A missed opportunity for DRM+?

In many major cities, the FM band is crammed with many tranmitters making it difficult to find room for DRM+
transmissions if one is trying to go digital by graceful simulcast. This hampers the roll-out of DRM+ in the FM
band as additional frequency spectrum needs to be licensed, if at all available. Redesigning the DRM+ radio
modem may partly solve this problem as redundancy is built-in at 96 kHz RF bandwith. This program channel
redundancy could be released for a more efficient lower RF bandwith mode, that could offer only one program
channel in a sparse bandwith mode, instead of multiple programme channels in the mini-multiplex.

Digital Radio Sweden intends to evaluate whether it is possible to transmit both analog and digital content within
the so-called FM spectrum mask (the maximum frequency space legally permitted for an FM transmitter), an
opportunity that no one seems to yet have considered. The RF bandwidth of DRM+ is too wide (96 kHz) to
allow simultaneous analog and digital broadcasting within the FM spectrum mask. DRM30 however, a
transmission mode that is only 20 kHz wide, makes it possible to accommodate both analog and digital
broadcast within the FM spectrum mask. [See tag Transmission modes and Test C]. 

The DRM30 radio modem however, is not suitable for high frequencies but is optimised for lower frequencies
(LW/MW/SW), which creates problems with Doppler shift when using DRM30 in the FM band. This permits
DRM30 technology used in the FM-band to function relatively well for stationary receivers, whereas mobile
receivers create reception problems already at moderate vehicle speeds.

The problem with DRM30 on the FM band, is however unimportant since our evaluation in question is only
experimental and conceptual and will not be used in practice. If this method would prove to be useful, it should
be recommended to extend the DRM+ standard with an amendment (DRM+ version 2), which could upgrade
the modem to also have reduced RF bandwidth settings below the traditional 96 kHz. Thus, the FM band could
be more effectively utilised during a simulcast transition period and one would be able to squeeze in significantly
more DRM+ transmitters.

The concept to transmit narrowband (DRM30) experimentally in VHF band II seems novel. Therefore Digital
Radio Sweden will implement and test this in simulcast with FM modulation, within the same FM spectrum mask.
This also involves an evaluation of whether a sufficient protection level will be obtained in order not to interfere
with adjacent FM reception.



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