10 Feb
2025
10 Feb
'25
4:08 p.m.
On Feb 10, 2025, at 3:58 AM, Jim Brain via cctalk
<cctalk(a)classiccmp.org> wrote:
On 2/10/2025 1:14 AM, Steve Lewis via cctalk wrote:
Not 4 tones; 4 modulation states per signal element, that is what QPSK means.
The difference is that the 202 standard was designed to run half duplex over a standard
phone line, or full duplex if you had a 4 wire (leased line) circuit. It's a very
simple device that actually works at any speed up to 1200 bps (or a hair more, as PLATO
did). The 212 modem using QPSK is a clocked system, but it can carry 1200 bps full duplex
over a single phone line, with half the channel bandwidth used for one direction and half
for the other.
The 202 specification was used in early amateur radio packet radio systems, FSK over
shortwave radio links or AFSK (FSK modulated audio tones modulated onto an FM radio
channel) for VHF. That works nicely because amateur radio is normally half duplex, and
202 modems were readily available at the time or could be easily built by amateurs if
needed. The only additional work is receive clock recovery, because 202 modems aren't
clocked so for synchronous transmission (packet radio is HDLC) you need to recover the bit
clock.
paul
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.
Not sure if it's been linked, but I found a list of baud->bps mappings at
Wikipedia:
https://en.wikipedia.org/wiki/Modem
For those who are OK using that resource to answer questions. I found it interesting at
1200 bps had two options (1200baud * 2 tones or 600 baud * 4 tones)