Why did 405 look so good?

[Geordie McBoyne]

In the early days of 405 line TV a maximum video bandwidth of 2.7Mc/s was the ideal figure. The bandwidth figure was arrived at from an aspect ratio of 5:4 and an active video of 17/20 of the total line period, about 83microseconds. When the aspect ratio was changed to 4:3 more bandwidth was required so the figure is nearer 3Mc/s. Since the mid sixties 405 line television was sourced from a converted 625 waveform and to accommodate the characteristics of the 625 system the active video of a 405 line signal was reduced to about 80 to 81 Mc/s. Again slightly more video bandwidth was called for. The BBC digital 625 to 405 converter has a output video bandwidth of nearly 3.1Mc/s.

Geordie McBoyne.