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INTRODUCTION
ORBAN Model 6200
problem since then as a result of viewer complaints. It is against FCC rules to broadcast
irritatingly loud commercials.
Audio Processing for Consistency and Loudness Control
In current NTSC practice, all audio is applied to a transmission audio processor that
automatically controls the average modulation and the peak-to-average ratio. This en-
sures that the audio will be comfortably listenable. The audio processor also has another
crucial function — it smooths out transitions between one piece of program material and
the next. Orban's OPTIMOD-TV 8282 (for analog services) incorporates the CBS Loud-
ness Controller algorithm. This uses a complex algorithm that estimates the amount of
perceived loudness in a given piece of program material. If the loudness exceeds a preset
threshold, the controller automatically reduces it to that threshold. The main purpose of
this circuit is to control the loudness of commercials that have been processed to produce
irritating loudness without such control. The 6200's Two-Band structure also incorpo-
rates the CBS Loudness Controller algorithm, bringing automatic loudness control to
DTV audio.
A Hybrid Technique for DTV Processing
Knowing how broadcasters do successful processing in the analog world, we believe that
the most realistic approach to handling AC3 dialog normalization is a hybrid technique.
Most program material can be passed through an audio processor with a loudness con-
troller very much like the ones currently used for analog television. This material is typi-
cally either mono or two-channel stereo. It includes commercials, live news, game
shows, talk shows, soap operas; and many documentaries, sports, and pop music videos
and concerts. Processors used in analog TV control their maximum loudness level very
well, so a single dialog normalization value will apply to all program material whenever
the processor is online. The advantage of this strategy is that the processor will guarantee
that all of this material is comfortably listenable, and that commercials are not exces-
sively loud. With the possible exception of sports, this program material does not rely on
extreme dynamic range to make its point, so we do not believe that compression dam-
ages the artistic integrity of this programming. No one needs more dynamic range on the
local news!
Prime time dramatic shows, newer feature films, classical music concerts, and certain
sports broadcasts all use dynamic range for dramatic impact, and therefore are candi-
dates for full-blown exploitation of the AC3 metadata. Each show, film, and concert
must have a dialog normalization value pre-assigned to it, ideally derived by referring to
a calibrated loudness meter. The uncompressed audio is then applied to the AC3 en-
coder, along with Line-Mode and RF-Mode Dynamic Range Control signals to ensure
that the receiver can apply compression if the viewer prefers a narrower dynamic range.
Unless they have been produced in 5.1-channel, commercials should be processed
through the 6200 in the usual way. If the fixed dialog normalization value is correctly
chosen for all material passed through the audio processor, commercials will automati-
cally be limited in loudness to the average loudness of the dialog and will therefore be
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