Source: One To One Magazine, October 1999 Issue.
DVD-Audio Problems Resolved By MLP Features
The Meridian Lossles Packing (MLP) system, now adopted by the DVD-Forum, has proved to offer more than simple data compression that allows up to six channels of high quality audio onto the DVD-Audio disc. After originally miscalculating the requirements for high quality multi-channel PCM recordings, the forum had to investigate the use of an audio-data compression system to squeeze six channels of PMC audio on the disc. After a long debate in which several different compression schemes were assessed, MLP was adopted. It turns out that one of the key reasons for its adoption was that MLP fixed another problems that the Forum had built into the standard. The first issue was how to provide a perfectly controllable stereo as well as multi-channel PCM version of the mix. ""With MLP, when the signals come in they go through a matrix, which creates two sub-streams,"" explains Peter Craven, who has been heavily involved in the development of MLP. ""The stereo downmix is in one substream and all the other information goes into the second substream. To get multi-channel you need to decode both substreams, but the stereo decoder only has to decode the two-channel substream. This was the real killer as far as the DVD committee was concerned ñ that you can do clever matrixing so that the two channel listener is catered for."" The signal production by the two channel matrix can be verified by the producer and the fact that the MLP stereo stream produces predictable results is helping to increase the acceptance of stereo downmixes says Craven. ""The music industry was at first very suspicious about the downmix. But now they are clamouring for six-channel and really want to get maximum playing time."" The second issue resolved by MLP is that the DVD-Audio specification allows for three PMC channels at one set of sampling and bit rates and the other three at different rates. However, the digital to analogue converters that are used in DVD have to use huge amounts of processing power to trans-convert the PCM streams to handle the different data rates. ""They hadnít thought of that; the fact that the D/A converters are in stereo pairs,"" says Craven, ""and you are in real trouble if you have an odd number of channels at different rates. You can deal with two at one rate and four at another, but three and three is so messy. With MLP bit depth is an irrelevance. If one channel is 16 bit an another is 24 bit it will automatically detect that ñ you donít need anything special in the format to tell you how to deal with that"" Craven also points out that when the audio is data compressed, there is enough storage capacity to keep all the PCM channels at the higher bit and data rates. ""If you donít need the audio bandwidth, all you have to filter off the high frequencies and make the economies. So you can transmit everything at the same sampling rate. That uses very little more bit rate than if you had some of the channels at half the sampling rate. So there is a vast simplification there.""