Loudness monitoring: PPMs are not enough

As TV broadcasters complete their transition to an all-digital delivery, content providers may soon have to embrace a new means for monitoring audio output in order to conform to acceptable loudness standards.

Just as what the move from the analogue of tape to the digital of CD did for music, the move to digital TV broadcast has brought about a delivery system capable of much greater dynamic range. While that should make for a better all-round experience, it presents a whole new problem for broadcasters who need to maintain consistent volumes between programmes and advertisements, to prevent annoying jumps in volume when a programme breaks to adverts.

The current UK guide suggests a maximum peak level during a programme of 6dB, and 4.5dB for adverts, and while there isn’t a disagreement about that ratio, there is an issue with the way it is measured. Peak metering only looks at the loudest part of the programme material, which may not be representative of the content as a whole. If the very loudest sounds occur right at the start, that will set the level for the entire programme, and the more dynamic range present in the audio, the greater the effect.

Adverts

Adverts on the other hand, don’t have this problem. With a run time of just a matter of seconds, advert makers try to pack in as much visual and sonic impact. Dynamic range isn’t going to vary all that much in that time, so it’s already likely to sound proportionally louder. And with all the adverts jostling for attention, advert-makers will employ every trick available to make their product appear louder than their competitors. By utilising heavy compression and limiting (ideal, since dynamic range isn’t a concern), it is possible to massively increase the perceived volume of the sound in an advert while remaining within the decibel limit. This is exactly the technique employed in the music industry (arguably to its detriment) as both record labels and radio stations fight to produce louder-sounding music than their competitors. In the TV paradigm, while still remaining broadcast-legal and within the allotted limits, it’s possible to produce an advert which is subjectively much louder than the programme during which it appears. All of which makes for very annoying viewing.

TV broadcasters are aware of this, and they’re also aware of the risk of viewers changing channels just to avoid the adverts. The trouble is that it is an exploitation of the way the broadcast standard peak level metering works, which is not at all how human beings perceive volume. We don’t hear something as being loud based on just the loudest points, we form a perception of overall level based on the sonic content as a whole over time. We call this the ‘loudness’ and it’s easiest to think of it in terms of sonic density – by compressing a signal to a smaller dynamic range, we perceive an increase in density and thus it seems louder, even if the peak volume remains the same.

Measurement

Loudness has largely been a subjective matter for years, but standards now exist to measure it, which means there are ways to monitor broadcast output based on how the viewer will hear it. The ITU BS.1770/1 standard gives broadcasters and content-makers a means to measure the loudness of their product in Loudness Units, a unit entirely separate from the traditional dB scale. As it looks (effectively) at ‘sonic density’ over a duration of time, a programme can provide metadata for its loudness value, which makes it possible for the broadcaster to standardise output. Tests on music have shown that by matching the loudness coefficient of older music against modern releases (which have a tendency to be mastered much louder), the listener perceives them to be the same volume, sometimes even reversing the trend and producing a better response to the music which has retained the greater dynamic range.

Although there are still competing standards, it’s inevitable that before long it will become a requirement for loudness to be taken into account. To that end, broadcast content providers will need to invest in new methods of monitoring that include loudness. Software currently exists from TC Electronic and Dolby to achieve this within the popular music production platforms such as Pro Tools and Nuendo, and TC Electronic have also released two models of standalone hardware loudness monitors with their TM7 and TM9 products, with the same software available for their System 6000 mainframe processor already widely used in broadcast mastering.

So while PPM meters may still have a role in calibration, it could well be that their time as the de facto standard of level monitoring is at an end. Broadcasters looking to invest in PPM meters definitely need to be aware of these changes if they want to stay competitive, so if this sounds like you, then please get in touch with one of Jigsaw’s broadcast consultants to chat about your options.

 You can call us on 03332 409 306 or email broadcast@Jigsaw24.com. You can also view our entire broadcast range on Jigsaw24.

Rob H
Rob H
Call us: 03332 409 306

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