Video Frame Rate – how a fortunate technological accident is on the verge of being ‘corrected’ (Part 2)


What could this possibly have to do with a music web site? Guest contributor Ray Purchase explains:

Early TV

The movie frame rate would have been a shoe-in for the earliest television systems except for two reasons. The first was that early television was prone to visible interference stemming from the mains power supply, which ran at 50Hz in the UK and 60Hz in the US. It was found that this interference was less objectionable if rendered stationary by making the frame rate some exact factor or multiple of the mains AC frequency. In the UK, a slight adjustment from 24 to 25 frames per second would have been perfect.

The second - and, to my mind, serendipitous and highly significant - reason was that in the early days of television the medium was limited by the physical characteristics of phosphor cathode ray tubes. Raster scanning (breaking the image down into a horizontal strips projected sequentially down the screen) didn't work at 25 frames per second because of objectionable flicker. That is, the image needed refreshing at a rate faster than 25 times per second. There were no digital memories in those days, so no option to send one frame and project it twice at the receiver.

Merely to avoid flicker the system would need to send twice as many frames per second, but this would be a heavy overhead in terms of bandwidth and, as cathode ray tubes were limited in the speed at which they could scan, the achievable vertical resolution at high frame rates was limited. The brilliant idea of interlacing was born. By sending 'odd lines' then 'even', the overall effect was a full resolution picture updated 50 times per second, but with only the same bandwidth requirements as transmitting it at 25 frames per second.

Stationary images appeared at full vertical resolution, while motion was broken up into lower resolution images, but the eye didn't notice. (It's analogous to modern lossy digital encoding.) Interlacing, though, is incidental to my argument; the main point is that the television frame rate was effectively doubled from what would have been considered adequate, and indeed desirable, by the industry.

Continued tomorrow: