A Dolby Digital Primer (part 5)
When you upgrade your room to 5.1 (note when, not if) you will find, as well as losing space to the extra three speakers, that the setting up of the system is more critical than conventional two-channel stereo. The Dolby DP569 encoder will translate your discrete 16, 18 or 20-bit digital audio into a 5.1 channel Dolby Digital bitstream leading to whatever mastering medium you are working with, perhaps even a workstation recording .ac3 files on disk, so you don’t necessarily need to be anywhere near a DVD authoring system. The Dolby DP562 will sit in the monitoring path allowing you to audition the results of the process in a variety of ways typical of a home setup, whether a full home cinema system or common-or-garden TV set with only an RF input. Obtaining a 5.1 channel input signal for the DP569 encoder is no problem at all, an eight-track MDM is perfectly suitable, allowing two spare channels for a conventional stereo mix if desired. Perhaps a workstation capable of a higher resolution, such as a SADiE 24•96 (as I used to test the system) would be preferable. It is nice if all the speakers are identical, but they are unlikely to be so in the home environment. Importantly, even if the centre speaker is not as big as the left and right, it should have a comparable sound quality and should be from the same range of the same manufacturer. Surround speakers can be smaller, with the same character as the front speakers. Two pairs may be installed to cover both the engineer and the client seating area. Ideally also, all speakers should be equidistant from the listener, but since this may not be possible the DP562, and decent domestic decoders, allow delays to be set to compensate for discrepancies.
All speakers must play back at the same level, so calibration using an SPL meter is a necessity. For small mixing rooms, the level of the surround speakers may be reduced by 2dB. For film work, a reference level of -20dBFS (20dB under digital full scale) is set to an acoustic level of 85dB SPL C-weighted. For pure music mixing, level is up to the engineer, although when incorporated into a picture-related format, dialogue should be normalized as outlined above to 85dB Leq for film, 79dB Leq for home use to ensure the dialogue is clear at typical domestic levels. Many domestic systems incorporate bass redirection so that low frequency signals are sent to speakers that can reproduce them, at the user’s discretion. The DP562 decoder allows for this too and, for instance, low frequencies from the centre channel can be sent to the left and right speakers, which may typically be bigger and more capable of handling them adequately. There are a number of other options.
To conclude, Dolby Digital is a whole new world for people used to stereo, both professionally and in the home, and there are a lot of new concepts to absorb. Everything I have outlined here is important and cannot be ignored, if the end user is going to get the best sound his equipment is capable of. Needless to say, the DP569 encoder and DP562 decoder make all of this possible, and in fact they are surprisingly easy to operate considering the complexity of what they have to do. As 5.1 becomes more important in our lives, without doubt an understanding of the concepts behind Dolby Digital and AC-3 will become essential knowledge for every engineer.