Ensemble Designs 9670 Audio Automatic Gain and Loudness Control and 9690 Audio Compliance and Monitoring Software - Manual (Page 21)

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Avenue 9670 - page 21
LevelTrack™ Audio Loudness Control AGC Software Option
such as sports or news? For this type of programming, a dialnorm target is selected. This is usually
specified by network specs or distributor standard practices. The audio technician then mixes the
audio content to the target dialnorm using LKFS metering. The dialnorm metadata (value) is entered
and encoded into the SDI stream on the fly.
The AC-3 audio system defined in the ATSC Digital Television Standard uses dialnorm metadata to
control loudness and other audio parameters more effectively without permanently altering the
dynamic range of the content. The content provider or DTV operator encodes metadata (the dialnorm
value) along with the audio content into an AC-3 encoder. This metadata parameter, when extracted
at the decoder, sets different content to a uniform loudness transparently. Basically, it provides results
similar to the viewer using his remote control to set a comfortable volume between disparate TV
programs, commercials and channels.
So, what does this mean to the average content distributor who has to comply with the Calm Act?
The FCC is attempting to regulate differences in perceived loudness with particular attention being
paid to inter-channel audio levels; the difference between programming content and commercial
content. In an ideal world, all content providers would provide accurate dialnorm values in the audio
bitstream of their programming. These values, sent to the AC-3 encoder before transmission would
then be decoded at the consumer set top box or flat screen display, which automatically adjusts
“volume” levels at the receiver based on the values received in the metadata. This works by Dynamic
Range Control “gain words” calculated in the encoder and then applied at the decoder. These DRC
calculations are relative to and based on the indicated loudness of content represented by the
dialnorm metadata parameter. In other words, the encoder needs to know how loud the content
is intended to be so it can determine when the content is either “too loud” or “too quiet”. Dialnorm
effectively sets this target. Therefore, it’s very important that the dialnorm accurately indicates the
loudness of the content.
The concept of Fixed metadata is simply to “fix” the AC-3 encoder dialnorm setting to a single value
and to bring the loudness of the encoder audio input signal into conformance with this setting. This is
the simplest method with no requirement for additional metadata equipment or data management. It
is the only approach possible when using an encoder without metadata input or external GPI control.
The Preset metadata concept uses GPI triggers to set predetermined preset values to be loaded into
the AC-3 encoder to accommodate known differences in content loudness. For example, known
differences between network feed and local playout. Some AC-3 encoders however, reset and disrupt
the audio bit stream output when a preset is changed. Depending on the encoder, this may result in
an audible “glitch” on air. To avoid this potential problem, it may be necessary to provide a framesync
for the output of the AC-3 encoder to stabilize the AC-3 source.
The Agile metadata system allows setting different dialnorm values for different content that has
different loudness. This is accomplished by embedding the dialnorm parameter within the metadata
bit stream accompanying the content at an upstream location. The metadata is dis-embedded just
prior to the AC-3 encoder and then connected to the encoder’s external serial metadata input. The
encoder dialnorm setting then changes appropriately on the boundaries of the content. The downside
of the Agile metadata system is the potential for a severe discrepancy in loudness between programs
and between stations if metadata is lost. Encoders with external metadata input provide a “reversion”
feature to mitigate the impact of metadata loss. It can be configured to either retain the most recent
metadata value, or revert to an operator-defined preset. While this feature can minimize the impact on
the consumer, the error in loudness can still be significant. This method requires each piece of content
submitted for broadcast to have its unique dialnorm value embedded into the audio bit stream.