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.
Theatre 3 in Pinewood Studios near London uses a Digidesign ICON console with Pro Tools. A DK-Technologies MSD600M monitors audio levels.
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.
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