The PicturePhone was connected to the Central Office via 3 standard wire pairs (for comparison, a regular telephone line uses a single wire pair). One pair carried the 1 MHz PicturePhone video signal in one direction, the other carried video the in the opposite direction. These had to be specially equalized to carry the signal but were still otherwise just ordinary telephone wire. The third pair carried the normal 2-way voice call plus carried the TouchTone dialing to set up the call.
A PicturePhone Central Office had a second switch (an electromechanical "crossbar" switch in those days!) operating in parallel to the regular switch for those making PicturePhone calls. That takes care of local calls.
The fun comes when a call has to be connected to someone served via another Central Office. Telephone calls are multiplexed together to go from one office to another. Many calls share the same communications system whether microwave, coaxial cable, or (nowadays) fiber optic is the medium. Even when offices are connected via ordinary wire pairs many calls share the same wires. A voice channel is allotted only 3000 Hz (this is after all telephony, not 20,000 Hz high-fidelity audio!) for each direction. But a PicturePhone video signal takes 1,000,000 Hz. That's 333 times the bandwidth! A few video calls would fill up all available bandwidth. How could they possibly have handled it? Necessity is the mother of invention so in all likelyhood they would have invented video data compression in a big hurry! After all, Bell Labs was working on digital audio as far back as WWII. (You should see the electron beam CODEC tube of 1947!)
Picture of one showing internal parts
I'm looking for a PicturePhone unit as a collector's item. If you know of anyone who has one please email me. Obviously it need not work!
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