The Basic IdeaIf one points a cine camera at a TV screen an image will be recorded, but a black bar will run up and down the picture due to the lack of synchronisation between the CRT (cathode ray tube) scanning lines and the camera shutter. This is well demonstrated by the uneven exposure on off-screen footage of Doctor Who taken in the late 1960s by an Australian fan.
Kits were available for
electronics buffs in the 1960s to modify 8mm cine cameras to lock the
shutter rate to the same frequency as a TV set, with a motor phase
adjustment to move the "black bar" out of the visible area; it is
uncertain whether examples exist in private hands. Some years earlier,
primitive recording techniques at the BBC used the same principle.
However, further problems existed which needed to be solved to give good
quality film recordings. These involved electronic, mechanical, optical
and photographic aspects. These can be summarised as
1 The Pulldown Problem
2 Synchronising the Camera and TV signal
3 Maintaining Good Gradation
4 Maintaining Good Resolution
5 Photographing an Interlaced Image
6 Movement Rendering
7 Grain and Noise
8 The Incoming Image
1 The Pulldown ProblemThis was always the major problem in recording a TV image. In a normal cinema film, half of the 1/24th second frame period is used for exposure and half to pull down, settle and register the film in time to record the next image. Thus, around 20 milliseconds (msec) is available for pulldown.
Suppressed FieldIn this system, used during the 1950s and early 1960s, of the 50 fields per second in the original image, only alternate fields were recorded. The picture thus consisted of a maximum of 188.5 lines. 20msec was available for film pulldown and this simple arrangement gave surprisingly good results.
Note the obvious line structure on this section of a BBC Continuity caption card from the mid-1950s, accentuated by the suppressed field recording method.
Stored FieldHere the camera lens was shuttered during alternate fields, giving 20msec for pulldown, but the lines written on the screen during the shuttered period were preserved as an afterglow in specially produced phosphor which retained brightness, so the two fields were on screen simultaneously. The "stored" field had to be pre-emphasised electronically so that, when it had decayed in brightness it matched the "live" field. Of course the situation was more complex, as the amount of pre-emphasis needed varied depending on the position of a line on the screen (the first line of the stored field was photographed after 20msec of decay, the bottom-most line after only 1.4msec). The same principles applied to lines in the "live" field. Thus, contrast and gain modulation of the incoming signal had to be varied continuously through a 2 field period. This modulation was applied to the CRT's vision amplifier as a voltage, dependent on the phase of the camera shutter mechanism.
Partial Stored FieldHere a faster pull-down time of approximately 5msec was exploited (at the time, the fastest 35mm cameras could achieve), which meant that only 1/5 of an active field was displayed when the camera shutter was closed. Again, brightness and contrast had to be modulated to give equal exposure from different screen areas, but as most of both fields were exposed "live", this was able to be achieved with a fixed neutral density filter over a portion of the display monitor.
Quick Pull-DownHere, with this Marconi-developed 16mm camera, the pull-down time approached 1.4msec so no pre-emphasis or phosphor storage was required. More accurately, pull-down time was actually 2msec, so 12 lines per complete picture were sacrificed, ideally 3 lines from the top and bottom of each field. A disadvantage of the high-speed pull-down camera was that the braking mechanism could scratch the film emulsion, causing a gradual build-up of emusion which could further scratch, or cause unsteadiness in the film (especially with long running times of up to 60 minutes with 16mm film). By the mid 1970s, film recording apparatus was available for colour or monochrome recordings which could pull down in around 1.6msec.
2 Synchronisation of Camera and ShutterWith suppressed field recording, shuttering and pull-down had to take place during the field suppression period. If not, a thin, light grey horizontal line (due to a duble exposure) or a dark band (due to under-exposure) would be visible. Also movement rendering would be affected if some lines were recorded from field A (upper, or dominant) and some from field B (lower or non-dominant).
3 GradationThe TV signal from a film recording was passed along a chain, from camera tube, via control unit, (sometimes vision mixer and possibly Quad VTR) to non-linear amplifier, to CRT, thence to film negative, optical printer, positive film and finally, telecine output. Any part of this chain introduced the potential for picture degradation. The contrast of video differs from film and different settings would be used depending on whether a print was destined to be telecined for transmission or shown optically in a theatre (for example, to servicemen abroad). Prints produced for projection tend to look poor on telecine, and vice versa; often "compromise" prints would be produced for maximum flexibility (see the examples below left, of low-contrast print destined for telecine and right, of high-contrast viewing print).
4 ResolutionResolution, or sharpness, could also be affected at any of the signal chain. The first loss tended to occur at the phosphor of the CRT, where the beam could spread to adjacent phosphor (especially at the edge of highlights) and resulting flare could remove clarity from the picture - rather like looking through smeared spectacles or a diffusing filter. Horizontal resolution could be corrected to an extent by boosting the high frequencies of the signal before display, but vertical loss of resolution was a problem until the late 1960s. Of course, suppressed field recording involved the most major loss of vertical resolution (50%)! Further loss tended to occur during film printing, especially if done in a continuous motion printer, where any slight film slippage would cause vertical smearing and loss of resolution.
5 The Interlaced ImageIn good CRT displays, the diameter of the scanning spot is less than the gap between adjacent lines, i.e. there is a strip of black between each line. If these lines were recorded on film, during projection the image would be reasonable. However, if the print were to be telecined, the reading scan lines would not exactly match the written lines and moire patterning would occur. This can be compounded on telecine transfer as (in the 60s) the film print would be underscanned by 3% in the fast pull-down system, and 377 lines would be created from 365 on the film. Nowadays, in the PAL system, 572 lines read from 365. To reduce the visibility of individual lines, spot wobble was developed. A high frequency (20MHz) alternating field was superimposed on the CRT spot beam, causing a small vertical strip of phosphor to be illuminated rather than a thin line. Hence, there should be no black between adjacent lines. Spot wobble was also used in the telecine transfer of film recordings, reducing further line structure but, again, at the expense of vertical resolution. Modern telecine apparatus does not have spot wobble facility, which can make replay of archive material difficult. However, the spot astigmatism controls can be adjusted to defocus the spot in the vertical direction to achieve a satisfactory result.
Note the very obvious moire patterning on this film recording which has been transferred on telecine apparatus without due attention to the elimination of scan lines.
6 Movement RenderingNormally, with cinema film, each 1/24th second frame is displayed twice (to reduce visible flicker). To achieve this on TV each 1/50th sec field is displayed sequentially. When committed to a film recording, subsequent fields become "locked" together and, in reality, cannot be recovered independently (an "ideal" film recording reproduced on an "ideal" telecine could do this and the original smooth video-like image would be obtained. This theoretical possibility is, in fact, effectively impossible). In video, with 50 discrete images per second, movement rendering is twice as smooth as with film (including film recordings). This is what gives film and video different "looks" and the recent "filmising" technique (or at least the simplest variant of it) mimics the look of film by discarding alternate fields (thus halving vertical resolution akin to suppressed field recordings - "what goes around comes around"!!) and using higher than normal contrast lighting.
7 Grain and NoiseThree types of grain and noise can be apparent on film recordings.
8 Incoming FaultsGarbage in, garbage out. Obviously any defects in the incoming signal, whether of brightness or contrast, noise or geometry, would be recorded. For example, vertical lines could slant; line velocity errors (usually due to incorrect adjustment of tape guide height during quad playback) manifest as zig-zag distortion especially noticeable on verticals; tape dropout could be recorded; excessive underscan would give a zoomed-in picture with loss of picture area and reduced resolution.
Further DevelopmentsDuring the 1970s the techniques were refined further and modified to allow colour film recordings to be made. Independent TV companies and facilities companies used this more than the BBC, which increasingly used videotape with analogue (and subsequently digital) standards conversion for foreign sales by the mid 1970s. One interesting system, called Vidtronics colour separation, used a 35mm fast pulldown camera developed by Moy and the BBC where the red, green and blue components were each recorded on monochrome film in different passes, then subsequently combined in printing to make a colour internegative or positive print. Other systems used the same principle as partial stored field recordings, but with a special shutter to compensate for afterglow differences from a high grade colour monitor. A system called Trinoscope (or videoprinting) used CRT projectors for R, G and B combined into a fast pulldown camera via semi-silvered (or dichroic) mirrors. Direct electron beam recording onto film was developed in the mid 1970s, giving excellent results, and with digital frame store capabilities came the ability to fully interlace an image which could be recorded by a fairly standard camera (in effect a digital stored field system). Further discussion of these systems is beyond the scope of this site.
Various sources have been used in the preparation of this page. Particularly of note are the following: "The Technical Problems of Television Film Recording" by A.B.Palmer, Journal of SMPTE, Vol 74, Dec 1965, "Television's Story & Challenge" by Derek Horton, Harrap & Co Ltd, 1951, "Practical Television" by T.J.Morgan, Ward Lock & Co Ltd, 1959, "Recording Colour Television on Film" by Angus Robertson, Video & Audio-Visual Review, Vol 2 No 5, May 1976. Thanks to Steve Roberts for invaluable help.