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Recording and Reproducing Characteristics Prints released by commercial film studios have a sound frequency characteristic that is designed, not unnaturally, to give the best results in large auditoria having widely diffcrent acoustical properties, at a reproduction level considerably above the level of the original sound. Films of this type when reproduced in a small room at low level, as they usually are by a television receiver, sound deficient in bass and somewhat ‘hard’ in character. Hence the question arises, what frequency characteristic should be used when films are specially produced for television? The highest fidelity would be attained by adopting a characteristic that is best suited to home reproduction, but there are two arguments against this course. First, there is no guarantee that films produced by a television organisation will not be shown in cinemas at some future date*; and secondly, the introduction of a non-standard recording characteristic would lead to complication in the operation of reproducing equipment at a television centre, since commercial films are used as well as films made specifically for television. Accordingly, all BBC films are recorded with a frequency characteristic that conforms as closely as possible to the standard commercial recording characteristic shown in Fig. 4, but equalisation to suit the home listener is introduced on all reproducers. The degree of equalisation found acceptable is shown in Fig. 4.
A = Speech equaliser; B = Standard commercial recording characteristic; C = 'Home Listener' equalisation; F = Film-loss equaliser; G = Film-loss decompensation Though the results are a compromise, high-quality sound at low level is obtained from BBC film recordings made to the characteristic shown in Fig. 4 and reproduced through the equaliser; and, furthermore, the reproduction at low level of most commercial recordings is considerably improved. |
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An interesting point is that it has been found beneficial to retain most of the upper-frequency lift introduced into commercial sound-on-film recordings over and above the amount required to compensate for normal film losses. This lift is introduced in order to attain a high degree of intelligibility in the cinema irrespective of whether the quality of the sound is natural. Speech in a television studio production also has a rising characteristic in the treble owing to the acoustic effect of the solid sets that are used, and it so happens that the two characteristics match each other very closely, thus preventing a disturbing change in speech quality when switching from a live transmission to a film production. Although the frequency characteristic of BBC television films conforms to commercial standards, they are balanced for low-level reproduction because a compromise in this respect appears difficult, and good results in the home must be the first consideration. Volume compression is used on all commentary work. It is essential for theatre reproduction, and has, moreover, been found to be an improvement even for low-level reproduction in the home. The compression starts 10 decibels below 100 per cent modulation, and a compression ratio of 20 decibels into 10 decibels is applied above this level. Limitation is also used in the recording chain to prevent overload and to assist the sound-mixer in maintaining a consistent peak programme volume.
Magnetic Recording
This machine cannot be used for quality checking; nor can the recording be reproduced with the picture, since the tape is unperforated and therefore non-synchronous. |
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