AP mast (2K)


by Arthur Dungate


The AP Film Dubbing Suite

  Projection room
Two 35mm Ross projectors with carbon arcs. On the right hand wall are the Selsyn selection units.

Description of the Installation

Dubbing Section
Power Room. This contains the power-supply equipment for driving all the film-production apparatus and also the mains-voltage stabilisers and power units for the projection-room and recording-room amplifiers.

power room (22K)
Power room

Projection Room. The equipment here consists of an RCA type 230R Studio Sound Reproducing Equipment of British manufacture, modified to suit television conditions, and two Ross Projectors, Type GC1, equipped with Ross ‘Streemlite’ high-intensity arcs.

dubbing projection room (27K)
Dubbing projection room

Each projector has an RCA combined soundhead and preview attachment, by means of which combined or separate, standard or push-pull, sound tracks can be reproduced. Two additional RCA sound-on-film reproducers are installed for sound-track reproduction only. All the machines are driven by Selsyn motors, which can be grouped and connected to the power-room Selsyn distributors as required.

  Projection room
The projection room as seen from the 16mm projector position. On the right are the two separate 35mm film reproducers.

In the Review Theatre it was found in practice that the two separate 35mm film reproducers were not used there and they were moved to the dubbing projection room which from then on had a total of four separate mag-opt reproducers.

  Dubbing Theatre
Immediately under the screen is the RCA loudspeaker.

Later, red velvet curtains were added at the sides of the screen.

A roller blind could be lowered to cover the mixer window when the theatre was used independently for the viewing of films.

  Silent films
The facility of adding sound on transmission was in practice not used and so the tv monitors were removed.

dubbing projection room (24K)
Dubbing projection room

Thus it is possible to run one silent picture film with up to four separate sound tracks in synchronism, or alternatively to use the twin projectors for continuous projection. Each soundhead is connected directly to a head amplifier, which raises the level to -30 decibels, an arrangement designed to eliminate interference from the television transmitter.

The outputs of the head amplifiers can be switched either to the mixer room for dubbing operations or by way of the reproducing amplifiers in the projection room to the loudspeaker in the dubbing theatre for review work. During dubbing operations the input to the reproducing amplifiers and to the projection-room monitoring loudspeaker is automatically connected to the monitoring output of the mixer desk, and the loudspeaker in the theatre is disconnected, since the theatre is virtually being used as a studio. A third projection position is provided for a 16mm projector or alternatively for a special film-band projector carrying cues to assist in foreign language dubbing or post-synchronisation work. All the equipment has been mounted on special anti-vibration beds, sunk into the floor, in order to prevent noise transmission to other rooms in the suite.

Rewind Room. Four rewind positions are provided, three for 35mm and one for 16mm film, each with illuminated ground-glass screens for film inspection.

Theatre. The theatre is acoustically treated and insulated so that it can be used as a studio for recording or dubbing. A commentary table is equipped with a miscellany of microphone positions, lighting fittings, script racks, head-phone jacks, and cue lamps, which can be arranged to suit a variety of conditions.

dubbing theatre (18K)
Dubbing theatre seen from the mixer room

Additional microphone, headphone, and cue-lamp positions are available around the theatre for effect or post-synchronisation work with floor-stand microphones.

The commentators’ table is also provided with communication equipment to the projection room and a loudspeaker level control for use when films are being reviewed in the theatre. Television monitors are fitted in positions easily visible to the commentator and to the mixer and gramophone-turntable operators so that sound can be added to silent films during transmission.

  Mixer room
The RCA loudspeaker can be seen to the left of the clock.

Underneath and to the left of the desk are the four audio equalisers or Variable Correction Units (VCUs) of standard BBC design, with the jackfield situated below them.

On the right is one of the two TD/7 disc reproducers. A shelf was later added on top as temporary storage for the piles of discs which were used in a dubbing session.

The record shelves (not shown) contained BBC sound effects discs (78rpm). Music discs were brought in as required by the dubbing editors.

The non-sync tape recorder was not used and was removed when magnetic film recording was installed.

When the main lighting was dimmed down during a dubbing session, low-level red lamps were on. This could have an unfortunate side effect! more

  Recording room
The 35mm RCA film recorder was later modified by RCA and fitted with magnetic record and replay facilities. Behind the recorder is the window which gave visual access to the mixer room.

Above the window is a metal roller shutter to make the room light-tight for use as a darkroom for loading the undeveloped film stock.

When the Film Transfer Suite was built later in the 1950s, it had its own darkroom and so the shutters in here were not subsequently used.

Mixer Room. The equipment consists of a mixer desk with an equalisation cabinet alongside; a bank of four 78rpm gramophone turntable units; the non-synchronous magnetic tape recorder; three amplifier bays, and a twin-unit loudspeaker for monitoring.

mixer room (20K)
Mixer room

Record racks containing approximately 500 special effects and music records are installed alongside the turntable units.

The mixer desk is basically a standard BBC Type A studio control desk modified for film dubbing work. Eight channels can be mixed, namely, the four sound-head outputs from the projection room, the gramophone outputs, the commentators’ microphone, and two other channels which can be connected to additional microphones in the theatre or to any other source. Mixing is carried out at zero level* in order to avoid interference from the television transmission. Each channel has a key as well as a fader so that it can be cut in or out of circuit instantaneously.

Pressing the buttons on the mixer desk that set up the equipment for dubbing or transmission automatically illuminates red signal lights throughout the suite, disconnects the PBX telephones, and locks the doors to the theatre. For this purpose a special electric door lock has been designed which prevents entry to the theatre, but allows immediate exit without de-operating the lock, in order to conform with the regulations concerning fire exits.

Variable treble and bass equaliser units are fitted in the cabinet alongside the mixer desk. These equalisers are normally connected in the soundhead channels, but they can be plugged to any other channel. On the mixer desk is a peak volume indicator, which, with the monitoring loudspeaker, can be switched to the output from the desk or to the recording channel output. The indicator is connected to the output of the recording channel when dubbing is in progress, because in the photographic system of recording, in which an immediate playback is impossible, it is essential that the sound should be checked at the last possible point in the chain - in this case across the light-beam modulator on the recorder.

The lighting in the mixer room can be switched to the theatre dimming circuit — an essential arrangement during dubbing. The mixer desk and gramophone turntables are, however, provided with independent lights, which are used during dubbing. Any reflection of this lighting in the windows separating the dubbing theatre and the mixer room is prevented by the tilt at which the windows are set and by a non-reflecting surface on the mixer-room ceiling.

Recording Room. This is equipped with a basically standard RCA 35mm film recording channel arranged for standard or push-pull recording and fitted with magnetic film drive to eliminate flutter and wow.

recording room (15K)
Recording room

A feature of the installation is that the output frequency of the motor alternator in the power room can be remotely controlled from the recording room, the recording engineer being provided with a frequency meter of wide-scale and high accuracy to enable him to check the speed of all machines to within 0.1 percent. of 24 or 25 frames per second immediately before a ‘take’. A separate self-contained ‘floor’ mixer on wheels enables recording to be carried out direct from other points in Alexandra Palace, such as the television studios, without using the dubbing-suite mixer, which can thus be freed for other purposes.

Review Theatre
Projection Room. Two projectors are installed, similar to those in the dubbing section. These projectors are fitted with synchronous and Selsyn motors for the reasons given earlier in this article. The sound equipment also is similar to that fitted in the dubbing projection room, except that the two additional separate soundheads are omitted.

Theatre. This is used solely for film review and is equipped with a screen, a loudspeaker, and a producer’s table fitted with loudspeaker level control and communication equipment to the projection room.

plan of review theatre (12K)

Plan of Review Theatre

All clocks in BBC premises were Gents sync slaves controlled by pulses fed from a master unit, corrected at regular intervals with GTS (Greenwich Time Signal). At AP this master clock was upstairs in CAR.

  * Zero level
'Zero level' was 1 milliwatt in 600 ohms and was standard throughout the BBC. A tone at 'zero level' was shown at 4 on the PPM scale, 100% modulation being 6 (8dB above 4).

Future Developments