DRM

Digital Radio Mondiale (DRM) is a system for digital radio in the long, medium and short wave broadcasting bands. As such, DRM is a digital replacement for the old analogue AM radio for the broadcasting bands below 30 MHz. In a later stage the DRM consortium took on the task to extend the DRM standard to the frequency bands up to 120 MHz. This makes it possible to replace the analogue FM-radio with DRM as well. Originally, DRM was an European initiative. Nowadays, it has world-wide support.

The analogue AM radio suffers from a poor quality of the audio and poor reception quality. DRM is a huge improvement compared to the analogue AM radio. Its main advantages for the listener are:

• Audio quality comparable to analogue FM,
  The audio quality of the analogue AM radio is rather poor due to the limited bandwidth. State of the art audio compression makes it possible to deliver a good sound quality in this limited bandwidth.
• Improved quality of reception,
  In an analogue AM radio transmission all disturbances as noise, interference and multipath fading have an effect on the amplitude of the reception and therefore cause audible impairments. DRM can cope much better with these disturbances.
• Easy to tune in,
  Part of the data capacity of DRM is used for service data, to make it possible to tune to a station by selection out of a list
• Data services,
  The data capacity can also be used for data services like traffic and text information as well as pictures.

The listener will need a new radio to receive DRM.

DRM has also advantageous for the broadcaster. First of all DRM saves on transmission power. In an analogue AM transmission most of the energy is in the carrier. Huge power savings are achievable while maintaining the coverage area. Another advantage is that it is possible to use DRM in a Single Frequency Network (SFN). This makes it possible to extend the coverage area without the need for additional frequencies.

The DRM system is fully compatible with the current radio channel arrangements of the long, medium and short wave broadcasting bands. This makes a smooth transition from analogue to digital possible. The chances that have to be made to the analogue radio transmitters are moderate.

Audio coding

DRM uses MPEG-4 Advanced Audio Coding (AAC). To reach a sound quality that is comparable to FM-radio the AAC is extended by Spectral Band Replication (SBR), an enhancement to improve the quality of the high tones.

For speech, there are two speech codecs available, a CELP (Code Excited Linear Prediction) code for higher bit rates and HVXC (Harmonic Vector eXcitation Coding) for low bit rates. Both codes are part of the MPEG-4 standard.

Transmission modes

DRM uses the available radio channels with a radio channel bandwidth of 9 or 10 kHz. It is also possible to use only half of the channel (4.5 or 5 kHz) or two channels (18 or 20 kHz). In this channel, the data is transmitted through the use of a large number of carriers (e.g. 88 - 226 carriers in a 10 kHz channel). Each carrier is used to transmit a small portion of the data. This is called Orthogonal Frequency Division Multiplexing (OFDM). The amount of carriers and the amount of data per carrier can be varied upon the quality of the transmission channel available.