With these sampling frequencies, the duration of the audio frame corresponds to :
The ISO/IEC 11172-3 audio coding standard can be used together with both, MPEG-1 and MPEG-2 Video as long as only two-channel stereo is required. MPEG- 2 Audio (ISO/IEC 13818-3) provides the extension to 3/2 multichannel audio and an optional low frequency enhancement channel (LFE).
Multichannel audio systems provide enhanced stereophonic stereo performance compared to conventional two channel audio systems. It is recognised that improved presentation performance is desirable not only for applications with accompanying picture but also for audio-only applications. A universal and compatible multichannel audio system applicable to satellite or terrestrial television broadcasting, digital audio broadcasting (terrestrial and satellite), as well as other non-broadcasting media, e.g.,
| CATV | Cable TV Distribution |
| CDAD | Cable Digital Audio Distribution |
| ENG | Electronic News Gathering (including Satellite News Gathering) |
| IPC | Interpersonal Communications (video conference, videophone, etc.) |
| ISM | Interactive Storage Media (optical disks, etc.) |
| NDB | Network Database Services (via ATM, etc.) |
| DSM | Digital Storage Media (digital VTR, etc.) |
| EC | Electronic Cinema |
| HTT | Home Television Theatre |
| ISDN | Integrated Services Digital Network |
This document describes an audio subband coding system called ISO/MPEG-Audio Multichannel, which can be used to transfer high quality digital multichannel and/or multilingual audio information on storage media or transmission channels with limited capacity. One of the basic features is the backwards compatibility to ISO/IEC 11172-3 coded mono, stereo or dual channel audio programs. It is designed for use in different applications as considered by the ISO/MPEG audio group and the specialist groups TG10/1, 10/2 and 10/3 of the ITU-R (previously CCIR).
As regards stereophonic presentation, specialist groups of ITU-R, SMPTE, and EBU recommend the use of an additional center loudspeaker channel C and two surround loudspeaker channels LS and RS, augmenting the front left and right loudspeaker channels L and R. This reference audio format is referred to as "3/2- stereo" (3 front / 2 surround loudspeaker channels), and requires the transmission of five appropriately formatted audio signals.
For picture accompanying audio applications (e.g. HDTV), the three front loudspeaker channels ensure sufficient directional stability and clarity of the picture related frontal images, according to the common practice in the cinema. The dominant benefit is the "stable center", which is guaranteed at any location of the listener and important for most of the dialogue.
Additionally, for audio-only applications, the 3/2-stereo format has been found to be an improvement over two-channel stereophony. The addition of one pair of surround loudspeaker channels allows improved realism of auditory ambience.
A low frequency enhancement channel (in this document called LFE channel) can, optionally, be added to any of these configurations. The purpose of this channel is to enable listeners, who choose to, to extend the low frequency content of the reproduced program in terms of both frequency and level. In this way it is the same as the LFE channel proposed by the film industry for their digital sound systems.
The LFE channel should not be used for the entire low frequency content of the multichannel sound presentation. The LFE channel is an option at the receiver, and thus should only carry low frequency sound effects, which may have a high level. The LFE channel is not included in any dematrixing operation in the decoder. The sampling frequency of the LFE channel corresponds to the sampling frequency of the main channels, divided by a factor of 96. This provides 12 LFE samples within one audio frame. The LFE channel is capable of handling signals in the range from 15 Hz to 120 Hz.
Downwards compatibility.
A hierarchy of audio formats providing a lower number of loudspeaker channels and reduced presentation performance (down to 2/0-stereo or even mono) and a corresponding set of downwards mixing equations are recommended in ITU-R Recommendation 775 : "Multichannel stereophonic audio system with and without accompanying picture", November 1992. Useful alternative lower level audio formats are 3/1, 3/0, 2/2, 2/1, 2/0, and 1/0, which may be used in circumstances where economic or channel capacity constraints apply. Corresponding loudspeaker arrangements are 3/2, 3/1, 3/0, 2/2, 2/1, 2/0, and 1/0.
Backwards compatibility.
For several applications, it is the intention to extend the existing 2/0-stereo sound system by transmitting additional audio channels (center, surround), without making use of simulcast operation. This provision of backwards compatibility with existing receivers implies the use of compatibility matrices: the decoder of the previous generation must reproduce the two conventional basic stereo signals Lo/Ro, and the multichannel decoder produces the complete 3/2-stereo presentation L´/C´/R´/LS´/RS´ from the basic stereo signal, and the extension signals.
Particularly for HDTV applications, multichannel stereo performance and bilingual programs or multilingual commentaries are required. This standard provides for alternative audio channel configurations in the five-channel sound system, for example a bilingual 2/0 stereo program or one 2/0, 3/0 stereo sound plus accompanying services (e.g. "clean dialogue" for the hard-of-hearing, commentary for the visually impaired, multilingual commentary etc.). An important configuration is the reproduction of commentary dialogue (e.g. via center loudspeaker) together with the common music/effect stereo downmix (examples are documentation film, sport reports).
The transmission of the five audio signals of a 3/2 sound system requires five transmission channels (although, in the context of bitrate reduced signals, these are not necessarily independent). In order that two of the transmitted signals can provide a stereo service on their own, the source sound signals are generally combined in a linear matrix prior to encoding. These combined signals (and their transmission channels) are identified by the notation T0, T1, T2, T3 and T4.
Backwards and forwards compatibility with an ISO/IEC 11172-3 decoder is provided. For a multichannel audio bit stream, backwards compatibility means, that an ISO/IEC 11172-3 audio decoder properly decodes the basic stereo information. The basic stereo information consists of a left and right channel that constitute an appropriate downmix of the audio information in all channels, or, optionally, the basic stereo information may consist only of the left and right channel of the multichannel audio configuration. Appropriate downmix equations are given by equation pairs (1) and (2), (3) and (4), and (5) and (6).
| Lo = L + x * C + y * LS | (1) | |
| Ro = R + x * C + z * RS | (2) |
| Lo = L | (3) | |
| Ro = R | (4) |
| Lo = L + x * C - y * jS | (5) | |
| Ro = R + x * C + y * jS | (6) |
Forwards compatibility means that an MPEG 2 multichannel audio decoder is able to decode properly an ISO/IEC 11172-3 audio bit stream.
The following combinations are possible:
| Basic Lo, Ro Stereo | Multichannel Extension |
| Layer II | Layer II mc |
| Layer III | Layer III mc |
| Layer I | Layer II mc |
This document describes the combinations of the basic Lo, Ro stereo of Layer I, II and III and the multichannel extension of Layer II mc and Layer III mc.
The ISO/MPEG-Audio Multichannel system provides full compatibility with the ISO Standard 11172-3. This compatibility is realised by coding the basic stereo information in conformance with ISO/IEC 11172-3 and exploiting the ancillary data field of the ISO/IEC 11172-3 audio frame for the multichannel extension.
The complete ISO/IEC 11172-3 frame incorporates four different types of information:
The variable length of the ancillary data field enables packing the complete extension information of the channels T2/T3/T4 into the first part of the ancillary data field. If the MC encoder does not use all of the ancillary data field for the multichannel extension information, the remaining part of the field can be used for other ancillary data.
The bit rate required for the multichannel extension information may vary on a frame by frame basis, depending on the sound signals. The overall bit rate may be increased above that provided for in ISO/IEC 11172-3 by the use of an optional extension bit stream. The maximum bit rate, including the extension bit stream, is given by the following table:
| Sampling Frequency | Layer | Maximum Total Bit Rate |
| 32 kHz | I | 903 kbits/s |
| 32 kHz | II | 839 kbits/s |
| 32 kHz | III | 775 kbits/s |
| 44.1 kHz | I | 1075 kbits/s |
| 44.1 kHz | II | 1011 kbits/s |
| 44.1 kHz | III | 947 kbits/s |
| 48 kHz | I | 1130 kbits/s |
| 48 kHz | II | 1066 kbits/s |
| 48 kHz | III | 1002 kbits/s |
| Sampling frequencies : | 48, 44,1 or 32 kHz |
| Quantisation : | up to 24 bits/sample PCM resolution |
The following combinations of audio channels can be applied as inputs to the audio encoder:
The different combinations of audio input signals are encoded and transmitted within the up to five available transmission channels T0, T1, T2, T3, and T4, of which channels T0 and T1 are the two basic channels of ISO/IEC 11172-3 and convey the backwards compatible signals Lo and Ro. Transmission channels T2, T3, and T4 together form the multichannel extension information, which is compatibly transmitted within the ISO/IEC 11172-3 ancillary data field.
After multichannel decoding, the up to five audio channels are recovered and can then be presented in any convenient format at the choice of the listeners:
A low frequency enhancement channel can, optionally, be added to any of these configurations.
Outputs may be required to provide discrete signals, or may be combined in accordance with downward mixing, or upwards conversion equations, as defined in ITU-R Recommendation 775.
Dynamic Transmission Channel Switching
In order to provide a better orthogonality between the two compatible signals T0 and T1, and the three additionally transmitted signals T2, T3 and T4, it is necessary to have flexibility in the choice of the channels T2, T3 and T4. ISO/IEC 13818-3 allows, independently for a number of frequency regions, the selection of any three out of the five signals L, C, R, LS, RS to be transmitted in T2, T3, T4.
Dynamic Crosstalk
According to a binaural hearing model, it is possible to determine the portion of the stereophonic signal which is irrelevant with respect to the spatial perception of the stereophonic presentation. The stereo-irrelevant signal components are not masked, but they do not contribute to the localisation of sound sources. They are ignored in the binaural processor of the human auditory system. Therefore, stereo- irrelevant components of any stereo signal (L, C, R, LS or RS) may be reproduced via any loudspeaker, or via several loudspeakers of the arrangement, without affecting the stereophonic impression. This can be done independently for a number of frequency regions.
Adaptive Multichannel Prediction
In order to make use of the statistical inter-channel dependencies, adaptive multichannel prediction is used for redundancy reduction. Instead of transmitting the actual signals in the transmission channels T2, T3, T4, the corresponding prediction error signals are transmitted. A predictor of up to 2nd order with delay compensation is used.
Phantom Coding of Center
Due to the fact that the human auditory system uses only intensity cues of the audio signal for localisation at higher frequencies , it is possible to transmit the high frequency part of the center channel in the front left and right channels, constituting a phantom source at the location of the center loudspeaker.
Encoding and decoding are similar to ISO/IEC 11172-3.
Coding modes :
| 3/2, 3/0 + 2/0, 3/1, 2/0 + 2/0, 3/0, 2/2, 2/1, 2/0, 1/0 second stereo program, |
|
| up to 7 additional multilingual or commentary channels, associated services |
Subband filter transforms:
| Number of subbands: | 32 | |
| Sampling frequency: | Fs/32 | |
| Bandwidth of subbands: | Fs/64 |
Additional decomposition by MDCT (Layer III only):
| Frequency Resolution: | 6 or 18 components per subband |
LFEC filter transform:
| Number of LFECs: | 1 | |
| Sampling frequency: | Fs/96 | |
| Bandwidth of LFEC: | 125 Hz |
| Dynamic range : | more than 20 bits |