AM bandplan - medium wave
The current broadcast bandplan for medium wave in Europe is based on 9 kHz channel spacing - starting at
531 kHz and ending 1611 kHz. This give 120 potential radio channels but to avoid interference not all channels can be used in each
country. In the UK about 76 channels are used.
AM bandplan - long wave
Long wave also uses a 9 kHz bandplan (153 to 279 kHz)
- however there are some existing long wave radio stations that are unlikely to convert to DRM as their AM carrier is used for data
The BBC uses three synchronised long wave transmitters on 198 kHz (1515 metres) to provide national coverage of Radio
4. These transmitter sites are located at Burghead (50 kW) north Scotland, Westerglen (50 kW) in south Scotland, with the main transmitter
located at Droitwich in central England (500 kW). The stability of the long wave transmitter carrier is very accurate and can be used
as a frequency reference. This long wave transmission also carries a data signal.
Digital data is transmitted by directly modulating
the 198 kHz carrier using bi-phase encoding and provides for 16 different data channels. One data channel is used to transmit an accurate
time code. Another channel is used by the electricity companies for switching domestic storage heaters and setting off-peak electricity
AM bandplan - short wave
Short wave is divided up into many bands allocated to broadcasting and uses a 10 kHz interleaved
channel spacing. Radio stations broadcast on a frequency that ends in either nnnn0 or nnnn5 kHz.
With a COFDM broadcast it it not
necessary to have a frequency guard band between DRM radio stations. On medium wave there is a 9 kHz gap between stations so that
the analogue IF filters in the receiver can demodulate an AM station without the risk of interference from any adjacent co-channel
With COFDM the demodulation is done in the ‘frequency domain’ using Digital Signal Processing software where a brick wall
type filter can be implemented in software.
It is possible for DRM stations to be broadcasting right next to each other. The actual
bandwidth of a ‘10 kHz’ Mode B DRM station is nominally 9.657 kHz so there is no overlap or interference between adjacent two DRM
One of the ‘killer features’ of DRM is the ability to broadcast multimedia but any data stream transmitted
has an adverse impact on the quality of the bit rate available to the audio which has to be reduced accordingly.
To overcome this
I would be in favour of allowing medium wave stations to broadcast using 18 kHz (double channel), this is defined within the ETSI
DRM specification so a DRM radio showing the DRM logo should already incorporate appropriate filters. Some radio stations could continue
to broadcast in 9 kHz channel such as sports or news stations.
Shortwave stations that are broadcasting mainly speech (which
constitutes the majority of broadcasts) or news would find the 10 kHz channel adequate but any station transmitting music or
multimedia should be allowed to broadcast in 20 kHz.
Obviously using a double channel (18 kHz instead of 9 kHz on medium wave)
halves the number of radio channels but more than doubles the available data rate per radio station.
As already explained
Single Frequency Networks makes more efficient use of the radio spectrum so this increases the available frequencies when compared
to AM. Only a DRM station broadcasting in mode A or mode B using 18/20 kHz spectrum can deliver near ‘FM quality’ audio. The audio
quality will be greatly improved and the DRM station can broadcasts other services without having a detrimental effect on the quality
of the main audio. Bi-lingual broadcasts become a possibility as well as transmitting text (news, weather, etc.) or a second audio
voice giving traffic news, weather, etc. This would be an attractive proposition particular for the ‘drive-time’ shows where commercial
radio makes most of its money.
I would prefer to have fewer medium wave DRM radio stations broadcasting good quality audio than
many stations broadcasting mediocre quality (i.e. DAB in the UK). More stations does not necessarily mean more choice just more duplication
of the same radio format.
A medium wave radio station broadcasting DRM will probably need to increase the protection level at
night to compensate for increased reception errors caused by co-channel/skywave interference.
At night time medium wave reception
can deteriorate due to changes in the ionosphere that allow sky wave propagation. As a result more distant radio stations become audible,
these stations are not normally heard during the day and can cause interference. However this change in protection level results
in a reduced bit rate being available. Consequently the quality of the audio will be far from the promised ‘near FM quality’ if single
channel occupancy is being used. (See modes page for more examples of typical bit rates).
Obviously using a double channel (18
kHz instead of 9 kHz on medium wave) halves the number of radio channels but more than doubles the available data rate per radio station.
As already explained Single Frequency Networks makes more efficient use of the radio spectrum so this increases the available frequencies
when compared to AM.
In the UK frequencies between 1642 - 1782 kHz was allocated to analogue cordless telephones (CT1).
There were 8 channels spaced every 20 kHz, these 'cordless' phones were withdrawn from sale in 2000 with a 5 year moratorium before
the frequencies could be re-allocated. In the US these frequencies have been allocated to AM radio.
From ‘Responses to the Consultative
document on the Future of Analogue and CT2 Cordless Telephony in the United Kingdom’
..... the Government has confirmed proposals
to phase-out the use of analogue and CT2 cordless telephony in order to make under-utilised spectrum available for new and innovative
If the radio authorities (Ofcom in the UK) want to promote DRM as an alternative to AM then they should consider allocating
this band exclusively to new DRM radio stations.
This way DRM could be introduced without the need to replace/move existing AM stations.
A DRM radio station broadcasting using 18 kHz radio bandwidth will easily demonstrate the superiority of DRM over AM in terms of audio
quality and data services. This will encourage sales of DRM receivers which in turn will convince the owners of AM radio stations
to convert to DRM. When broadcasting in a 9 kHz channel the advantages of DRM over AM is not so overwhelming with the audio, particularly
rock and pop music, sounding so heavily compressed.