> On Feb 10, 2025, at 3:58 AM, Jim Brain via cctalk <cctalk@classiccmp.org>
> wrote:
>
> On 2/10/2025 1:14 AM, Steve Lewis via cctalk wrote:
>> 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)
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
