One thing that you can say about the guys at Dolby Laboratories is that they are deep thinkers. They plan ahead to cover all possibilities, and look backwards to maintain compatibility with existing technologies where possible. AC-3 is no exception. Dolby Digital, incorporating the AC-3 algorithm, in the cinema is backwards compatible with old technology since it doesn’t displace the traditional optical sound track. Cinema owners who don’t care to equip themselves with the latest technology can still show the same print in Dolby Stereo (now known simply as Dolby) – and that doesn’t sound too bad even when played without Dolby decoding. That’s backwards compatibility for you! Looking forward, Dolby Digital as a discrete 5.1 channel encoding system has sufficient flexibility to incorporate additional data that can be used to reinterpret the dynamic range and channel format of the original in a variety of ways to suit a variety of listening circumstances, as outlined above. Some options are present only in the decoder, but must be anticipated during the encoding process, others are within the domain of the engineer to modify as necessary to achieve optimum quality under a wide variety of playback conditions.
Of course, one is entitled to wonder how it is possible to slim down the traditionally accepted data rate of around 700kbits/s per channel (CD quality) to 448kbits/s or less for 5.1 channels for delivery into the home. If you examine these figures then it is evident that AC-3 must discard around seven-eighths of the information originally contained in a recording made at CD quality in each channel, retaining only one eighth, or less if a lower bit rate is used (possible rates range from 56kbits/s to 640kbits/s). It makes you wonder whether we really pay attention to what we hear. Still, the nutritional content of the pudding is best appreciated by its consumption, and we find that for most people, AC-3 is subjectively equivalent to CD quality. Indeed, in terms of its dynamic range it can be better and, as we shall see, it is very much more versatile.
As mentioned above, dynamic range is a major problem in domestic listening. Some people want cinema quality sound, and have appropriate circumstances to enjoy the high levels involved. Others require a reduced dynamic range for, say, late night listening. Dolby Digital, of which the AC-3 algorithm is a component, will accept a word length of 16, 18 or 20 bits as its input, and the algorithm handles a full 24-bit dynamic range. The advantage of coding to such a wide dynamic range is that during the decoding process, the dynamic range can be reduced if necessary in a variety of ways to suit the user. The dynamic range processing of Dolby Digital consists of two components – Dialogue Normalization and Dynamic Range Compression. Let’s consider Dialogue Normalization first. Standardization of playback levels in the cinema has been pursued as a goal for years and, in part thanks to THX, it is now seen as a very desirable thing that the cinema audience should be treated to sound levels that are the same as those heard in the dubbing theatre. Levels in both the cinema and domestic environments are most usefully measured according to the equivalent loudness method, Leq, which is the long term average of sound pressure level (SPL) with A-weighting. Leq, according to Dolby, corresponds well to perceived loudness and from it can be derived a measurement in terms of dBFS Leq, which is the level of a digital signal with reference to a full scale digital signal, measured according to the Leq method.