A: Second question first: Yes, if you do the math, 6 (or 5.1) channels of surround audio at 24-bit/96kHz is more data than a DVD can transfer at one time. The maximum data transfer rate of a DVD is 9.6MB/second. A 5.1 surround track at 24-bit/96kHz requires a data transfer rate of about 13.8MB/second. So the data is compressed using a lossless codec called Meridian Lossless Packing, or MLP for short.
The DVD Forum, the industry group that sets standards for all types of DVD production, selected Meridian Lossless Packing as the specified standard for audio compression on DVD-Audio discs and players. The important word in all this is “lossless;” to maintain the full frequency response and dynamic range of the 24-bit/96kHz surround signal, MLP does not alter the final signal; it just “packs” the audio data to fit a smaller rate and space.
How does MLP work?
The first concept MLP uses is this: audio that is of interest to the human listener contains some redundancy. Second, in music signals, the information content varies with time, and each input channel’s bandwidth is rarely fully used. The MLP encoder uses a variable data rate on most audio content, reducing both the average data rate (i.e. compressed file size) and the instantaneous peak data rate (for high sampling rates such as 96kHz or 192kHz). MLP tackles this by forcing the audio data into two substreams and 6 channels and by maximizing compression at all times using three basic techniques:
- Looking for “dead air” – channels that do not exercise all the available word size, and channels that do not use all the available bandwidth.
- Removing inter-channel correlations (similar data on more than one channel), and efficiently coding the remaining information.
- Smoothing the transfer of high-bandwidth information with buffering.
MLP accomplishes these tasks by using a series of data-manipulation processes:
Prediction – If the values of future audio samples can be predicted, then it is only necessary to transmit the rules of prediction along with the difference between the estimated and actual signals. MLP “assumes” that audio data changes only slightly from sample to sample and predicts what the next sample will contain. Then it encodes only the data that differs from its prediction.
Entropy Coding – Following the prediction step, MLP identifies low-bandwidth material (data that reflects few or no changes, such as silence) and audio data that is duplicated on multiple channels and aggressively compresses that data.
Buffering – Most audio signals can be predicted, but there will be occasional fragments like sibilants, synthesized noise or percussive transients that require high bandwidth. MLP uses a particular form of stream buffering that can reduce the variations in transmitted data rate, absorbing transients that are hard to compress. This buffering creates a constant delay across encoding and decoding. The overall delay is small – typically around 75ms.
Other techniques such as low-pass filtering and bit rate reduction on some channels also factor into the MLP process.
Playing time on DVD-Audio
The MLP process also reduces the size of audio files by 30 to 50%, thus allowing more than the standard 74 minutes of surround audio to fit on a DVD-A. MLP can also be used to simply get longer stereo recording times onto the disc, whether the files are at 96kHz or 192kHz sampling rates. Here are some typical examples of playing times:
- 5.1 channels 24-bit/96kHz: 100 minutes
- 6 channels 24-bit/96kHz: 86 minutes
- 2 channels 24-bit/96kHz: 4 hours
- 2 channels 24-bit/192kHz: 2 hours
- 2 channels 16-bit/44.1kHz: 10 hours
- 1 channel 16-bit/44.1kHz: 20 hours (talking book)
Since Meridian Audio is primarily a manufacturer of high-end home theater hardware, they licensed MLP to Dolby Laboratories for management. Dolby handles further licensing of MLP to DVD player manufacturers and software publishers who incorporate it into their DVD encoding programs.