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Managing loudness for TV

Sep 1, 2010 12:00 PM, By Tim Carroll

Arguably the hottest topic in television these days is loudness. The subject has been beaten to death since the serious introduction of legislation to specifically mandate what was already on the books as law, albeit confusingly.

Broadcasters are in a panic because as license holders, they are ultimately responsible for obeying the law and could face fines or worse if not compliant. Rubbing the other side of this issue raw is the desire to preserve program integrity. We could take the route used in the NTSC days and just blindly process the audio to prevent this modern digital version of “over-modulation.” This would stem complaints of loudness shifts, but at the expense of changing the content.

So, the industry finds itself between a rock and a loud place. The ATSC document “A/85: ATSC Recommended Practice: Techniques for Establishing and Maintaining Audio Loudness for Digital Television” (available at www.atsc.org under Standards) lays out in detail some suggested methods for approaching the problem.

Loudness

Loudness is a frequency-weighted measurement integrated over some time. To do this, the ITU standard BS.1770 describes a measurement method where audio is first filtered to remove the extreme low frequencies while tipping up higher frequencies (to emulate the physiology of our heads). Then samples are stored and averaged to produce a result. This is why the notation looks somewhat strange: -24LKFS means Loudness (integrated over time), K-weighted (filtered), with respect to Full Scale digital.

What exactly gets measured to determine loudness? This is one of the most debated topics within standards organizations around the world right now. The ATSC long ago specified that it should be the average level of spoken dialog, because speech is the most common feature in the majority of television programming. This is the so-called anchor element. The new A/85 recommended practice extends this to encompass any anchor element so that music programs or music-only commercials are not left out. There is wisdom to this idea of an anchor element that might not be readily apparent. Ignoring the anchor and just measuring the overall loudness of all channels all the time can produce results that do not match perception with some programming.

Metadata

The golden and simple rule is this: The loudness of transmitted audio and the transmitted loudness metadata indicator (a.k.a. dialnorm) must match. A summary of the four common techniques of metadata control is useful as no single method is universally applicable:

Dynamic
Measure each piece of content — programs, commercials, interstitials, etc. — either during production or during ingest. Store the audio with a valid loudness reference indication (dialnorm metadata), and make sure the audio and the metadata make it to the consumer.

While not for everyone, this method has proven useful for broadcasters with straightforward and reliable distribution paths, such as movie channels, and has worked well for nearly a decade. The content is not changed and can remain intact to the consumer.
Static
Pick a loudness target, and set the facility's dialnorm value at this number. Then, measure the overall average loudness of each piece of content, and adjust or scale the content if necessary to have it match the target.

The ATSC A/85 recommends a target level of -24LKFS ±1 or 2dB (over time, hopefully) for content that does not have or cannot have metadata, as it was found by terrestrial broadcasters that most of their legacy content fell within this range and had no metadata. It may not be completely appropriate for all broadcasters, especially movie channels where content is likely quieter on average, so some are using -27LKFS. Remember though, as long as the content loudness matches the metadata value, it does not really matter what that actual metadata value is. Scaling is an overall gain adjustment performed once to realign each piece of audio to the target. Compared with the original, the result may be louder or softer overall, but it has not been otherwise changed.
Traditional audio processing
Similar in many ways to the devices we used in analog television, a traditional processor employs wideband and or multi-band automatic gain controls to constantly adjust the audio signal, effectively reducing the peak-to-average ratio. The loudest sections are decreased, and the softest sections are increased toward some center target. This target can be aligned with a static dialnorm metadata value.

Managing loudness automatically but not wanting to modify the content is like swimming without getting wet. The former necessitates the latter, and until time machines become more practical, it is what it is. While some techniques are better than others at preserving a consistent balance, all of these systems change the content compared with the original, although sometimes this is a good thing.
Hybrid metadata audio processing
This consists of a combination of metadata and traditional audio processing, where present metadata is used to guide processing to be applied only when and if necessary and to generate new metadata based on the measurement of incoming audio. The degree to which the original content is changed is adjustable, from purely traditional with fixed metadata to only protection limiting with dynamic metadata that can be bypassed by the savvy viewer or program producer.