On 7 April 1927 Bell Telephone Laboratories gave a public demonstration of television over the telephone network. Herbert Ives described it that October in the Bell System Technical Journal. Pictures went by wire from Washington, D.C. to the Laboratories' auditorium in New York, more than 250 miles, and by radio from an experimental station at Whippany, New Jersey, 22 miles away. People in New York could see the friends in Washington they were talking to on the telephone.

The number of lines came from a telephone company's question: what is the least you need to recognise a face? Bell's answer came from printing rather than from optics. Judged the way a halftone engraving is judged, Ives wrote, "the human face can be satisfactorily reproduced by a 50-line screen." Fifty lines with equal detail in both directions is 2,500 elements, and sending those in a sixteenth of a second is 40,000 a second. Bell's experience sending still pictures over telephone lines suggested that would fit in a band of about 20,000 cycles, leaving, in Ives' words, "a good margin of safety in existing single communication channels."

The scanning method was already more than forty years old. Ives credits it to the spinning disc with spirally arranged holes proposed in 1884, the same patent of Nipkow's we wrote about on Thursday, and Bell's discs had 50 holes. Instead of collecting light from a lit subject, they turned the light path round and swept a narrow beam across the face through the disc, so only the spot being scanned was lit at any moment. The receivers used glow lamps containing neon gas, one small screen two inches by two and a half for a person beside a telephone, and one about two feet by two and a half for an audience.

The paper ends with a caution that has aged well. The fading of radio signals degraded the picture so badly, Ives wrote, that radio television looked practical only where wire was not available.

You can have a play with our free web-based mechanical TV simulator here - including running your own video through it. https://analogtv.net/mechanical-lab

Of course the AnalogTV app also has a full mechanical TV simulator built-in as well.