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Sun 1 Apr 2007 12:40 PM

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Driving Dolby Digital

Broadcasters are being driven by both the consumer and the movie studios to provide higher quality sound, says Martin Dyster of TSL.

There was a famous English quote made by Brian Sullivan of BSkyB a few years ago where he stated that, "HD is the picture quality that Dolby Digital surround sound has been waiting for". This may not be an opinion shared by all but it's true to say that the consumer market has been used to surround sound using high quality systems like Dolby D for a long time. Broadcasters are being driven by both the consumer and the movie studios to provide higher quality sound. So, what are the challenges for broadcasters and systems designers when shipping Dolby Digital signals around?

First, the widespread migration to Dolby Digital coincides with the transition to HD. This may also require hybrid working in SD and so, some corresponding video issues such as aspect ratio and additional video processing delays occur throughout the programme chain, particularly in presentation and transmission. Dolby Digital requires more audio channels (six or more) to transport it. The emergence of Dolby E as a possible solution in turn creates a new set of challenges.

To discover some of the issues, we must first examine how surround sound really works end to end. Dolby Digital relies on specific metadata about the encoded audio, which tells the consumers receiving equipment, how to decode the signal and reproduce it as it was intended to be heard. Whether listening through a state-of-the-art 5.1 AV system or in mono, the Set Top Box (STB) must present the correct audio mix from its RF, Scart, HDMI, stereo phono or digital output connectors. In the various successive stages of monitoring in a typical broadcast chain, such as production, post production, quality control and ingest for play-out, it is also necessary to easily and consistently check the audio, but now there are many more permutations than there were with stereo.

If using Dolby E as a transport stream, this source metadata is further embedded within the Dolby E bitstream and as such, can remain constant throughout a carefully designed transmission process. Whether transmitting live or recorded content this metadata is produced during the Dolby E encoding process using a compatible processor. Prior to final transmission encoding, Dolby E is decoded back into Dolby Digital and the metadata carried onto the consumer.

Among the problems that broadcasters perceive when using Dolby E is that video delay is introduced when you encode and a further delay when you decode; whatever you do with the audio you have to do with the video. Although you can carry out basic cut editing or splicing with Dolby E, you can't easily affect levels once audio is encoded so you have to make sure that what you're encoding and/or embedding is good for transmission. To do anything creative with it you may have to decode back to baseband and then re-encode whilst keeping video synchronised, you may also have to replace or reinsert the metadata synchronously.

If, for example, an HD OB is transmitting live sports remotely, they will produce a transmittable 5.1 mix (with specific metadata values) before uplinking it to the broadcaster. The broadcaster can then monitor the audio quality, ensure metadata values are correct and communicate any technical problems back to the sound supervisor in the truck to make the necessary adjustments.

Dolby E audio should be kept frame accurate with its associated video source when embedding the Dolby E stream within the HD/SDI, which can make this job easier. Manufacturers of useful Dolby E compatible mux/demux interfaces have integrated synchronisers into their products. Two Dolby E streams can be ‘cut' on a Presentation Mixer (whether embedded or not).


The resultant transition will produce a basic cross fade so that any major differences in metadata values will be minimised. If a broadcaster uses live voiceover during transmission to accompany trails or over end credits where the transmitted material is a Dolby E source then some careful thought is required.

A few Dolby E processors on the market allow the insertion or mixing of an external audio source into the bitstream but the functionality doesn't integrate into a system in the usual way that manual voice over using a fader channel on the Pres Mixer has allowed with conventional stereo broadcasts. To perform this trick, you have to decode back down to AES or analogue and present multiple audio channels to the mixer.

You are then faced with the decision of whether to mix a voiceover across left/right, centre or all three channels and then ensure that your mix level and metadata values are compatible so that the viewer isn't reaching for the remote control every time the continuity announcer has something to say, just before or after the commercials (which are always so loud, but that's another story).

There are a few other pitfalls. Most video frame synchronisers repeat or drop video frames in sync mode. When this happens, a downstream Dolby E decoder or STB may ‘pop'. To counter this problem a system designer can either de-embed/decode the Dolby E, retime it and then re-encode or be very careful when choosing video processing equipment to ensure that devices are fully Dolby E compliant.

Dolby E is relatively new but it is already being adopted worldwide as the production and transport standard for multi channel and surround sound audio. For system designers, broadcasters and manufacturers alike, starting to get to grips with HD and Dolby E together make our jobs more challenging than ever before. For audio systems designers, being pushed into the spotlight alongside our more glamorous video colleagues this has been a long time coming. Audio presents just as many exciting challenges as video, as far as the viewing experience is concerned.

TSL

TSL has an audio monitoring unit called the AMU2-8HD Dolby. It comes with a number of audio and video features aimed at the operator who is using embedded audio, Dolby D and Dolby E in an HD/SD environment. For the foreseeable future, it will be necessary to decode and monitor these HD/Dolby sources at any number of technical monitoring positions throughout broadcast facilities. The unit can act as a basic tool for monitoring stereo level, both visually and audibly using external stereo loudspeakers or, as part of a full blown 5.1, HD monitoring position using a set of active surround loudspeakers.

The AMU2-8HD Dolby also provides a reclocked video output to use with an HD monitor, and a downconverted SD output with a letterbox ARC feature when using a 4:3 display with a widescreen source.

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