Magazine article American Cinematographer

Editing Motion Picture Sound Tracks Using Ordinary ¼'' Magnetic Tape

Magazine article American Cinematographer

Editing Motion Picture Sound Tracks Using Ordinary ¼'' Magnetic Tape

Article excerpt

A suggested alternative to recording on full-coat magnetic film for 16mm and Super-8 when the budget must be given prime consideration

While the text that follows is primarily devoted to Super-8 film editing, the system can also be used for 16mm.

It is possible to move ¼'' magnetic tape tape back and forth synchronously with motion picture film on the editing bench in much the same way that one works with magnetic full-coat recording film.

The savings in cost of ¼'' tape and hardware as compared to magnetic recording film and hardware is obvious. Magnetic tape, which is much thinner than recording film, provides a more intimate contact with the record and play heads. This is particularly important in the case of Super-8 where tape recorders are being adapted by removing the tape guides and replacing them with Super-8 film guides. On more than one occasion I have had the unhappy experience of a Super-8 splice hanging up at the pinch wheel and capstan of the recorder during a sound transfer. This does not happen when using ¼'' magnetic tape-which is what a tape recorder was designed to use. The secret and heart of the system is to wrap the ¼'' tape with the film on the SAME REEL! The tape is wrapped with the film as the film is rewound onto the supply reel, as shown in ILLUSTRATION A.

In order to maintain perfect sync, a sync pulse is recorded on the tape as the tape is being wound up with the film. The pulses are equal to film perforations and are, in effect, magnetic sprocket holes. The tape sound is the same length as the picture film, plus the slight outer circumference of the tape wrap.

The sync pulse can be derived from two places; the perf edge of the film, or from the viewer screen, using a photocell. The sound reader comprises two tape heads, one for audio and the other for pulse. The pulse head is used both for recording and playing back the pulses.

After the tape is pulsed, the pulses are used to fire a small neon bulb that is placed next to the film perfs at the film viewer, as shown in ILLUSTRATION A.

A toggle switch is used to switch either of the two heads to the input of an amplifier. During rewind, the switch is thrown to the left, which activates the pulse head and also switches the amp output to the neon strobe circuit.

In practice, the strobe pattern drift is very slight and is easily controlled, using more or less back-tension on the tape, as needed to keep the tape in sync with the film during rewind.

The reel of tape, mounted on a torque motor and the track reader, is located immediately to the left of the left film wind. The tape, after passing over the heads of the track reader, is fed up to the film on the left wind. The variable voltage fed to the torque motor controls the back-tension needed for synchronization. A variable voltage transformer (variac) is used to provide a variable voltage to the torque motor for this purpose.

There are several configurations that the film and tape can take. First, and the least expensive, is to wrap the ¼'' tape with the picture film, but since sync can only be established during rewind, in this mode all syncing must be done tail first.

Unless a work print is being used, the abrasive action of the tape oxide against the film will be objectionable. The use of a liner between the tape oxide and film, such as Scotch 23-W, will stop the scratching, but unless the liner is used out of a bin, another torque motor must be used to hold the reel of liner.

The second configuration-and by far the best-requires a two or more gang synchronizer. …

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