Surround audio production metering

Accurate surround-sound metering if critical to delivering high-quality audio tracks.
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The rapidly expanding drive toward HDTV is satisfying, representing the biggest quality improvement in domestic television in 50 years. And yet for camera operators, vision mixers, lighting technicians, picture editors and the whole production team, very little has changed in terms of the requirements of their skill base or operating practices. The equipment is more expensive, and the pictures look fabulous, but HD equipment is used in nearly the same way as the legacy SD equipment it replaces.


Triple expansion

The situation for the sound department — particularly in the live on-air sector — is entirely different, and steep learning curves must be climbed in short timescales. Although the film industry discovered surround sound 70 years ago and has employed it routinely for the last 40 years, it is still a challenging and often mysterious new element of TV sound. Post-production departments may be au fait with the requirements of creating 5.1 sound tracks if they have been involved in the production of commercial DVD releases of SD material, but the escalation of live HD production brings with it the expectation of live surround sound too.

An eight-camera SD studio upgrading to HD will usually still have eight cameras. However, if the studio's sound console was originally designed to cope with, let's say, 48 stereo sources, providing the same number of 5.1-capable channels would require a threefold expansion of the console. That's a daunting 288 audio channels! This scale of audio signal processing is simply not practical using analog technology, so in addition to coping with the operational and practical requirements of surround acquisition, the sound engineers must also master the radical changes associated with digital audio consoles, along with many other relatively unfamiliar technologie like multichannel embedded audio and Dolby E.

Watching surround

While the old adage “if it sounds right, it is right” is still an appropriate mantra, ears alone are not sufficient to judge and control the technical aspects of sound production. In particular, the human sense of hearing isn't too good at recognizing absolute signal levels or the phase relationships between channels. That's why VU meters, PPMs and stereo phase correlation meters were invented, and equivalent visual aids are an essential aspect of working in surround sound too. The problem is that with six or more channels to monitor, the level of complexity rises. There are a variety of sophisticated multichannel metering systems on the market, both as stand-alone hardware and software plug-ins, all with different approaches to displaying the relevant information.

There is universal agreement that bar graph meters provide the most practical way of displaying the levels of six or more channels simultaneously, and in most designs, the display can be configured to show the average or peak levels. However, the latter is still a cause of some confusion as analog PPMs are deliberately slugged (for historical reasons) to ignore brief transients, whereas digital peak meters tend to indicate true peak levels no matter how short. Programs mixed to the appropriate conventions using PPMs (whether analog or virtual digital representations conforming to the published standards) will inevitably appear to peak too high when monitored with digital true-peak meters — often by as much as 4dB. This is inherent given the different nature of the metering systems involved, but few program delivery specification documents take adequate account of it.

Bar graph meters are excellent when it comes to identifying and aligning channel levels with lineup tones, and they are also good at illustrating the relative balance of channels. Having a visual confirmation of how the various channels balance against one another, and what (if any) contribution is being made by the LFE channel, provides confidence in a busy sound control room. However, some mental translation is required to relate the vertical bar graph displays to the horizontal acoustic surround environment.

Indicating phase correlation

Bar graph meters — useful as they are — don't tell the whole story. In the stereo world, level meters are often supplemented with a phase correlation meter or a sum and difference meter system because the phase relationships between channels cannot be determined from level meters alone. Moreover, it can be extremely difficult to judge the coherence between channels aurally, yet the phase correlation between channels is critical whenever a downmix is created, such as mono from stereo, or stereo from surround. These downmixes may be required for simultaneous feeds to legacy transmission chains, or by a consumer wanting to listen to a stereo program on a mono radio, or a surround program on a stereo TV. Correlation problems between channels will typically produce quiet, colored and generally unacceptable results in the downmix. Good practice requires the sound engineer to audition downmixes occasionally during a production, but it is impractical to listen all the time while creating a stereo or surround mix. Therefore, it's essential to have some form of metering to warn of a potential problem.

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.

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.