Audio Loudness and 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 that increases perceived loudness in programing 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.
Dialnorm and LKFS
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. The dialnorm values of 1 to 31 are interpreted as -1 to -31 LKFS.”
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 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. Incidentally, -22 LKFS is considered a Dialnorm of 22.
Decoders at the consumer’s location, take the embedded dialnorm value from the encoder and adjusts the receiver volume automatically. For example, if the decoder receives a dialnorm value of 5, it understands it’s a loud program, and ramps down the receiver volume. If the decoder receives a dialnorm of 30, it understands it’s a quiet program and turns up the volume at the receiver. The change take place at content transition points and is transparent to the consumer.
There are three different methods of complying with the requirement for accurate dialnorm. Each can be used at the discretion of the operator and is dependent on the type of AC-3 encoder being used:
• Fixed Dialnorm Metadata Encoder: The AC-3 encoder Dialog Level is “fixed” to a single value and the content Dialog Levels are conformed to that setting prior to reaching the encoder.
• Preset Dialnorm Metadata Encoder: AC-3 encoder “presets” are programmed, each with different dialnorm values and engaged via a “GPI” or other control interface from automation.
• Agile Dialnorm Metadata Encoder: The AC-3 encoder is configured to receive external metadata. An upstream “agile” dialnorm metadata system may be used to deliver dynamically changing dialnorm values to the encoder, corresponding to the changing loudness at the content boundaries.
Use LevelTrack Audio Loudness Control – AGC Software and stay in compliance
Staying in compliance with the Calm Act has never been easier. The Ensemble Designs LevelTrack™ Audio AGC option adds a user configurable audio level management system to Avenue signal processing modules. LevelTrack will correct mismatched audio levels between different program sources or segments within a program. LevelTrack provides two different approaches for adjusting audio levels: VU algorithms and the ITU-R1770 loudness algorithm. Simply enter the LKFS value to be maintained, the acceptable range and audio channel grouping and LevelTrack does the rest. By adjusting the overall level of the signal, rather than masking errors with compression, LevelTrack does not upset the internal dynamics of the program material – it maintains the full dynamic range.
For facilities utilizing a preset dialnorm AC-3 Encoder, the LevelTrack Audio AGC has presets which can be accessed via GPI trigger or serial control.
Use Audio Compliance and Monitoring Software to log your audio levels for compliance requirements.
Audio Compliance and Monitoring Software provides compliance verification and archiving with the Avenue system. The software gives you the ability to monitor and log levels of audio occurring in selected Avenue modules so that you can show whether audio levels are exceeding a certain configurable limit. In addition to monitoring and logging, the software can be configured to send email alerts or text messages when audio levels are too high. This software will provide a long-term record to prove compliance with government regulations.
- 9670 LevelTrack Audio Automatic Gain Control Software Key Option
- 9690 Audio Compliance and Monitoring Software