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Optical sound troubleshooting
These are some examples of common optical sound issues, and their potential causes and remedies.
Buzz and Hum can be caused by:
Ground Loop: If there is a hum in the system with no film running, there is likely a ground loop.
Light leak: Check for light from an overhead light or a reflection from the port window spilling into the optical sound system.
Lateral Guide Roller Alignment: Film is not properly aligned in the soundhead, and the solar cell is picking up parts of the image, DTS timecode, or perforations in addition to the soundtrack. Align with Buzz Track test film as described in your projector's manual.
Damage to Optical Soundtrack: scratches, gouges, or sprocket run can cause audible buzzing. If damage is limited to only one side of a mono track, the undamaged side can be isolated by blocking off half of the slit lens or solar cell with a business card, or bringing the gain all the way down on the left or right channel.
Shrunken Optical Soundtrack: Print is shrunken and not aligned laterally in the soundhead. As with soundtrack damage, part of the track can be blocked off with a business card. The lateral guide roller can also be adjusted to the shrunken film, but must be set back afterwards. Consult your projector's manual for this adjustment.
One of the most common types of distortion in optical sound is the distortion of sibilant sounds (s, sh), which can be very apparent in dialog. This can be caused by playing a film in the incorrect sound format (most notably playing an A type film in SR).
Wow & Flutter
Common causes of Wow and Flutter include:
-Worn bearings or other mechanical resistance on sound drum shafts, which cause the sound drum to spin at an inconsistent rate. This can be caused by bearings sitting idle for too long and supporting the weight of a heavy flywheel, which is why it's important to run film through projectors at least once a month if they're sitting idle.
-Stuck sound drum. Sound drum may not be spinning at all, putting resistance on the film and causing audible flutter. A stuck sound drum will also cause severe film scratching, unfortunately not visible on screen until the print is run again. It is a good idea to check that the sound drum is spinning freely during the process of threading the projector (note that some projectors have an electronically or mechanically coupled sound drum which does not turn until the projector is in motion).
-Screw holding sound drum flywheel onto sound drum shaft has become loose or fallen out entirely, causing sound drum, flywheel, and film to all spin at different speeds.
-Worn sound head rollers. Sound head rollers or rubber O rings may be disintegrating or have worn a flat spot, causing the film to advance at a slightly different speed at every revolution. Such rollers should be replaced. TIP: If projector has a rubber roller in the sound head and is not used regularly, put a piece of felt between the roller and sound drum so it doesn't wear a flat spot.
-Misthread. The dreaded operator error. Sound head roller is not engaged, lower loop is too tight, or film is not secure in film path.
The optical sound head must be kept spotless and rollers must turn freely to eliminate wow and flutter.