A Dolby Digital Primer (part 1)
1989 was a very good year. Something very important happened and now, ten years later, all of our working lives are affected by it - or soon will be. In 1989 a SMPTE subgroup looked into the pressing issue of how many channels a digital film soundtrack should have. They came up with the magic number of 5.1 - Left, Centre, Right, Left Surround, Right Surround and Low Frequency Effects or LFE (sometimes known as the subwoofer channel). Meanwhile, Dolby Laboratories had been hard at work developing digital audio data compression systems such as the AC-1 delta modulation coding system which employed 4-2-4 matrixing technology, similar to the original analogue Dolby Stereo, to achieve four channel surround sound suitable for the high definition television systems then being proposed. Also in 1989, Dolby raised their game to the AC-2 standard which lowered the bit rate and increased the quality, while still retaining matrixed surround. A year later and Dolby had taken a giant leap forward conceptually to AC-3 - a discrete multichannel system with yet a further reduction in bit rate (per channel). AC-3 and the cinema were natural partners. Dolby developed a digital cinema sound system that employed AC-3 5.1 channel coding at a bit rate of 320kbits/s with the digital audio data printed between the sprocket holes - an area of the film that had proved in tests to be surprisingly resistant to damage. The first AC-3 film was Star Trek VI in 1991 followed by the formal announcement of Dolby SR-D cinema sound for Batman Returns in the following year. Dolby SR-D is now known simply as Dolby Digital. Achieving this much can hardly have been easy, but cinema sound is quite specific in its requirements. Firstly, if a sound track is encoded in 5.1 channels, then it will be played in 5.1 channels - there is no point in doing anything else. Secondly, 35mm film shown in a cinema is a medium that is consistent in its requirements, so the bit rate need never be any other than 320kbits/s. Additionally, cinema audiences have a simple requirement of the sound that they hear - they want the best sound possible, at a standardized level with an impressive dynamic range. The soundtrack will hopefully achieve this, but this is the only use to which it will be put.
The application of AC-3 to the cinema is therefore a straightforward matter of solving a single set of problems. Applying AC-3 to domestic media is a totally different matter. It would be nice if at a stroke all current home entertainment systems could be deactivated, necessitating replacement with the latest and best systems that technology can offer. Unfortunately, this isn’t likely to happen. Relatively few enthusiasts have a full 5.1 channel home cinema installation. Rather more are content with their existing Dolby Surround and Pro Logic equipment. Most of us however settle for old fashioned two channel stereo or even , in the most die-hard of cases, glorious mono! Any digital encoding system would have to cater for all of these options. Even if a home cinema enthusiast has equipped himself with a splendid array of loudspeakers, are they all going to be identical - as ideally they should be - or will the surround speakers be smaller? Will the centre speaker be smaller to fit under or above the TV? Will there be a subwoofer, or are the other speakers expected to handle the low frequency component of the sound track? Take another case - a would-be home cinema owner is building up his system incrementally, starting with a DVD player perhaps. He doesn’t want to buy any additional equipment at this stage so he plays it through the RF (aerial) input of his TV. Not only is it mono, but it is mono that is modulated onto an RF carrier, and demodulated in the set. How is a signal best heard in 5.1 format going to withstand this kind of treatment? Add another scenario - a late night listener who doesn’t want to annoy his neighbours with excessively loud action sequences played at a level that makes best use of the available dynamic range. If he turns the level down, he won’t be able to hear the quiet bits. And what about that age-old problem - you set the sound to an appropriate level to watch a feature film and hear the dialogue easily - then on comes a commercial and people can hear it all the way down the street? These are all real and significant problems that are not encountered in the cinema. Basic AC-3 therefore has to be extended to cope.