How fast?

One of the aspects of video that doesn't get talked about much is frame rate. So I will now proceed to talk about it - at length. If you work in the TV or film industry you may be aware of the delights of 3:2 pulldown, PAL vs NTSC incompatibilities and the delightful judder much sought-after by TV and video directors wanting "the film look". If you don't, you may be aware that film is shot at 24 frames per second and old-fashioned TV at 50 fields per second (in Europe), but that's probably it.

If you don't understand any of the aforementioned, let me just explain that "frame rate" is the term used to describe the rate at which the individual pictures that make up a video sequence are shown to the viewer. Pick a frame rate that's too slow, and you can't display motion accurately - things move jerkily around the screen, or blur unrecognisably, or appear as double images - the faster the motion, the bigger the problem.

One of the things we're looking into is what the frame rate for a hypothetical successor to HDTV should be. In order to understand why the figures in use today were chosen, I've been reading lots of documents from the early days of television and film - both from our library and from the web. For you, the lucky reader, I will summarise them here.

James Card wrote an article entitled "Silent Film Speed" in the October 1955 issue of "Image" that is available online here. In it, he gives the projection speeds for about thirty silent films made between 1916 and 1928. There is considerable variation: between eleven and fourteen minutes for a 1000-foot reel (400-foot for 16mm stock). This is due in part to the use of hand-cranked cameras, but also due to advice given to the projectionist to vary the speed in different sections of the film:

'Silent films were usually released with musical cue sheets supplied in many cases by the producing company itself. As early as 1916, Triangle published special instructions to the projectionist. Here are some samples: "The best effects in The Captive God will be had by timing the film to run from 13 to 13 1/2 minutes to the reel. The two big battle scenes... should be speeded up considerably. Following the sub-title 'The Alarm,' shoot it through fast."' - James Card

At 16 frames to the foot (for 35mm), that implies an estimated average frame rate of between 19 and 24 fps for the films of the period quoted.

In his "Projection Department" column in "The Moving Picture World" dated 02/12/1911, F R Richardson advises:

'Speed is of very very great importance and a comprehension of this fact is absolutely necessary to do really fine projection. The operator "renders" a film, if he is a real operator, exactly as does the musician render a piece of music, in that, within limits, the action of the scene being portrayed depends entirely on his judgment.'

and

'Watch the scene closely and by variation of speed bring out everything there is in it. No set rule applies. Only the application of brains to the matter of speed can properly render a film. I have often changed speed half a dozen times on one film of 1000 feet.'

With the development of sound-on-film processes in the 1920s by inventors such as Lee de Forest and Theodore Case, film speeds and hence frame rates standardised at the now ubiquitous 24 fps - an uneasy compromise between cost and audio quality. The subsequent exact adherence to this figure may be due to the limitations imposed by the more complex audio-capable projectors, but I suspect that desires to de-skill the role of the projectionist and mechanise (and electrify) the cranking of the camera may also have been issues. Provision of a uniform viewing experience - quality control - may also have been a priority. To avoid visible flicker (probably the most annoying of all the temporal aliasing artefacts), a double or treble-bladed shutter was used to display each image two or three times in quick succession.

The 30-line television system developed by Baird (and the BBC) in the late 1920s and early 1930s ran at 12.5fps. It may be worth noting that the system used an optomechanical scanning system whereby the "scene" was illuminated by a scanning beam of light from the camera, however. More versatile cameras were adopted relatively quickly, but not before Baird had experimented with an infra-red version that was less irritating to performers.

The Marconi-EMI television system (now known as "405-line", the old monochrome standard that the BBC continued broadcasting until as recently as 1986) was adopted by the BBC in 1937 after broadcast trials against an improved 240-line (progressive-scan!) Baird system. These systems were described contemporaneously as "high-definition television"... This and all subsequent TV systems have used a frame rate that is the same as the mains frequency (50Hz in Europe). The reasons given for this in the BBC's "Technical Manual, M-EMI System of Television, London Television Station", published in April 1939 are that it is to avoid "beating" against the 100Hz brightness fluctuation in AC-driven studio lights and the 50Hz fluctuation induced by poor ripple-suppression in the HT generation circuitry of early CRT televisions. In addition, a 50Hz refresh rate is about the minimum necessary to avoid visible flicker in the displayed image; a fact well-established by cinema at that time. At least on small screens of limited brightness - TV manufacturers in the 1980s started experimenting with 100Hz refresh rates (120Hz in 60Hz-mains countries) to reduce flicker on large, bright CRT televisions. (At the expense of accurate motion portrayal, in many cases, although Philips claim to have cracked that problem in the previously linked article.)

• Posted: 29/03/07 04:50PM
• Category: BBC

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