All India Radio tested the AM single channel Digital Radio Mondiale (DRM) simulcast system on a 100 kW transmitter last year. Unlike the DRM simulcast system that was tested around 2002 which used two adjacent channels, which clearly would not work in the USA, this system used operates on a single channel using an ingenious method of squeezing the digital carriers completely within a +/- 5 kHz channel!
This means that the DRM simulcast signal will occupy LESS bandwidth than a normal AM 10 kHz analog signal. Good-bye first adjacent channel hash! The DRM signal described in US Patent 7170950.
The technique starts by bandlimiting the analog signal to 5 kHz and forming it into a modulation that has the structure of Leonard Kahn’s “compatible single sideband” from the early sixties. The big difference is that today we can use a Digital Signal Processor (DSP) to perform the job that Leonard’s analog gear could never quite get right and stay in alignment. The DSP can also learn the transmitter distortions and compensate for them. The DRM signal is a high level OFDM spectrum of the same magnitude as the average analog modulation (not some little trashy signal on your neighbor’s front porch) that is placed in the unoccupied sideband.
The ODFM signal will interfere with the analog sideband, you say! Here comes the elegant part – A mirror image of the digital signal is placed under the analog sideband. This signal, when received by an envelope detector, will cancel out the ODFM signal in the other sideband!
Apparently, the tests in India went well, with the analog signal receiving no apparent self-interference, and the digital signal having somewhat more range than the analog signal. On the face of it, the analog signal should perform similarly to a well adjusted Powerside ® facility. Remember, compatible single sideband does not intrinsically decrease modulation power, as the total amount of power is doubled in the one sideband. Regular transmitters will not be able to produce the same analog power in a combined modulation scheme because the ODFM sideband power must be included in the peak power that the transmitter develops.
Important considerations of this system:
- Intrinsically single channel – digital sidebands stay within assigned channel
- No adjacent channel interference
- Digital signal range exceeds analog range for single digital program
- ODFM carrier strength is high, providing lots of data capacity in a narrow bandwidth
- Analog reception works fine with a conventional envelope detector
- Decoding the digital signal is very simple in a non-fading environment.
- A Zero-IF receiver with a sound card class Analog to Digital converter and slow DSP will be able to decode simulcast DRM.
- Cheap digital receiver with potentially only two chips and no ceramic filters.
- Low power battery powered radios are practical
- Probably will work with 8 – 10 kHz analog audio in the US
- Current class of DSP digital transmitters may be reprogrammed to transmit this signal.
- Should work will in Single Frequency Networks.
I am surprised that this development has not been publicized here, as this promises to be the first truly workable analog-digital broadcasting system for the AM band. AM broadcasters have been left out in the cold, with no battery powered medium wave digital radios. This technology might just be the solution.
I find this news heartening, seeing the slow to non-existence of growth in AM HD. Perhaps a little competition on the technical field will help AM broadcasters to have a stake in the future.
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