SMPTE 2013: Loudness Legislation Continues To Evolve
When the transition to digital TV took place more than a decade ago, it meant beautiful pictures but also a new problem: digital audio signals perceived to be up to 32 times louder than analog audio signals. Those early woes kicked off a movement to find a comfort zone so that the difference between one channel and the next or one commercial to the next is less than 2.4 dB louder or 5.4 dB quieter. It is an issue that every American TV household is familiar with: at one time or another, someone has scrambled for the remote to turn the volume up or down when there is a commercial.
The CALM Act, passed in 2010, did much to alleviate that scrambling. But there is still much work to be done, according to Jim Starzynski, director/principal audio engineer, NBCUniversal, and chair of ATSC TG1/S6-3 work group, and Patrick Waddell, manager, standards and regulatory, Harmonic, and chair of ATSC TG1/S6. Both cited ongoing development during an update of the Commercial Audio Loudness Mitigation (CALM) Act at SMPTE’s Annual Technical Conference this week in Los Angeles.
The CALM Act has been a mixed blessing, according to Waddell. It definitely lays out some workflows and best practices designed to tighten the range of loudness so that viewers have a more enjoyable viewing experience. But the passage of the Act resulted in ATSC recommended practices’ (A/85) being turned into a standard, not the ideal way to finalize a standard. In fact, as the A/85 evolves, so too does the CALM Act.
“At our first opportunity, we went in and tweaked the paper,” said Waddell. “So every subsequent version of A85 will become the law.”
A recent win for the industry has been the continued acceptance of certificates validating that a given piece of content adheres to the CALM Act.
“Those give the TV station a safe harbor, as the liability can be pushed upstream,” explained Starzynski. “The good part is, it meets the needs of the suppliers and is a win-win because the downstream suppliers don’t need to alter the content. And the certificates are easily available to any distributor who needs access to them.”
In 2013, there have been a number of changes, including an update of the reference to ITU-R BS.1770 to version three. One advantage of that change is that there is a more accurate reading of loudness levels. For example, loudness is measured across the entire piece of content, and silent or quiet passages can skew the overall loudness reading. The new version uses gating to remove those silent passages from overall rating so that the loudness reading is concerned with the loudest parts of the content.
“The stuff that had the biggest variation of measured levels were things like a David Attenborough narration that was followed by the sound of feet on the beach for 30 seconds,” explained Waddell. “Those sections tended to be the outliers, so the gating certainly helps in measuring loudness properly.”
Also new are some careful edits to Section 5 that are concerned with mixing techniques. More specifically, they address the challenge of downmixing to stereo without raising the audio loudness level.
There is also better definition of the term comfort zone so that other industry associations, such as SCTE, can cite it in their own documents.
And the latest changes are definitely not the last. Noted Starzynski, “We could be doing this for a while because the paper is still active.”