Managing audio loudness across multiple platforms

With the CALM Act now in effect, stations need strategies for managing audio loudness — preferably for a wide variety of broadcast platforms. This article details the loop spanning from production to multi-platform delivery, paving the way for high-quality audio across genres, platforms and borders.

As the number of listeners per stream goes down, and in combination with a more dynamic and less predictable consumer environment, it is important for broadcasters to consider five factors before committing to any change of station operating procedure: 1) Are we addressing listener concerns? 2) How well does a technique cater to the station’s majority of programs? 3) Does it reduce content creation time? 4) How does any procedure facilitate cross-platform distribution? and 5) Will any decision limit potential options, or will we retain the freedom to maneuver in the future?

Because the same measurement may be applied in production, ingest, transmission and logging, a transparent loop can be established from production to multi-platform. The loop may even be closed, with feedback from logging used to improve step by step.

To help this transparent loop, Loudness Range (LRA) is a statistical tool for making objective mixing and processing decisions. It quantifies the level variation with a time-varying loudness measurement. LRA is supplementary to the main audio measure, Program Loudness, of ITU BS.1770-3.LRA measures the variation of loudness on a macroscopic time-scale, in LU (loudness units).

Normally, broadcast should not be mixed like a cinema movie, nor like a pumped-up commercial, and LRA provides a simple value at which to aim. Figure 1 shows loudness changes in a clip from the movie “Pulp Fiction”: Relatively loud music plays until halfway through the clip, when the scene changes into dialogue. Both scenes sound even in loudness, but the first scene is noticeably louder than the second. The 3s time scale seems ideal for measuring the magnitude of that macro dynamic change; the 1s time scale shows the same tendency but more noisily, and the 10s time scale blurs the change unnecessarily. LRA catches this difference because it is tuned to time scales relevant to film, broadcast and music.

Used during ingest or on a broadcast server, LRA is an objective measure used for deciding when programs for delivery to certain platforms require range restriction. HD platforms may be set to tolerate any LRA value, though a limit such as 12-, 15- or 20LU, depending on genre, may be recommended in delivery guidelines. Downstream of production, LRA doesn’t change as long as gain offsets only are applied (normalization), but the number reveals if any significant range processing has taken place between two points in the broadcast chain. LRA may also serve as a logging tool, verifying that no range processing has happened during distribution, or in a codec.

For programs shorter than 30s, LRA is not suitable. Short-term or Momentary loudness are the metrics to use for preventing such programs from becoming too loud.