The DRM System

Digital Modulation

The modulation in DRM is a coded Orthogonal Frequency Division Multiplexing (OFDM), which is also used for DVB and DAB. It spreads a digital bit stream over a large number of lower bitrate carriers. Errors in the transmission path are suppressed by interleaving the bit stream in frequency and time and adding error correction. The final data stream is transmitted several bits at a time with radio symbols that are combinations of phase angle and amplitude.

>> The magic of COFDM

Signal Configuration

The DRM signal may contain several data streams. Fast access (FAC) is coded onto four radio symbols and describes signal configuration. The next (SDC) is coded with four or 16 symbols and describes content so that the receicer may decode it. The main signal (MSC) is coded with 16 or 64 symbols (QAM) and contains streams such as radio programmes, textradio, graphics, photos and data. At reception, the total delay is
0.8-2.4 seconds.

>> DRM system specification

Robustness Parameters

The signal's parameters decide how resilient it is to noise and distortion in the transmission path. For stable audio, the bit error rate shall be less than 0,0001. With robust parameters (Code Rate 0,5 and SDC/MSC 4/16 QAM) the station name may be seen from approximately 5 dB signal/noise ratio, and audio decoded from 9 dB. Thus the modest 0,2 kW of the DR Kalundborg 243 kHz long wave DRM test reaches more than 500 km.

>> DRM data applications

Receiver Sensitity

A DRM bit error rate of 0.0001 corresponds to 36 dB carrier/noise ratio for AM at 30 percent modulation. ITU-R BS.1615 states conversion factors for receiver sensitity on internal antennas that correspond to DRM from 39-52 dBµV/m on long waves, 33-46 dBµV/m on medium waves and 13-26 dBµV/m on short waves. The equivalent figures in ITU-R BS.703 for AM are 66, 60 and 40 dBµV/m (at least 14 dB or 25 times more powerful transmitters).

>> DRM coverage planning