AP mast (2K)

DIRECT TELEVISION from ALEXANDRA PALACE

by Arthur Dungate

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The AP Film Dubbing Suite






  RCA soundtrack
This was a single bilateral "variable area" track (actually variable width) with noise reduction effected by shutters which masked down the track area when the sound level was low.

RCA soundtrack (4K)


Projector Illuminant

A careful investigation into the most suitable form of projector illuminant for small theatres of this type showed that the medium high-intensity arc is still superior to other light sources. This result is interesting, since it would appear at first sight to be an unnecessarily complicated and uneconomical light source for illuminating a screen measuring only 6 feet across. The advantages of the arc are as follows:

  1. Its colour range is complete.

  2. The high-intensity light that it produces gives a projected picture of good contrast with excellent high-light detail. The high lights are a good hard white with no tendency towards yellow.

  3. Reliability and economy in running costs are unequalled.

In the dubbing theatre a screen illumination of 16 foot candles is obtained. This illumination is slightly in excess of the optimum value photographically, but is used in order to provide a well-graded picture with the high ]evel of stray illumination from desk lamps which obtains during a dubbing session. A plain white duck screen is used in preference to a perforated screen, because the perforations are visible on screens below 6 feet in width. The loudspeaker is mounted beneath the screen.

Recording System
The standard RCA variable-area system is used, and as details of this system have been published elsewhere*, a description is not included here. Though all the film production equipment at Alexandra Palace is capable of recording and reproducing standard or push-pull sound tracks, the standard system is being adhered to at present in order to reduce the possibility of error resulting from the presence of non-standard tracks on BBC films among the standard tracks used on all commercial release prints.

Accurate delineation of the sound modulation envelope on the final print is essential if good high-frequency quality is to be achieved. Under normal photographic developing conditions it is difficult to achieve accurate delineation, and it is now standard practice with variable area recording to arrange the exposure, developing, and printing conditions so that a heavy image spread is obtained on the negative, and to cancel this spread by an equally heavy image spread on the print. The method used with RCA equipment to ascertain the processing conditions that will ensure accurate cancellation may be of interest to those unfamiliar with the technique.

A special oscillator provides a 9,000-c/s tone, which is modulated from zero to 80 per cent, by a 400-c/s tone. A number of recordings are made, using this signal, with varying exposure times produced by varying the current in the recorder lamp. The complete series is developed in the normal manner to produce a number of negatives of varying densities. Each of these negatives is printed with different printer-light values so that a family of prints is obtained. Clearly, the optimum conditions will obtain when an audible 400-c/s note produced by image spread on the negative is cancelled out by the positive image spread. To discover the optimum conditions, all the prints are reproduced through a 400-c/s band-pass filter, which eliminates the 9,000-c/s note, and the print which has the minimum 400-c/s content will provide the best negative and positive densities for the emulsion used and the development conditions. The recording channel at Alexandra Palace is equipped with an oscillator for this check, and both the dubbing and review theatre reproduction chains are equipped with a 400-c/s band-pass filter which can be switched in circuit when required for this cross-modulation check.



  * RCA system
The system is described in Elements of Sound Recording by Frayne and Wolfe (Chapman and Hall, London).

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