Digital Radio Mondiale (DRM). Unlike the DAB standard, which uses MPEG II, DRM uses a more modern version of MPEG-4 compression. It includes an adaptive signal compression mechanism AAC (Advanced Audio Coding) in mono and stereo variants, as well as CELP (Code-exited Linear Prediction) for high-quality speech encoding and noise-like signals. In MPEG-4, the long-term prediction is performed not in the time plane, but in the spectral plane. The encoder makes a prediction and then encodes either the difference between the real and predicted signal or the input signal itself if its meaning can be coded more compactly than the difference. In addition, the encoder supports several new mechanisms related to the ability of the stream to adapt to changes in channel parameters. Any of the options can be supplemented with SBR (Spectral Band Replication) technique, designed to improve the quality of high-frequency transmission. When transmitting at frequencies below 30 MHz, all formats except stereo use the 9/10 MHz band. The use of SBR technique requires a wider band.
In addition to audio signals, data can be transmitted in the digital stream. Multiplexed stream of audio and data form the main service channel Main Service Channel (MSC). Up to 4 streams are transmitted to the MSC, each of which carries either audio or data. The MSC channel information is split into logical frames of 400 ms each. In addition to the MSC, two additional channels are formed. The main and service channels are multiplexed in a certain way, resulting in the formation of transport superframes with a duration of 1200 ms.
The first additional channel, Fast Access Channel – FAC, transfers data about the radio frequency signal parameters and information that allows to allocate individual services. The signal parameters include the stream identifier, bandwidth, modulation type, encoding type, the alternation depth index, the number of services transmitted. These parameters are sent in each FAC frame. The parameters characterizing the services include specifying the type of service (audio/data), conditional access flag, language pointer and some others. They are transmitted sequentially: in one frame there are parameters that belong to the same service.
The second additional channel, Service Description Channel – SDC, contains information related to conditional access, program schedule, copyright information, supporting information for some applications, and links to alternative frequencies on which the same channel is transmitted. The SDC information is placed at the beginning of each superframe and starts with links to alternate frequencies, which allows to automatically select the channel that is currently being received in the best way.
DRM, like DAB, uses the COFDM modulation system. This system is very effective for transmitting signals over a radio channel with multi-beam propagation of radio waves and selective fading of the signal, which is typical for short waves. A guard interval is used to compensate for multipath interference. It should not exceed 20% of the total duration of the symbol, in order not to reduce the bandwidth of the channel. The number of carriers placed in the channel frequency band is limited by the signal frequency Doppler shift occurring in the mobile reception mode. Taking these factors into account, about 200 carriers are used in the 9/10 kHz band. Their exact number, as well as the duration of the symbol and the guard interval, depends on the nature of the radio waves propagation (surface or spatial), the estimated transmission distance and the required reliability.
The channels included in the MSC are divided into 2 parts, which differ in the importance of information for proper decoding. They are subjected to separate noise-resistant coding, characterized by varying degrees of interference immunity. As noise-resistant coding, data interlacing and convolution coding with code rates from 0.5 to 0.8 are used. Data alternation in COFDM systems is carried out both in time and frequency, which allows to restore the signal at a high level of selective fading in a radio channel. In addition, to combat this phenomenon, pilot signals are introduced into the stream, allowing the receiver to assess the degree of signal attenuation at each carrier frequency. The level of protection imposed also depends on the band and expected range of the signal. In particular, when transmitting on short waves, the depth of interleave is 2.4 s, and on long and medium waves — 0.8 s. Moreover, convolutional encoding with a lower speed code is used on short waves, and a larger number of pilot signals is entered.
DAB/DRM/RAVIS comparative analysis- spectral efficiency (bit/sec/Hz): 0,38-1,1/0,39-1,9/0,77-3,64