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DRM modesDRM has 4 transmission modes, referred to as Mode A, B, C, and D.
Mode A is most likely to be used on medium and long wave using ground wave propagation for local broadcasts. Mode B is the most common mode for single hop short wave broadcasts and most broadcasts within Europe use this mode. Night time or international medium wave broadcasts (via sky wave) may use Mode B instead of Mode A. Mode C for long path multi-hop broadcasts on short wave. For example direct broadcasts from New Zealand or Australia being beamed into Europe. Mode D is the most robust and could be used for Near Vertical Incidence Skywave (NVIS) broadcasts where the radio signal is broadcast upward and the ionosphere reflects the signal back down. This mode is little used for broadcasting in Europe but radio amateurs have recently been allowed to broadcast in the 5 MHz band to asses whether NVIS could be used for national coverage. NVIS is common in the tropical short wave bands as it gives considerable area coverage with a single transmitter located in the centre of the broadcast area. Mode A and BAt the bit rates available for Mode A and Mode B using AAC and SBR frequency enhancement, gives 15.2 kHz audio bandwidth (equivalent to VHF-FM radio) at the DRM receiver but thanks to AACplus compression only requires 9/10 kHz of radio spectrum to achieve this. For most DRM broadcasts in Mode A and Mode B protection level 1 is typical. Mode B is the most common mode used for short-wave broadcasts. The lower the protection level number the more robust the signal is against selective fading and multi-path interference. Only the higher bit rates available in Mode A and Mode B could be described as ‘near FM quality’ - the main selling point of DRM. However to achieve these higher bit rates the protection ratio has to be kept low so the received signal is more susceptible to interference or fading. The following tables show the maximum aacPlus bit rate available for the given mode and protection level. There is overlap between the available bit rates on different modes. It is up to the broadcaster to decide which mode/bit rate best suits the propagation path from the transmitter to the target area. If the radio station is also broadcasting a data service then the bit rate allocated to the audio is reduced accordingly. All figures given assume EEP (Equal Error Protection).
Typical bit rates available for DRM broadcasting in 9 kHz channel (medium wave) channel using 64-QAM for the MSC.
Typical bit rates available for Mode B DRM broadcasting in 10 kHz channel (short-wave) channel. For a more robust Mode B short-wave broadcast using 16-QAM for the MSC then only the AAC codec can be used - no SBR. Mode C and DDRM Mode C and Mode D are only used on short-wave and demonstrate the trade off between making the signal robust against fading and multi-path signals and the actual data carrying capacity. Audio quality can be poor and comparable with AM broadcasts as many broadcasts can only use the AAC codec as there is insufficient data rate to support both AAC and SBR codecs. With Mode C and Mode D broadcasts using a high protection level then the audio quality (bandwidth) can sound worse than AM and obviously there is no possibility of including multimedia with these broadcast. Although the audio may be poor quality, provided there is sufficient signal-to-noise ratio at the receiver then the audio will not have any of the characteristics propagation fading or distortion of an equivalent AM signal.
Mode C transmitting in 10 kHz channel.
Mode D transmitting in 10 kHz channel.
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