Ensemble Designs 9670 Audio Automatic Gain and Loudness Control and 9690 Audio Compliance and Monitoring Software - Manual (Page 20)

www.ensembledesigns.com
Avenue 9670 - page 20
LevelTrack™ Audio Loudness Control AGC Software Option
Ensemble White Paper on Dialnorm - Broadcaster
Compliance with the Calm Act
In December of 2011, Congress enacted and the President signed the “Calm Act” regulating perceived
loudness of programming at broadcast facilities in the U.S. This regulation -based on the ATSC A/85
Recommended Practice: Techniques for Establishing and Maintaining Audio Loudness for Digital
Television – has caused a great deal of confusion in the market place. The key to understanding this
legislation is the phrase “Perceived Loudness”. Perceived loudness goes beyond simple VU gain values
and AGC control. It also takes into account audio processing such as compression, expansion, gating,
limiting, etc., that increases perceived loudness in program audio without increasing gain. The purpose
of this legislation is to maintain consistent audio loudness to the consumer, both between channels
and between programming and commercial content. Broadcasters, satellite providers, cable operators
and other multi channel content providers have until December 2012 to comply. Ensemble Designs
has products that can help broadcasters maintain compliance with the Calm Act.
Perceived loudness compliance is based on Dialog Normalization – dialnorm. Dialnorm is defined in
ATSC A/85 as “An AC-3 metadata parameter, numerically equal to the absolute value of the Dialog
Level, carried in the AC-3 bit stream. This unsigned 5-bit code indicates how far the average Dialog
Level is below 0 LKFS. Valid values are 1-31. (zero value is reserved) The dialnorm values of 1 to 31 are
interpreted as -1 to -31 LKFS.”
To many, LKFS is a relatively new term, which means “Loudness K-weighted relative to Full Scale.”
It’s a scale for audio measurement similar to VU or Peak. However, rather than measuring gain, it’s
measuring loudness. LKFS is based on the ITU-R BS.1770 Loudness Measurement Method. The ITU-R
group performed tests to groups of people around the world with various content and listening
environments and were able to construct an algorithm that accurately measured the loudness of
audio content. It was quite an undertaking, and the details of the process take up the majority of the
BS.1770 document. Have a look sometime. It covers everything from speaker placement to noise in the
environment, to the type of content used for the testing. The loudness unit of LKFS is dB and is used
the same way as a dB of gain. For example, a -15 LKFS program can be made to match the loudness of
a -22 LKFS program by attenuating 7 dB.
There is a defined process for determining the dialnorm value for a particular piece of content. It’s
a fairly involved process depending on whether dialog is present and the length of the sample that
must be used to determine the dialnorm value for the entire program. It can be found in the ATSC
A/85 document if you’d like more detail. Dialnorm relates to the level of an “Anchor Element” which
is usually dialog. It disregards momentary intentional loud elements such as gunshots or car crashes.
Here’s another way to think about this. When you are home watching TV, what do you adjust the
volume to hear? It’s usually the talking part of the program. Sound effects may be louder, whispering
softer. But when you adjust the TV volume, you’re setting it to comfortably hear what’s being spoken.
This is essentially dialnorm. If there is no dialog present, as in music programming, it relates to the
viewer focus element. For example: the level of the featured pianist rather than the level of the entire
orchestra. A single dialnorm value is determined for the entire program content, and is embedded
into the metadata bit stream of the SDI signal. To simplify the process, companies such as Dolby,
Sony, Tektronix and others, have a box that will “listen” to the program content and then report a
dialnorm value for that piece of programming. Another box is often used to encode this value into
the SDI stream. File based content solutions are also available from companies such as Telestream and
Masstech which analyze a particular file’s content and then embeds a dialnorm into the metadata of
the file. All this is well and good for content that is already produced, but what about live productions