Teletext: the BBC stuffed a news magazine into TV's invisible spare lines

In 1971 John Adams, Philips' lead designer for video display units, proposed putting pages of text into the vertical blanking interval of the TV picture — the lines nothing displays. His spec already had 24 rows of 40 characters, page selection and sub-pages, and one hard constraint: it had to be cheap enough for a home set, which 1971 electronics made genuinely hard.

The alternatives were all worse. A viewdata service over phone lines (which the GPO launched as Prestel) needed a central computer and a connection per viewer; new data spectrum needed licences and towers. The BBC demoed its Ceefax system wired in 1972; the ITA's rival Oracle gave the world's first live broadcast teletext demonstration from Crystal Palace in April 1973; Ceefax went on air on 23 September 1974 with thirty pages and an editorial team of nine.

The two services were born incompatible — Ceefax used 24×32 pages, Oracle 22×40 — but instead of a format war they agreed a single 24×40 display standard in 1974, before either had a commercial audience. The joint Broadcast Teletext Specification followed in September 1976 from the BBC, the IBA and the manufacturers' association, and became the World System Teletext used worldwide.

The TV signal already carries invisible spare lines

The spectrum was already paid for: spare lines ride inside a signal every transmitter was already sending.

One-way broadcast scales perfectly — the millionth viewer costs the same as the first.

Cheap memory in the set turned a slow data rate into page recall that felt instantaneous.

The 1974 truce gave TV makers a single decoder spec years before teletext sets became normal.

Look for capacity that exists and is being thrown away: the cheapest new product rides infrastructure the incumbent already pays for, and a format truce before launch beats a standards war.

Decoder sets cost about £1,000 in 1976 and only a few thousand existed by 1977; after a big promotional push in 1981 there were two million teletext sets by 1982, and by the mid-1980s the decoder was an option on nearly every European TV. Subtitling switched on by the viewer followed (the Queen's Christmas speech was subtitled from 1979), Prestel borrowed the 24×40 screen, and Philips built millions of teletext chips on Adams' design. Ceefax itself broadcast until 23 October 2012 — thirty-eight years — ending with analogue TV.