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Surround audio production metering

May 1, 2009 12:00 PM, By Thomas Holm Hansen

Accurate surround-sound metering if critical to delivering high-quality audio tracks.

DK-Technologies developed the StarFish display to show the volume of sound from the loudspeakers as perceived by the listener.

Stereo phase correlation metering (and the BBC's preference for sum and difference metering) is well-established, but the same is not yet true for the surround market. Some current solutions involve multiple phase correlation meters placed around a display screen alongside a graphical representation of the surround channel levels, drawn as a horizontal map. However, this approach results in what some people perceive as a busy and confusing display, with the eye having to scan several different graphics in various parts of the overall screen to obtain all the information. An alternative and visually simpler approach is to make the surround channel display incorporate multichannel phase correlation information as an integral aspect, using different colors. In this way, the user can observe information about relative levels, balance, coherence and phase errors in one place, at the same time, unambiguously.

Display of acoustic reproduction

The next issue to consider is whether the surround metering display should indicate the electrical signal levels in each channel — as is the case in traditional stereo metering configurations — or the acoustic reproduction of the multichannel source. Bar graph meters already display electrical signal levels adequately, so it makes sense to expand on the information presented by using the multichannel surround phase correlation display to illustrate the true acoustic reproduction of the sound stage.

This display shows ITU loudness in accordance with BS1770/1771.

As a result, what is seen relates much more closely to what is heard. For example, the same signal fed to two loudspeakers will produce a central phantom image midway between them, while inverting the polarity of one signal will result in the listener perceiving a hole in the middle, with the sound pushed outward to the edges of the sound stage. A display illustrating the same effects — phantom sounds emitted midway between speakers and inverted polarity signals resulting in a hole — makes intuitive sense to the user. In this way, the meter serves as an informative tool for live balancing of surround material in situations where the monitoring may be less than ideal, or where additional unrelated sound sources have to be auditioned — such as talkback — which will confuse the surround-sound monitoring.

With modern LCD screens, configuring a display to provide metering for both surround and downmixed stereo signals simultaneously is easy to achieve, with bar graph meters and a multichannel phase correlation display for the surround source, plus additional bar graphs and a normal stereo phase correlation meter for the stereo downmix. This kind of arrangement enables the sound balancer to keep an eye on both formats at the same time, in the same place, making it far easier to monitor and control the mixes.

Shout it loud!

In today's sound control room, even this level of sophistication is not enough. The issue of perceived loudness during and between programs and advertisements has long been a concern, but until recently it was impossible to provide a consistent means of quantitatively measuring loudness. Several standards organizations and manufacturers have tried to address this issue over the years but the ITU's new BS.1770 and BS.1771 recommendations for a standardized form of loudness assessment metering (LEQ-RLB) are now being widely and rapidly adopted across the broadcast industry. Indeed, in the UK, the Broadcast Committee of Advertising Practice (BCAP) introduced a ruling on sound levels last year (ASA Rule 6.9) specifically to “minimize the annoyance that can be caused to viewers by TV ads … being generally perceived as too loud.”

The ITU recommendations provide a basis for measuring loudness that tallies closely with human perception, and most metering manufacturers are incorporating these new standards into their displays. The standard implementation provides a single loudness bar graph with a simple numerical readout. Various options and weightings accommodate both stereo and surround material, and a gated mode ignores quiet sections of programming (such as in golf tournaments) to avoid a falsely low loudness reading.

A well-produced surround sound track adds enormously to most programs, especially sports, music and drama, and the industry is slowly moving into routine surround-sound production where budgets allow. However, the technical challenges are nontrivial, and new working practices are still evolving. The growing awareness of and requirements for loudness control adds to the pressures on the sound engineers, and accurate, versatile and informative metering is more critical than ever to the successful creation of top quality sound tracks.

Thomas Holm Hansen is vice president of DK-Technologies.