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Chapter 5: Trouble in Paradise? Digital
Common Analog Problems, Symptoms, Errors and Bad Fortune
The digital reader appears to be dead.
A Dolby Digital reader is at the heart a video camera. You can test it by shining a light source into the
lens and watching for a signal on your oscilloscope. If it is not functioning at all it is likely that the reader
was "hot plugged" into the processor. If this has happened a replacement digital circuit board is required.
The Dolby Digital reader has an odd scope trace.
It is possible to have a foreign object in three places. The scope trace would look similar in all cases. It
could be on the LED itself, the reader lens, or on the CCD chip that is inside the reader. Pictured below
are three traces of what you might see. If the dip is in sharp focus it is on the LED or the CCD. All parts
should be cleaned with a clean dry Q-tip.
These look very similar to a film scratch. The film scratch would show a dip in the digital burst but not in
the envelope. The sprocket hole would be untouched so the digital burst would show problems but not
the frame it appears to be in.
The Dolby Digital has a high error rate.
A Dolby Digital reader depends on the proper performance of the mechanical parts of the sound head.
The film must be properly registered , at a constant speed, with even and sufficient illumination. If you
have performed all the alignment steps successfully and the error rate reported is not satisfactory it is
likely that there is a mechanical problem with the projector/sound head. It could be making the film
move side-to-side or causing vertical jitter. In either case you will see the movement on your scope.
Either the trace itself will move side to side or the right hand side of the sync pulse on channel 2 will
have movement.
There are many causes for poor sound head performance. Worn or damaged mechanical parts, film
guidance parts, sprockets, bearings, etc. If the Digital reader is capable of making scope traces that
closely resemble those shown in the alignment procedures but play with a high error rate it is safe to
assume that the problem is mechanical and proceed in that direction.
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