Here are some of the most frequently asked questions we receive about sync sound filmmaking together with their answers in, what we hope you will find to be, easy to understand language.
What is meant by "60hz. sync" or "pilotone sync" ?
What is "crystal sync" and how does it work ?
What is a "resolver" and what is meant by "resolving" ?
What is a magnetic film recorder ?
What is "slating" ?
What is the difference between recording sound for transfer to magnetic film as opposed to sound for transferring to video ?
Can I use a stereo cassette recorder to record lip sync sound ?
Can I use a DAT recorder to record lip sync sound ?
If I am shooting at 30fps for video transfer, how will this affect the sync sound recorder?
How can I tell if my camera is running at 24fps?
Q. What is meant by "60hz. sync" or "pilotone sync"?
A. The term 60hz. sync is a method of producing lip-synchronized motion pictures. In the early days of double-system sound recording, both the motion picture camera and the magnetic film recorder (see "What is a magnetic film recorder") were driven by "synchronous" AC motors. In North America we use an electrical frequency of 60hz. and a voltage of around 120vac. A synchronous motor is designed in such a way that it's rotational speed is in direct relation to the frequency of the AC current powering it rather than the voltage level of this current.
So, even though the voltage might vary up or down, a synchronous motor would always run at a constant speed regardless of these fluctuations. Since both the film camera and film recorder were equipped with synchronous motors, they would both always run at the same speed (i.e. 24fps @60hz.). Thus, early picture and sound synchronization was achieved by the ability of the synchronous motor to maintain an accurate speed.
This was all well and good as long as you remained in the studio and had a source of AC current to plug everything into. When it became necessary to take the camera out of the studio and away from any source of AC current a method had to be devised that remained compatible with the equipment already in wide use throughout the industry. Early attempts at so called "portable" double-system sync had the camera and recorder tied together by a cable. What would become known as cable sync was accomplished by deriving a signal from the camera to indicate the exact speed at which it was running from moment to moment. Therefore, when the camera was running at exactly 24fps, a small electrical generator fitted to the camera's drive motor would produce a signal of exactly 60hz. (again the 24/60 relationship!) This sync signal was then sent by a cable to the battery operated tape recorder where it was recorded on a separate channel of the tape as a sync track. Cameras now had to only have a motor that ran relatively constant (i.e. constant speed motors) since any speed fluctuations between the camera and the recorder would be reproducible when the sync track on the tape was played back, later. (see "What is a resolver...").
The term Pilotone was originally a trade name for this process and is now more or less synonymous with the phrase 60hz. sync.
Q. What is "crystal sync" and how does it work ?
A. Crystal sync is another method for producing lip synchronized motion pictures. The main difference between this method and cable sync (see above) is the elimination of the umbilical cord between the camera and tape recorder.
In crystal controlled sync systems both the camera and recorder are electronically referenced to their own independent crystal oscillators. A crystal oscillator produces a very precise output which can be calibrated to a high degree of accuracy. The speed of the camera is locked to exactly 24fps. by a controlling circuit which has a crystal oscillator as a reference. The crystal at the recorder only has to produce a precise signal from its crystal oscillator, it does not actually control the speed recorder (this is a bit different with DAT recorders). The final output from this crystal sync generator is a signal having a frequency of exactly 60hz. This sync signal is then recorded on the tape as a sync track, just as in the cable sync system we described, above. Since there are no interconnecting cables to contend with the cameraperson is free to move about without having to drag the soundperson as well. Also, because of the crystal accuracy involved, any number of crystal cameras can be covering the action and the film from each freely intercut in complete synchronization with the sound track.
Q. What is a "resolver" and what is meant by "resolving"?
A. Resolving is the process used to obtain lip-synchronization between two different mediums. A resolver is an electronic device that matches the speed of the sound recorder it is connected to (usually called the slave) to some other reference (known as the master). The most common use for the resolver is transferring audio recorded in the field to an editable medium such as magnetic film or video tape. Without the resolver it would be impossible to provide for an accurate, frame-for-frame relation between two dissimilar devices since their running speeds would be completely independent of each other. The resolver can overcome this problem by comparing the frequency of the 60hz. signal from the sync track recorded on the tape to the master frequency it has been set to. If the resolver detects a difference between these two frequencies it will create an error signal equal to this difference. The error signal is returned to the slave recorder causing a speed correction to take place until the two sync frequencies match each other exactly.
Q. What is a magnetic film recorder ?
A. Magnetic film recorders or mag film recorders (or fullcoat recorders) are basically audio tape recorders that record on sprocketed magnetic film. This magnetic film has the identical physical properties as motion picture film and comes in sizes of Super-8mm, 16mm and 35mm. The difference is there is no image. The magnetic film is either completely coated with magnetic oxide or has a clear film base with one or more magnetic stripes running down it's length. If the film project is to be edited as a film (as opposed to being transferred to and edited on video), the field audio tapes will need to be resolved to magnetic film. At that point the sound can be easily edited frame for frame with the picture using a conventional horizontal editing bench or flatbed table.
Earlier mag film recorders used synchronous motors to lock their speed to 24fps referenced to the 60hz. AC line frequency. Current models use internal servo systems to control the speed of their motors and have more flexibility in terms of what they can be referenced to, be it the AC line, an internal crystal oscillator or an external pilotone signal from the field audio tapes.
Q. What is "slating"?
A. Slating is the process of providing positive identification marks for the start of a lip-sync take on both the picture film and the sound track tape. These markings are very important in the editing stage to greatly simplify the process of locating the exact sync position between picture and sound. There are various methods for creating these markings although the original "clapboard" technique is the simplest and is still quite common. Electronic slating devices are now often used especially in documentary film production where the clapboard would prove impractical. These include everything from the low-cost blinking LED versions to devices with digital readouts for take numbers to the more complex and more costly time code systems.
Q. What is the difference between recording sound for transfer to video and sound for transfer to magnetic film?
A. As far as the recording process, there is no difference. It is in the resolving process that you must be careful to select the correct master sync frequency! If you shoot film at 24fps and have it transferred to video on Rank, the film will actually be transferred at 23.976fps. Therefore, to maintain the correct sync relationship, the field audio cassettes must be transferred against a master clock frequency of 59.94hz. This is easily accomplished with a resolver by simply connecting a video reference to the external sync input. The resolver will substitute the external video reference at 59.94hz for the internal reference of 60hz. The field audio cassettes will now be resolved to video at the same rate as the film to video transfer.
Q. If I am shooting at 30fps for video transfer, how will this affect the sync sound recorder?
A. It will not matter to the sync sound recorder whether you shoot at 24 or 30fps for video transfer. The crystal oscillator will remain at 60hz either way. If you are shooting at 30fps and will have your film transferred to video on a Rank scanner it will be transferred at 29.97fps. Therefore, to maintain the correct sync relationship, the field audio cassettes must be transferred against a master clock frequency of 59.94hz. This is easily accomplished with a resolver by simply connecting a video reference to the external sync input. The resolver will substitute the external video reference at 59.94hz for the internal reference of 60hz. The field audio cassettes will now be resolved to video at the same rate as the film is being transferred to video.
Q. Can I use a stereo cassette recorder to record lip sync sound?
A. Yes. Stereo cassette recorders have been used professionally for sync sound field recording since the early '70's with excellent results. Normally, one of the stereo channels is reserved for recording the 60hz. sync signal from either the camera (if it is set up for cable sync) of from a crystal sync generator. This leaves the other stereo channel for the microphone signal. If the cassette recorder has been so modified, it can also be used to resolve the field audio tapes to either magnetic film or videocassette. Otherwise, a sound lab can be used to accomplish these transfers.
Q. Can I use a DAT recorder to record lip sync sound?
A. Yes. A DAT recorder can be used as long as the field audio cassettes are resolved correctly. A DAT recorder is very similar to a videocassette recorder in the way that it scans the tape during record and playback. A servo system is required to keep the spinning head and linear tape transport locked together. DAT recorders must have their own internal crystal oscillator in order for this servo system to operate. Inexpensive DAT recorders, however, do not allow for any resolving other than their innate ability to play back in real time against their own internal clock. If the DAT recorder will be used only for transferring the field audio cassettes to magnetic film and if the mag film recorder (see "What is a magnetic film recorder") is also referenced to crystal, there is usually no problem. They cannot be slaved to some other external master source such as a videocassette recorder, for instance (see "...recording sound for...video"). The more sophisticated DAT recorders (certain portable models and some studio decks) do accept external sync references. These models are commonly used by sound labs for resolving DAT cassettes to magnetic film or videocassette.
Q. How can I tell if my camera is running at 24fps?
A. If the camera is crystal controlled there will be some form of sync indicator to inform you of an out-of-sync condition between the crystal system and the camera. Usually a lamp or an LED is provided on the back of the camera, the camera's motor or the crystal control unit. It is also possible to test the camera for proper sync speed using any of the various speed-check devices on the market. These units use their own crystal oscillator circuit to drive either a strobe light or a series of LED's that, when properly positioned or viewed through the viewfinder will clearly indicate whether the camera is running at 24fps.