AP mast (2K)

DIRECT TELEVISION from ALEXANDRA PALACE

by Arthur Dungate

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The AP Film Dubbing Suite





  * Tv films in cinemas
This did happen in the 1950s when BBC film documentaries were shown in Australian cinemas.     more

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.

35mm optical film characteristics (15K)

Fig 4: Frequency characteristics
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.

  Compression
In practice the level of compression used on the narrator's channel (commentary) was a ratio of 8dB into 2dB, and usually set to peak just under PPM 6. To eliminate low-frequency thumps from the compressor an 80Hz high-pass filter was inserted after it.

  *Non-sync recorder
This ¼ in. tape recorder was removed when magnetic film recording was introduced.




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
It will be possible to overcome the disadvantages of photographic sound recording for television film production due to the processing delays involved as soon as 35mm perforated magnetic film and the necessary equipment become available. In the meantime, in order to reap some of the advantages of magnetic sound recording, a non-synchronous magnetic recorder* operating in parallel with the photographic sound-on-film recorder has been installed. The advantages of having a machine of this type are as follows:

  1. It makes possible an immediate playback after a dubbing session for checking the programme content.

  2. Any errors, on the part of the commentator for example, can be checked to ascertain whether they can be eradicated by editing the final print, thus saving the time and cost of a ‘re-take’.

  3. Sections of the ‘take’ can be played back after the session to a typist for the production of file records.

  4. The engineer at the mixer desk can check levels and mixes immediately after the session, before releasing the ‘take’ for processing.

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.

  Magnetic Recording
When the facility of recording onto 35mm mgnetic film was first introduced, the tracks were recorded to RCA's characteristic. During the late 1950s it was decided to adopt the CCIR curve and film recording circuits and telecine replay circuits were modified. Until those at AP were modified, it was necessary to specify on the recording sheet that accompanied each film which curve it had been recorded to - in our case I would mark it "Old RCA curve".

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