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Some info on TV broadcast Tim Carol wrote this...

Posted on February 28, 2010 at 12:56 AM

Some info on TV broadcast Tim Carol wrote this...

BEYOND DOLBY DIGITAL

There is little sense in having an emission coding system such as DolbyDigital (AC-3) that can carry 5.1 channels of audio if you can't get5.1 channels to the encoder. In 1996, all commonly used VTRs had onlyfour audio channels; servers could in theory do more but were notgenerally configured that way, and digital plants rarely had more thantwo AES pairs available for routing. Once the Dolby Digital (AC-3)system was in place as part of the ATSC standard, Craig Todd, LouisFielder, Kent Terry and others at Dolby began to investigate ways toefficiently distribute the multiple channels of audio-they foresawissues that were still a few years away from becoming a really bigproblem.

So what is Dolby E? Contrary to some rumors, it is not high-rate DolbyDigital (AC-3). This approach was considered, but there were too manybenefits to be had for starting over with a different set of goals.What I mean by that is the goal of the Dolby Digital (AC-3) system isto deliver up to 5.1 channels of audio to consumers using the fewestbits possible while still preserving excellent audio quality. As wewill see, this is not the goal of Dolby E. To meet this goal, the DolbyDigital (AC-3) encoder is rather complex and takes about 187milliseconds from the time it receives audio until the time it producesa Dolby Digital (AC-3) output. This is analogous to the video encodingprocess-high-quality but low bit-rate means the encoder is going toneed processing time. Although this encoding latency is small (far lessthan video encoding latency) and is taken into account in themultiplexer, this amount of delay is difficult to deal with inproduction and distribution.

Also, while the audio quality of Dolby Digital (AC-3) is very good, itwould not be appropriate to use it for multiple encode/decode cycles.This might enable coding artifacts to become audible. I say mightbecause the artifacts are unpredictable-sometimes you might hear themwith certain material, sometimes you might not. Again, high-rate DolbyDigital (AC-3) minimizes the chance of this occurring, but it could.

Another drawback of using Dolby Digital (AC-3) for distribution is thatalthough its data is packetized into frames, these frames do notregularly line up with video frames (see Fig. 1).

You might be thinking, "PCM audio is packetized into AES frames that donot line up exactly with video frames either so what is the problem?"Good point, but Dolby Digital (AC-3) frames carry bit-rate reduced(i.e. compressed) audio, not baseband audio. Although a video editwould cause little problem for baseband audio, cutting a mid-DolbyDigital (AC-3) frame will cause major problems. After decoding, theresults will be audio mutes if you are lucky, clicks and pops if youare not. Dolby Digital (AC-3) is simply not intended to be used thisway.

This did not stop some early adopters, however, and at least one majorDBS provider used Dolby Digital (AC-3) recorded on one of the AES pairsof a Digital Betacam recorder to carry the 5.1 channel audio of movies.Did it work? Absolutely, and even when the Digital Betacam tapes werenot long enough to hold an entire film and the Dolby Digital (AC-3)stream had to be switched mid-movie, it hardly ever caused a glitch.They were lucky! It can be done, but it is not easy and is notrecommended. Clearly, it was time for a new system designedspecifically for the task.

A TALL ORDER

Some of the original goals for this new system were that it had to bevideo frame-bound so that it could be easily edited or switched, had tobe able to handle multiple encode/decode cycles (at least 10) whilecausing no audible degradation, had to carry eight channels of audioand metadata, had to fit into a standard size AES pair of channels andhad to do its encoding and decoding in less than a video frame.

Dolby E satisfies this tall order

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. The system will accept up to eight channels of baseband PCM audio andmetadata and fit them onto a single 20-bit, 48kHz AES pair (i.e., 1.92Mbps), or it will fit six channels plus metadata into a 16-bit, 48kHzAES pair (i.e., 1.536 Mbps). After decode, PCM audio and metadata areoutput to feed a Dolby Digital (AC-3) encoder.

Fig. 2

In Fig. 2 you can see how Dolby E frames match video frames. Althoughonly NTSC and PAL rates are shown, the system will also work with23.976 and 24 fps material.

Notice the small gap between the Dolby E frames. This is called theGuard Band and is a measurable gap in the bitstream. Switching hereallows a seamless splice of the audio signals. In fact, upon decode,the audio is actually crossfaded at the switch point, which is aremarkable feat for compressed audio.

As the goal for the Dolby E bit-rate reduction algorithm was to enablemultiple encode/decode cycles or concatenations, the audio quality ismaintained for a minimum of 10 generations. This does not mean that at11 generations the audio falls apart, but rather that absence of anyartifacts is no longer guaranteed. I was one of the listening testparticipants at Dolby during the development of the "E" algorithm. Ispent two rather unpleasant afternoons listening to all kinds of audiosamples both pre- and post-ten generation Dolby E. I consider myself acritical listener, but those were some of the hardest listening tests Ihave ever participated in. This was not like comparing apples andoranges, more like comparing two perfect apples-one was ever soslightly different than the other. In a word: Maddening!

WATCH OUT

Dolby E, like Dolby Digital (AC-3), is carried on a standard AES pair.It can be recorded, routed and switched just like standard PCM audio inan AES pair (see Fig. 3).

Fig. 3

However, there are some strict requirements. The path for the AES pairmust be bit-for-bit accurate. This means that there can be no levelchanges, sample rate converters, channel swaps or error concealment inthe path. Remember that although the Dolby E data is in an AES pair, itis not audio until it is eventually decoded. Any processing that causeschanges in the data will destroy the information. These "Gotchas" arehidden everywhere, especially sample rate converters, so be prepared toreally evaluate your facility. An invaluable resource is the Dolby Epartner program, run by Jeff Nelson at Dolby. You can find a bunch ofvery useful information at www.dolby.com/tech/dolbyE_prtnr_prgrm.html. Manufacturers and individual products that have been tested to pass Dolby E are listed here.

When baseband audio is not possible, Dolby E has become the de factostandard. Since it began shipping in September 1999, more than 1,000encoders and decoders have been sold, and the system is the source forvirtually all 5.1 channel Dolby Digital (AC-3) broadcasts here andabroad.

One last point. The question I was probably asked most often was: "Whyis it called Dolby E?" Simple: "E" comes after "D." So, as Steve Lymanlikes to say "Dolby E is for Distribution and Dolby D (i.e. AC-3) isfor Emission."




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