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Audio Coding
DVD-Audio uses high resolution coding with lossless
compression for the highest quality multi-channel surround
sound and maximum flexibility. |
The DVD-Audio specification makes use of a scalable linear PCM multi-channel
and stereo encoding format, down-mixing control and optional audio formats.
The video objects use the same audio encoding as DVD-Video discs.
| Parameter
|
Audio
Object |
Video
Object |
| Coding mode |
LPCM or MLP |
LPCM or Dolby
Digital |
| Sampling frequency (kHz) |
44.1/48/88.2/96/
176.4/192 |
48/96 |
| Bits per sample |
16/20/24 |
16/20/24 |
| Max channels |
6 (@ 96 kHz)
or
2 (@ 176.4/192 kHz) |
6 or 8 |
| Max bit rate |
9.6 Mb/s |
6.144 Mb/s |
As this table shows, DVD-Audio offers sampling rates up to 192kHz for
stereo audio and up to 6 channels at 96kHz maximum for surround sound.
Audio bandwidths of up to 96kHz are therefore possible with signal to
noise ratios up to 144 dB.
Channel Groups
The maximum data rate for the audio data is 9.6Mb/s, which means that
the sampling frequency for multi-channel audio is limited to 96kHz or
less. To make best use of the bandwidth available, multi-channel DVD-Audio
can be encoded as two Channel Groups with different parameters (ie sampling
frequency and quantisations) for each group.
The channel groups can be used for any number of channels from three (L,
R and C) upwards. Note that the sampling frequencies and quantisations
(bits per sample) used must be as shown in the table below.
| Parameter |
Channel Group 1 |
Channel Group 2 |
| Sampling
Frequency |
48kHz |
48kHz |
| 96kHz |
96 or 48kHz |
| 44.1kHz |
44.1kHz |
| 88.2kHz |
88.2 or 44.1kHz |
| Bits per
sample |
16 bits |
16 bits |
| 20 bits |
20 or 16 bits |
| 24 bits |
24, 20 or 16 bits |
For sampling frequencies of 176.4 and 192kHz the number of channels
is two or less, so only one channel group can be used. Down-mixing Down-mixing
of multi-channel audio to stereo is facilitated by the inclusion of down-mix
coefficients to obtain best results. This technique is called Smart Content
(System Managed Audio Resource Technique). 16 coefficient tables can
be re-defined for each Album and the appropriate coefficient table selected
from these 16 for each Track. All players must implement down-mixing.
Meridian Lossless
Packing (MLP)
The use of 6-channel 96/24 audio with PCM encoding would imply a data
rate of over 13 Mb/s, which is higher than the maximum of 9.6kHz allowable. In
addition the playing time would be only about 40 minutes for a DVD-5
disc. Therefore some form of compression is needed to accommodate
the highest quality in surround sound.
For these reasons, the DVD Forum has chosen Meridian Lossless Packing (MLP),
developed by Meridian Audio, a UK company committed to the highest quality
digital audio. MLP is easy to implement and will not alter the decoded
signal in any way. Decoding MLP requires relatively little computing
power even for six channels of 24-bit/96kHz audio. MLP also provides additional
flexibility that allows, for example, quantisations in one-bit steps, eg
22 bits can be chosen where appropriate instead of 20 or 24, for optimal
quality and playing time.
The possible sampling rates, quantisation and numbers of channels
for both PCM and MLP audio are shown in
| Sample Rate (kHz) |
Quantisation |
2
ch |
4
ch |
6
ch |
| 44.1/48 |
16 to 24 |
LPCM/MLP |
| 96/88.2 |
16 |
LPCM/MLP |
| 96/88.2 |
20 or 24 |
LPCM/MLP |
MLP |
| 192/176.4 |
16 to 24 |
MLP |
No |
Examples of typical playback times with MLP are shown below for high quality
stereo, high quality surround sound and CD quality stereo.
Audio
channels |
Configuration |
Playing
Time |
| Single layer |
Dual layer |
| 2 ch |
192kHz, 24bits |
120 mins |
215 mins |
| 6 ch |
96kHz, 24bits |
86 mins |
156 mins |
| 2 ch |
44.1kHz, 16 bits |
13 hours |
23.6 hours |
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