Numbers stations : when enough DRM recivers become avalable, robust mode
would be best -- anciallary data streams avalable as well...

== FAQ ==
The DRM system uses a type of transmission called COFDM. This means that all
the data, produced from the digitally encoded audio and associated data
signals, is shared out for transmission across a large number of closely
spaced carriers. All these carriers are contained within the allotted
transmission channel. The DRM system is designed so that the number of
carriers can be varied, depending on factors such as the allotted channel
bandwidth and degree of robustness. The system is designed to work within
current 9 or 10kHz wide channels or multiples of these, should it be
possible in the future to use wider bandwidths.

The DRM system can use three different types of audio coding, depending on
the particular option chosen by the broadcaster. MPEG4 AAC audio coding is
used as a general-purpose audio coder and can provide the highest quality.
MPEG4 CELP speech coding is used for high quality speech coding where there
is no musical content and HVXC speech coding can be used to provide a very
low bit rate speech coder. In the case of AAC encoding the system can employ
a bandwidth extension tool, SBR, which allows the perceived audio bandwidth
to be increased over that which would be possible with pure AAC coding using
the same bit rate.

Because the DRM transmission system employs a multiplex to carry the
broadcast signals it is possible to carry audio signals with more than one
type of coding in the same transmission. For example, providing there was
sufficient data capacity available, it would be possible to carry a full
bandwidth AAC audio service alongside a low bit rate HVXC speech signal
carrying a news or information service.

Many transmitters currently in use today are capable of modification so that
they can carry DRM signals. This is the case with the transmitters being
used to provide DRM test transmissions at the moment. However it may not be
cost effective to modify older transmitters and so they may need to be
replaced with new DRM capable transmitters. Both the modified and the new
transmitters will retain the capability to transmit analogue AM if required.

A major factor in determining the suitability of transmitters using
non-linear modulation techniques (e.g. PDM and PSM modulators) for
modification will be that of the bandwidth of the audio modulator. In
general the modulator will need a bandwidth of at least 3 to 4 times the
bandwidth of the transmitted signal. For example a transmitter capable of
handling a 9 or 10kHz wide DRM signal will need a modulator bandwidth of 30
to 40kHz. This bandwidth is likely to be obtainable only with transmitters
having solid-state modulators. The reason for this wide bandwidth
requirement is due to the fact that the DRM signal is constructed by
applying phase modulation to the carrier synthesiser and an amplitude
component to the normal analogue audio input. The two signals pass through
the transmitter and are combined at the modulator to form the OFDM signal.
Delay compensation is applied to the amplitude signal component to
compensate for the different transition times of the two signals through the
transmitter. The lower the modulator bandwidth the less accurate will be the
combination of the two signals to form the OFDM signal.

In the case of linear transmitters a DRM signal can often be successfully
transmitted. This is achieved by applying the composite OFDM signal to the
same point at which the analogue low level signal would normally be applied.
However a high level of linearity will be required if unwanted
intermodulation products are not to be generated both within and outside the
allocated transmission channel. Some additional work may be needed to ensure
that this required level of linearity is obtained.

In some cases it may prove possible to linearise a non-linear transmitter
and then use it to transmit the composite signal as in the linear
transmitter case above. However this will generally considerably reduce the
efficiency of the transmitter.