Operating instructions

D-3
3. Why Adjusting Dolby Level Is Important
The accurate reproduction of Dolby soundtracks requires that the decoders in
the theatre act as precise mirror images of the encoders used when the tracks
were recorded. This precision is ensured in two ways. First, Dolby system
circuits are manufactured to very close tolerances. In fact, any A-type or SR
circuit, regardless of when manufactured, can be connected back-to-back to
any other circuit of the same type in an encode-decode configuration, and
provide flat response within 1 dB. Second, a reference Dolby Level has been
established so that any Dolby-encoded recording can be accurately decoded by
any Dolby decoder of the same type.
Remember that Dolby circuit action in both recording and playback is level-
dependent. Loud signals are untouched, while low-level signals are boosted
during recording and attenuated during playback, by an amount that depends
on their level. How does the playback Dolby decoder know when, and by how
much, to attenuate the previously-boosted signals?
The answer is that it doesn’t know. When the decoder sees any particular
playback voltage, it assumes what happened when the recording was originally
encoded. For that assumption to be accurate, however, it was necessary when
Dolby noise reduction was first developed to establish a standard reference
level to which all encoded recordings and all decoding circuits could be
calibrated.
This is where the Dolby Level Tone on the Cat. No. 69 film comes into play. The
tone on this film is very carefully recorded at the standard reference Dolby
Level. When you make the Dolby Level adjustment on the CP455 as this film
plays, you are making sure that the tone on the film is converted to a standard
reference voltage at the inputs to the decoders. Thereafter, signals from Dolby
soundtracks will be converted to the correct voltages for mirror-image decoding.
As long as the proper Dolby Level procedures are followed, any Dolby-
encoded soundtrack recorded anywhere in the world will be accurately decoded
by any Dolby cinema processor anywhere in the world.
Although large level-matching errors can cause audible decoding errors (such
as frequency response changes), both Dolby A-type and DolbySR are tolerant
of errors smaller than about 2 dB. Thus, Dolby Level must be readjusted only
if there is a significant change in the reproduction chain ahead of the Dolby
decoding circuit. In the theatre, exciter lamp brightness is the variable most
likely to affect Dolby Level calibration. The voltage present at the inputs to the
decoder circuitry is directly proportional to the amount of light that passes
through the slit and soundtracks onto the solar cell. Therefore, significant
changes in exciter lamp brightness — likely as the lamp ages over time or when
a new lamp is installed — should be compensated for by adjusting Dolby Level.
4. The Consumer Dolby Noise Reduction Systems
Dolby A-type noise reduction and DolbySR are used for professional
applications, including music recording, broadcasting, and preparing film
soundtracks. Other Dolby noise reduction systems—B-type, C-type, and S-
type—have been developed for use in such consumer products as cassette
recorders.