Surround sound has been around since the early 1990s. The concept of enhancing standard stereo with rear loudspeakers for effects and atmospheres developed in cinema during the 1940s. However, largely for technical reasons, it did not become a widespread commercial development until the late 1970s and into the 1980s. It was at that point that adding multiple audio channels to a soundtrack became easier and less expensive.
The recent home cinema boom resulted in more speakers being added to people's living rooms to play surround sound from video, LaserDisc and DVDs. Those post-production facilities working on broadcast television content soon learned they were expected to provide surround mixes for many of their projects.
Despite the availability of material and a small but growing enthusiasm for surround sound, some broadcasters were not keen on promoting programs featuring Dolby Surround or Pro Logic. The reasoning was that they were not officially offering a surround sound service; if there were any problems with the soundtrack, they didn't want to be held responsible.
Surround material wasn't being stripped from the programs. If the feed was mono-stereo compatible, the soundtrack was broadcast as it stood. Those viewers with decoders received the additional audio channels. Despite the lack of promotion from broadcasters, some producers made sure to include the Dolby logo in the opening titles of programs.
Dolby and DTS theater sound
Despite these issues, dubbing mixers working on television programs still had the opportunity to create a sound picture that immersed viewers in the action, even with the limitations imposed by analog, matrixed coding systems and consoles with only basic facilities to offer. While broadcasters were being circumspect about surround sound, Dolby and DTS introduced digital surround systems for the cinema with discrete channels.
The Dolby Digital and DTS formats both offer 5.1 channels — a front left, center (for dialog), front right, rear left, rear right and a subwoofer channel for low frequency effects. This arrangement is now common on DVDs. Part of the format's success has been attributed to 5.1 audio, so it was logical that when broadcasters decided to begin HD transmission, digital surround sound had to be part of the package.
By then, the production chain had matured, and new desk technology was allowed the proper mixing of surround audio, even with joysticks to allow engineers to accurately place sounds and effects. Grouping of effects and dialog became easier with digital consoles, memory, recalls and automation.
Live television production, both in the studio and particularly in OB trucks, resisted digital consoles for some time.
Many digital vision studio infrastructures were installed during the late 1990s, but analog audio mixing consoles continued to be used. The reasoning was that digital consoles had not proved themselves sufficiently reliable for live broadcasts, where there is no “take two.” Also engineers often preferred to have one function per fader rather than scroll through layers of menus.
Times have changed, and digital consoles are routinely installed in the audio sections of TV studio galleries and OB trucks. The driving force is surround sound, although the ability to recall settings is an advantage for broadcasts involving a number of configurations or acts. OB operators were moving towards digital consoles of their own accord but broadcasters, notably BSkyB in the UK, have forced the issue by specifying preferred technology and equipment.
Console manufacturers have responded by producing desks that fit better into the often cramped spaces in OB trucks, and even studio complexes. Greater CPU processing power means that mixing and equalization are faster than in the days of analog. All this means more power in less space.
Contribution and distribution links
With the creative/technical aspects of surround sound now well established, attention has been turning more to the practical matter of moving a greater number of audio channels around existing broadcast infrastructures. While new broadcast centers are often required to properly accommodate HD, 5.1 and multimedia, many broadcasters have to adapt what they have at the moment to handle technologies for which they were not designed.
DTS produced a compression system for the emission of multichannel signals but not for contributions and distribution. Dolby developed a system for both distribution, which would allow eight audio channels to be carried around traditional two-channel infrastructures, as well as for final transmission. The result was Dolby E and broadcasters soon discovered that it worked pretty well both as an in-house and private external distribution system.
Surround audio for live sport
As digital systems have grown, broadcasters have been wary of introducing too much compression into a chain. So, for instance, during the 2006 FIFA World Cup, HD pictures and 5.1 and stereo audio streams were passed between the stadiums and the International Broadcast Centre in the form of uncompressed signals over a 20Gb/s ATM link, with the audio embedded in the video carrier. However, Dolby E was used by the host broadcaster, HBS, and by some of the Broadcast Partners who customized the main audio feed for their own requirements. Each network could then distribute 5.1 mixes to their audiences over the limited bandwidth of the satellite links.
The experience of BSkyB
For its 5.1/HD service, BSkyB uses a mix of embedded audio and Dolby E. The network initially was concerned that it would have to migrate completely to the latter. Sky has been using discrete embedded audio for some time. Although its engineers have experience using Dolby E with the broadcaster's film channels, designing a new infrastructure for live sports coverage was a different proposition because many of audio circuits were stereo.
The presentation suite used for Sky's live Premier League football coverage, Studio 7, is equipped with 39 Dolby E decoder cards manufactured by Axon. The studio relies on 13 embedding cards, which makes audio switching more manageable, according to the designers. A bigger problem has been dealing with the latency introduced into the chain by the Sony HDCAMs they used on location. In the early days of Sky's coverage, allowances for sync problems were made by introducing a frame delay to the audio to match the video.
Routers can easily switch multichannel or Dolby E signals. However the problem comes in matching the audio delay caused by video coding processes. Dolby E is considered good for its main purpose of moving surround around down a stereo cable, but once video is involved, a frame of delay is usually introduced into the audio. That means trying to get the Dolby audio back into sync with the video before it's transmitted.
Some engineers question whether it is sensible to maintain Dolby E throughout a system. There is a view that an SDI (Serial Digital Interface) infrastructure or HD-SDI framework using embedded discrete audio channels, which obviates the need for Dolby E, eliminates the audio/video sync problem. However, even moving 5.1 audio as separate channels can result in delays being introduced by other devices. Part of the audio may go through one manufacturer's decoder, and another part of the audio may be routed through another. Each time this happens, about one frame of delay can be added. The result is that at the point where it's time to compress the final mix for transmission, the stereo or surround image has been changed.
While router manufacturers have products to detect and remove the delay, even if it's fixed in the professional chain, some of the new HD plasma screens will reintroduce it in the home as they scale and de-interlace the video.
Multiplexing multichannel audio
Another concern about using carrier systems, such as Dolby E, is the capacity. When the technology was originally developed, designers believed that eight channels would be enough for all eventualities. Today, detractors say that eight channels was probably an underestimation. As an example, live sport may need a full surround mix, plus stadium sound mix minus the commentator, which can quickly add up to nine or more channels.
There is also the matter of the point-one of 5.1. This designates the subwoofer channel used in cinemas, adding low frequencies effects (LFE) to action sequences. A few audio, and more video, engineers think that an LFE may be unnecessary for most TV broadcasts. This would allow HDTV audio systems to only need five channels — 5.0.
The matter of needing a center channel is similarly a cause of debate. A technique inherited from film and being used in some sports applications is to keep the commentary in the middle of the stereo front loudspeakers. Other wisdom prefers no fixed center channel but the use of a phantom centre created through the front stereo channels. The opposing argument is that this method is unstable, and center voices could wander either left or right.
Despite the always contentious world of audio, solutions continue to be developed. The technology gets better. And, because sound is a more subjective medium than vision, its likely that these arguments will continue long after 5.1 becomes the norm.
Kevin Hilton writes about emerging technologies for digital audio.
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