DVD-Video discs use audio compression giving high quality and, optionally, surround sound. DVD-Video discs can carry up to 8 streams of audio using a number of non-compressed and compressed audio coding formats. Each audio stream can contain from 2 (stereo) to 6 (surround sound) channels depending on the source material available. Multi-channel audio will be down-mixed to stereo in players where there is no surround sound decoder.
The audio coding formats specified differ slightly between PAL and NTSC.
Multiple streams allow multiple languages on a single disc. The number of streams available will depend on the number of channels used per stream, the coding method used and the video bit rate needed.
Dolby Digital (formerly known as Dolby AC-3) provides up to 5.1-channel surround sound and is currently used in some Laserdisc players and discs as well as DVD-Video. The coding format is lossy so some of the original audio quality will be lost. Bit rates from 64kb/s(mono) to 448kb/s are available. Full 5.1 channel surround sound requires at least 384kb/s, but Dolby recommends using the maximum 448kb/s. Stereo audio is normally encoded at 192kb/s. DVD-Video players will output the 5.1 channel Dolby Digital (excluding the low frequency effects channel) to Dolby Surround (ProLogic) via the analogue stereo outputs for use where there is no Dolby Digital decoder.
The MPEG video encoding formats include audio encoding, which use lossy compression.
MPEG-2 provides 5.1 or 7.1-channel surround sound and can be either CBR or VBR. CBR bit rates can be between 32kb/s and 912kb/s, 384kb/s being the average. The sampling rate is fixed at 48kHz. The 7.1 channel option adds left-centre and right centre speakers.
MPEG-1 layer II stereo audio, as used for Video CD. MPEG-1 can only be CBR with bit rates up to 224 kb/s. Layer III, known as MP3, is not supported in the DVD standard but some players will play MP3 files. MPEG-1 and MPEG-2 audio coding are identical for stereo audio, so MPEG-2 audio is backward compatible with MPEG-1 decoders.
Linear PCM (LPCM) offers an alternative uncompressed audio format that is similar to CD audio, but with higher sampling frequencies and quantization. LPCM offers up to 8 channels of 48kHz or 96kHz sampling frequency and 16, 20 or 24 bits per sample but not all at the same time. These values compare with 44.1kHz and 16 bits as used for CD audio. The maximum bit rate is 6.144 Mb/s, which is much higher than Dolby Digital or MPEG-2 coding. LPCM offers high quality (similar to DVD-Audio) but its high data rate leaves little bandwidth for video.
DTS (Digital Theater Systems) Digital Surround is an optional 5.1 channel audio format that has become quite popular for DVD. DTS uses lossy compression with a sampling frequency of 48 kHz at up to 20 bits per sample. The data rate can range from 64 kbps to 1.536 Mb/s, with typical rates of 768 and 1536.
|NTSC disc:||Linear PCM or Dolby AC-3||Linear PCM, Dolby Digital, MPEG or others|
|PAL/SECAM disc:||Linear PCM or MPEG-1 or MPEG-2 or Dolby AC-3||Linear PCM, Dolby Digital, MPEG or others|
Players for PAL/SECAM areas should be capable of decoding LPCM, Dolby Digital and MPEG and provide stereo outputs as a minimum.
Multi-language Audio At least three surround sound streams are possible while maintaining sufficient data for the video if either Dolby Digital or MPEG-2 encoding are used. Three examples for implementing multiple languages are shown in the table with the corresponding bit rates needed.
|1||1 surround sound channel||448 + (3 * 128) = 832 kb/s|
|3 mono (centre) speech channels|
|2||1 surround sound channel||448 + (3 * 256) = 1,216 kb/s|
|3 stereo speech channels|
|3||3 surround sound channels||3 * 448 = 1,344 kb/s|
In each case the data rate for surround sound Dolby Digital is the recommended 448kb/s.
|TV||TV or monitor|
|L||Left front speaker|
|R||Right front speaker|
|Ls||Left surround speaker|
|Rs||Right surround speaker|
|LFE||Low Frequency Effects speaker (subwoofer)|
Many movies include one or more surround sound channels usually encoded using Dolby Digital. With the correct surround sound amplifier and speaker setup very realistic results can be obtained. Some players include surround sound decoders but for those that do not separate decoder/amplifiers are available.