smarts to generate or decode the signals
needed for v..33, etc. Instead, the modulation and demodulation was all in
software that then was played much like a sound card. Way cheaper to make,
but that took a lot of CPU. And the mod/demod ran only under Windows at
first. Not under DOS, Linux or FreeBSD, at least not until much later for
Linux...
Warner
These easy-to-ask questions have very long
answers. But mostly, it was
*not*
a case of merely increasing the baud rate or the number of bits per baud
with a different modulation scheme, but multiple concurrent advances
which
did those *plus* some other techniques which
would maintain signal
integrity
despite the reduced SNR.
Apologies for the very open ended question. I found some old 1992
newsgroups thread that guided me towards the answer (including a long
excerpt from "Computer Select CD-ROM" with lots of CCITT referencing).
Interesting on comparing 1992 perspectives and contemporary confusion,
versus today's modern references on the topic.
If I'm understanding it right, a "sort of" answer to my own question is:
2400 baud (v.22bis) was an "amplification" (not the right word, but
"phase
magic") of 600 baud. While as has been mentioned, 9600 baud (v.32) was a
similar "amplification" of 2400 baud.
One thing I didn't realize was v.32bis (14400) was a dynamic speed (it
could slow down and speed up based on line quality). I don't recall
experiencing that, as far as varied transfer rates.
Someone here mentioned "Winmodem" and wow, I recall using one of those - I
think it was maybe one of the last modems I used, at around 28,800 baud? I
never got to experience the 33.6K or "57.6K" stuff. Even back then I
recall the v.42 "compression" stuff being confusing (to me), since I
didn't
know what the dial-up ISP at the time would support. And if I recall
correctly, the "WinModem" was somehow tied or dependent on Windows OS (or
they only build drivers for Windows) and it started to hit me how "DOS is
dead."
Neat review.
-Steve
On Sun, Feb 9, 2025 at 6:18 PM Peter Corlett via cctalk <
cctalk(a)classiccmp.org> wrote:
screeches
which
On Sun, Feb 09, 2025 at 12:08:47PM -0600, Steve
Lewis via cctalk wrote:
[...]
> So.. If you had a slow system that couldn't really take advantage of a
> ~7MHz 16550 serial card (or I guess like a laptop that was stuck with
an
older
UART) That might be the use-case where this parallel v.fast might
help (by being able to "feed the modem" fast enough to actually take
advantage of the faster modem speed?) Or is there some other scenario
Note that all but the dumbest modems reclock the data before transmission
and the XON/XOFF or CTS/RTS flow control is handled locally, buffering in
the modem as necessary. At faster speeds there's no longer a 1:1
relationship between the signal level on the RS-232 cable and the
going down the phone line. Start and stop bits
are not transmitted,
giving
a
25% speed boost from that alone.
A parallel-connected modem is a bit pointless except in weird
environments
where one's serial port is broken or
otherwise unusable. Information
theory
tells us that you can't get more than 64kb/s
out of a dialup link,
because
that's the speed of the underlying digital
channel used by the PSTN. Due
to
the reclocking, you actually need a serial port
capable of 80kbaud to not
drop data, and the next-highest standard baud rate is 115,200 baud, which
any half-decent PC serial port can handle.
To pre-empt the obvious retorts from the peanut gallery, sure, one may
well
have an original IBM XT or BBC Micro or whatever
whose serial port drops
bytes when driven faster than 9600 baud. Guess what: the machine's so
slow
that it can't handle the firehose of data
even if its serial port wasn't
a
basket case.
I don't know where you get "~7MHz 16550 serial card" from. The 16550 (and
predecessors) sample the incoming serial signal with a clock 16x the baud
rate. For 115,200 baud, that's 1.8432MHz, and it is no coincidence that
this
is a standard crystal frequency and also the maximum speed of a lot of
UARTs.
[...]
> 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
These easy-to-ask questions have very long answers. But mostly, it was
*not*
a case of merely increasing the baud rate or the number of bits per baud
with a different modulation scheme, but multiple concurrent advances
did those *plus* some other techniques which
would maintain signal
integrity
despite the reduced SNR.
If you want the full gory detail, the relevant ITU standards are
freely-available. Bring a good signal-processing textbook.
It'd
(did any modem protocol go above 3KHz?).
V.90 used the full 4kHz analogue bandwidth for the downstream. Yes, even
the
frequency extremes which were heavily-attenuated by the line filters.
just listen much harder.