Using MPEG-4 audio for DRM digital narrowband broadcasting

DRM (Digital Radio Mondiale) is an international consortium, working on the definition of a world-wide standard for digital narrowband broadcasting in short- and medium-wave bands. One major requirement for a digital system in these bands is the bandwidth compatibility with existing analogue sen ices. This means that the system needs to operate with a channel bandwidth of 9 or 10 kHz. This translates to a net bit rate for audio services in the range of 20 to 24 kbit/s; even lower bit rates can be useful for special services. Therefore DRM has a need for highly efficient audio and speech coding. A further requirement is error robustness. Due to the fading characteristics of the channel, signal errors may occur in spite of digital modulation and channel coding. Last but not least DRM wants to use an international standard to make sure that widespread, well-tested and well-supported algorithms are being used. Following these requirements, DRM has chosen parts of MPEG-4 audio for the DRM system. Based on listening tests performed in cooperation with the NADIB project [2], the European predecessor of DRM, MPEG has proposed audio and speech coding algorithms suitable for use within DRM. Since then, more work on these algorithms has been done within version 2 of MPEG-4. MPEG-4 offers highly efficient audio coding by the AAC algorithm as well as state-of-the-art CELP speech coding. Both algorithms are very flexible in terms of codec configuration and bit rate. In addition to that, they support special error robust modes for use in error prone channels. DRM has chosen MPEG-4 AAC for audio coding. The MPEG-4 CELP is a very good candidate for the speech coding part of the system. The DRM specification will be finalized this year. By use of MPEG-II audio and a modulation and coding system custom tailored for the use in short- and medium-wave bands, DRM will offer a new quality of audio service to a huge, worldwide audience.