INTERNATIONAL ORGANISATION FOR STANDARDISATION
ORGANISATION INTERNATIONALE DE NORMALISATION
ISO/IEC JTC1/SC29/WG11
CODING OF MOVING PICTURES AND AUDIO

ISO/IEC JTC1/SC29/WG11 N8185
April 2006, Montreux, Switzerland

Source: Audio Subgroup
Status: Approved
Title: MPEG-2 Audio Layer I/II
Author: Werner Oomen, Leon van de Kerkhof

MPEG-2 Audio Layer I/II

The MPEG-2 Layers I and II formats define support for lower sampling rates. In addition the MPEG-2 standard defines an extension to multi-channel that is backwards compatible to MPEG-1, and inclusion of up to seven multi-lingual tracks for mixing with the multi channel audio content.

MPEG-2 adds support for the lower sampling frequencies 16, 22.05 and 24 kHz and bit rates down to 8 kb/s.

MPEG-2 audio also adds a compatible extension to MPEG-1 audio encoding, which enables the transmission of stereo and multichannel audio in a single bitstream. It can operate at a wide range of bitrates up to more than 1 Mbit/s. A 5.1 multichannel (five full bandwidth channels plus a Low Frequency Enhancement channel) movie soundtrack requires an average bit rate of 384-640 kbit/s.

The MPEG-2 standard was designed with compatibility as a major consideration. The core of the MPEG-2 bitstream is an MPEG1 bitstream, which enables fully compatible decoding of a multi-channel bitstream by a low complexity MPEG1 audio decoder. As illustrated in Figure 2, an MPEG-1decoder will decode the stereo part of the MPEG-2 frame, representing an appropriate (stereo) 'downmix' of all channels in the multichannel frame, and ignore the multichannel extension.

Figure  1 – MPEG-2 backwards compatibility to MPEG-1