1200/75 was very common in the UK. The whole of the BT Prestel network used it. (And Micronet 800 which was run under the Prestel system if anyone remembers that too!)
-Gordon
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Likewise in Aus, especially with the venerable AM7910 World Modem chip. I think I still have one somewhere in my pile in what I call “the junkbox”.
According to Waldrop, the initial speed on the Arpanet was 50 kilobits, but he says in the same sentence, “(later upped to 56 kilobits).” Now, he doesn’t say how much later that was done. I had the thought that the history told about it is that the internet came as an outgrowth of the Arpanet. So, maybe he meant when the internet came online in the early 1980s. From what I read about the internet, it was certainly 56K then, but the increase in speed may have come earlier. I don’t know.
From the linked spec of the 303 modems:
Three speed categories are available with this equipment. The highest speed capability is over a “Supergroup” channel which uses the bandwidth of 60 voice circuits, and is a convenient breakdown of bandwidth of telephone carrier systems. On a supergroup facility, a synchronous speed of 2.30.4 kilobits per second is available. The next lower convenient breakdown is the “group” channel which uses the bandwidth of 12 voice circuits. The 303-type equipment can transmit at a synchronous speed of 50 kilobits per second over group facilities.
It’s quite interesting to me how all this works: up to about 33kbit/s with modern signal processing we can use a single line with a conventional modem. When we, in our domestic lives, started to see higher speeds up to 56kbit/s it was made possible by having the other end at the local exchange, and the two ends absolutely synchronous with the digital sampling at the exchange ends.
Whereas, back in the day, that kind of signal processing was probably well out of reach, and end-to-end connections were sought, rather than end-to-exchange.
So, grouping voice lines was the way to go: at presumably somewhat high prices!
It would be interesting to know what the technical change was which took the Arpanet from 50 to 56 - just possibly again it was a change from end-to-end audio, to end-to-exchange.