Bryston SP 1 Series Instructions Manual page 7

A brief explanation follows:
There are two variables built into many Dolby Digital
bitstreams during the encoding process by the program
producers, that can enable decoders like the SP1 to provide
automatic gain control based upon the information supplied
by these data variables.
One of these two variables (labeled Dynrng by Dolby)
provides a type of compression useful for situations such as
late night viewing of programs with a wide dynamic range
(like many action movies).This function can also be used to
provide compression for program material that may require
enhancement of overall intelligibility (such as some older
soundtracks).
Another feature provided by this option is the necessary peak
limiting required avoiding signal overload when "downmixing"
functions are selected.
The second variable (labeled Compr by Dolby) provides
additional peak limiting to allow overall average program level
to be increased.
These two algorithms can provide sufficient overall gain
reduction to allow even high dynamic range soundtracks to
maintain good audibility at low volume levels.
In most cases as an integral part of the Dolby Digital encoding
process, a certain minimum amount of dynamic range
reduction will be automatically included in the bitstream to
ensure safe downmixing.The precise amount of this function
is selected by the program producers, and will to a degree
depend on the contents of the audio channels.
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