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The maximum data output rate of a DVD-Audio disc is 9.6 Mbits/second. Because this rate is not sufficient to deliver high resolution (24-bit word length at 88.2 or 96kHz) audio in 6 channels data compression is required. In the DVD-Audio specification, Meridian Lossless Packing (more commonly known as MLP encoding) was chosen as the best sounding scheme available. MLP, as its name implies, is a lossless data compression scheme, which means that the information is not changed in any way, the dynamic range of the music is not affected, and neither is any other aspect of the program. The system includes a method of checking that the final information is identical to the source. Both data rate and storage space requirements can be reduced. First the data is pre-processed, then redundant information is removed and the resulting smaller amount of data is packed into a smaller space. There is a further level of data compression in which a sort of "data shorthand" is used called "entropy coding", which is more commonly seen in computer file transfer methods like "Drop Stuff" and "Zip". From an audio purist standpoint less in the signal path is always better, so although the MLP scheme is lossless it is better if we can avoid compression altogether. In the mastering/authoring process we can control how data compression is used. The MLP encoding only takes place if the data rate would exceed 9.6 Mbits/second, so it is the producer's option to avoid using data compression by using lower sample/bit rates in the less important surround channels. There are three groups of sample rates that are allowed within the DVD-Audio specification: 44.1/48kHz, 88.2/96kHz, and 176.4/192kHz. 176.4/192kHz can only be used with 2 channels. Any of the other sample rates can used with up to 6 channels in combination with word lengths of 16, 20, or 24 bits. The format allows mixed sample rates in a 2:1 ratio to be played back on one internal clock. The most important channels for surround music are the front channels, so this is where the highest quality should be. For example we could have our left/center/right front channel audio at 88.2kHz/20bit and the rear channels could be 44.1kHz/20bit. Of course we do not need to use all 6 channels, we could use 3 front 2 rear, or even 2 front 2 rear at the sample rates described above. Part of the new job of producers/engineers is therefore to determine the best possible result based on the variables of length of program, sound quality, and channel configuration. (see the Data Rate Table) There is also a process called "down mixing". This is a process whereby the surround mix is automatically folded down to stereo. MLP can perform this function and so engineers have the option of using MLP's down mixing tools to design some of the characteristics of resulting stereo image. Unless there is a need for precise control over the stereo mix, this eliminates the need for a separate stereo track. For further reading on this subject, take a look at Meridian's on-line library: http://www.meridian-audio.com/m_lib.htm |
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