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116 KEITH
With variable-area sound tracks, displacement in either direction will also cause severe distortion at high amplitudes because of sharp cutoff of wave peaks. Track position should be checked by means of Z22.57-16-Mm Buzz-Track Test Film.
5. Uniformity of Illumination
Nonuniformity of scanning-beam illumination has no effect on a variable-density track, but may introduce considerable distortion in variable-area tracks. Harmonic distortion may be shown to be quite low for any reasonable nonuniformity of illumination. For example, with a bilateral track and symmetrical (parabolic) nonuniformity 20 per cent brighter in the center than at the sides, the second harmonic is about 3 per cent and the third about x/2 per cent. However, intermodulation of high and low frequencies is much higher. With the same light distribution intermodulation would be 15 to 20 per cent.
In some machines distortion is also produced in area tracks by nonuniform sensitivity of the cathode of the phototube. This occurs where the optical system is such that the illuminated area of the cathode varies as the width of the track. Intermodulation as high as 60 per cent has been measured because of this effect. No standard test film is now available to measure intermodulation caused by scanning deficiencies, but such a film can be made without difficulty.
6. Flutter
One of the most noticeable defects in the reproduction of films over television at this time is flutter. The flutter frequently is very much greater than would be tolerated in even the smallest theater. While it is possible that the flutter may be in the film, it may also be in the 16mm reproducer. This need not be the case since it is possible to build machines in which the flutter is consistently below audible levels. Although methods of flutter measurement have not been finally standardized, methods and equipment are well known in the film industry. A flutter test film Z22.43 is available for this purpose.