Frames are fundamental to the operation of video – there are no moving pictures in television, just individual frames that are played back with sufficient speed to give the perception of motion.
The frequency at which frames are played is fixed. In the UK and Europe the playback rate is 25 frames per second, and in the USA and countries using the NTSC system it’s 29.97 frames per second.
Video has its roots in film, and in the early days of the silver screen, researchers discovered that 24 frames per second was the slowest a film could be played whilst maintaining fluidity of motion.
Engineers discovered that playing a film back at 24 frames per second caused a visually unacceptable flicker, and to fix this they flashed the projection bulb twice for every frame displayed to increase the rate to 48 frames per second. The result was fluidity of motion without flicker.
Television engineers needed to replicate the film system as playing video at 25 or 29.97 frames per second also caused a similar unacceptable flicker. But doubling the frame rate would have doubled the frequency bandwidth required, resulting in fewer channels being broadcast, and the electronics at the time could not easily work at the higher frequencies.
Instead of doubling the frame rate, television engineers invented interlace – the number of frames is doubled, but each frame uses half the number of lines. A video line is similar to a row of pixels and is described in a later article. Frames are paired into two fields, field one represents the odd number lines, and field two represents the even numbered lines.
When fields are played back in a television set they are interleaved together, and by averaging over two fields we see one complete frame. Consequently, the field rate of UK and Europe is 50 fields per second and 25 frames per second. And the field rate of the USA is 59.94 fields per second, or 29.97 frames per second.
The field rates were originally chosen so camera scanning systems could be synchronized to A.C electrical frequencies. Before color was invented, USA used 60 fields per second and 30 frames per second. The advent of color caused an interference which manifested itself as flicker, a problem that is described in a later article. Without synchronizing the camera’s to the electrical A.C frequency, a strobing interference between the studio lights and video output of the camera could be seen.
Modern formats use a system called progressive, represented as “P”, such as “1080P” or “720P”, this removes interlace by doubling the frame rate. A USA system that broadcast at 59.97 fields per second, or 29.94 frames per second, could now transmit at 59.97 frames per second. This has shown to improve the fluidity of motion even more and there is no perceivable flicker. Higher frame rates are being introduced which improve fluidity of motion even more.
Compression and digital transmission allows much higher frame rates to be used than in the old analogue days of terrestrial broadcast.
The 1930’s was the birth of television and a great deal of research was undertaken during this time. As broadcasting needs to be forever backwards compatible, the decisions made at this time are still with us now.