I read through this article about the ongoing restoration work of an RCA SSB T-3 transmitter and found it interesting. The RCA T-3 transmitter is a 20 KW SSB/ISB HF (2-28 MHz) unit designed for point-to-point telephony service. Because SSB requires class A or AB low distortion amplifiers, this is a large unit, even for its age and power levels.
From the looks of the before pictures, this transmitter was in sorry shape.
Here is a brief video of the transmitter start-up:
These units were designed to be switched on and run at 100% duty cycle for most of their operating lives. That is some heavy iron there. This particular unit was made in 1959. More here and video part 2:
Anyway, before geosynchronous satellites, HF point-to-point transmitters were used to make long-distance phone call connections and send data and pictures back and forth over long distances. Out in Hicksville, Long Island, Press Wireless ran a data and fax system that used HF for long-haul data transmission. Much of the WWII reporting from Europe and the Pacific Theaters was carried over this system.
Text would be printed out on a mechanical teletype machine at something like 60 words per minute, which was considered fast for the time:
Tuning across the band, one can often hear Radio Teletype (RTTY or RATT) which uses a 5-bit Baudot code, 170 Hz shift with 2125 HZ representing a Mark or 1 bit and 2295 Hz representing a Space or 0 bit, which is a bit different from the Bell 103 modem specifications. This is what it sounds like at 75 Baud:
So slow you can almost copy that by hand.
The RCA H (SSB T-3) unit above was independent sideband (ISB), which means that either sideband or both could be modulated independently of the other, thus two channels of information could be transmitted. SSB bandwidth is about 2.7 KHz, which is good for telephone-grade audio or low-speed data.
I sort of wish I was living in California again, I’d lend a hand.