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SOUND ON FILM FOR TELEVISION 115
of noise and distortion products above 4 to 5 kilocycles degrades the sound quality more than it would be improved by reproducing the signal uniformly up to say 7 kilocycles. While this may be a reasonable assumption for many 16-mm films in circulation today, it does not allow for improvement as better recording and printing techniques are developed. It would be preferable to use a scanning system and amplifier which would provide essentially uniform response to at least 7 kilocycles and then use a suitable low-pass filter whose cutoff frequency could be adjusted depending on the noise and distortion on a particular record. This is similar to the practice used in broadcasting phonograph records. A test film such as Z22.44-16-mm Multifrequency Test Film should be used for determining reproducer response.
2. Scanning-Beam Focus
An out-of-focus condition reduces high-frequency response. Some reproducers focus in the center of the film to take care of both standard and nonstandard emulsion positions. This means neither position is quite right and may give a loss of about 3 decibels at 7 kilocycles depending on the optical system. Out-of-focus by thickness of film may cause a loss of 10 decibels at 7 kilocycles.
The projector should have quick change of focus between front and back surface of film. Focus should be checked periodically with Z22.42-16-Mm Sound Focusing Test Film, 7000 Cycle Laboratory Type.
3. Scanning-Beam Azimuth
Azimuth error causes loss at high frequencies similar to loss caused by slit width. It also introduces wave-form distortion in reproducingvariable-area sound records. For example, if azimuth is out by width of slit (0.5 mil), second harmonic of 3600 cycles would be about 12 per cent for area tracks. Azimuth should be checked with above Z22.42 Test Film.
4 Scanning-Beam Position
Improper position of scanning beam relative to sound track may cause noise on density films or both noise and distortion on area films. Scanning beam too close to the edge of the film causes random noise while beam too far from the edge may produce 24-cycle noise caused by picture-frame lines.