05.01.2013 01:41 PM
Originally featured on BroadcastEngineering.com
ATSC updates A/85 recommended practice for loudness


The ATSC has updated and delivered to the FCC its ATSC A/85, the core technology mandated by the Commercial Advertisement Loudness Mitigation (CALM) Act.

The A/85 Recommended Practice (Techniques for Establishing and Maintaining Audio Loudness for Digital Television) provides the industry with uniform operating strategies that optimize the TV audience listening experience by eliminating wide variation of television loudness.

The revised version of A/85 includes an update of the reference to the International Telecommunications Union (ITU) recommendation for "algorithms to measure audio programme loudness and true-peak audio level. "The revised A/85 now references ITU-R BS.1770-3. Version 3 of BS.1770 and adds "gating" (excluding low-level passages from the measured value) to the measurement algorithm. It also includes some minor editorial updates to the loudness measurement text and a minor correction to the true-peak measurement algorithm. Beyond the reference change, A/85 now includes improved guidance for measuring the loudness of surround programming in both its multichannel format and in its two-channel downmix.

This is important because many use the stereo downmix of surround programming, and it is possible to have loudness disparity between the two modes. In addition, A/85 is now specific about the differences between loudness and dynamic range.The updated ATSC Recommended Practice A/85:2013, dated March 12, 2013, is the successor document to Recommended Practice A/85:2011, dated May 25, 2011.

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