DRMradio
With DRM it’s not all about high power transmitters and international broadcasting.
There are a number of university and low power radio stations experimenting with DRM. Transmitters operate in the range 10 - 100 watts and broadcast in the 15 MHz (19m) or 26 MHz (11m) short wave bands.
The radio station uses an amateur radio linear power amplifier with nominal 2 kW power output. DRM signals have a very high ratio between average signal power and statistically occurring signal peaks of about 13 dB (Peak to Average Power Ratio). Also the requirement that the transmitter must be operating in the linear region of the amplifier means only approximately 100 watts output is available from a 2kW amplifier.
The 2 kW Arcomm power amplifier with antenna tuner, power and SWR meter.
Various aerials on the roof with the magnetic loop centre of the picture.
I received this Mode A broadcast using my indoor 1.5 metre vertical; which gave a better quality signal (higher SNR) than my external long wire. This reception was during the morning but have previously received this transmission at night. At the time the transmitter power was 45 watts using a dipole aerial.
The signal was just strong enough to decode some audio and multi-media. Not bad for a 900 km path from Erlangen, Germany to the UK and using Mode A which is the least robust DRM mode.
>>>>
DRM Demodulator/Decoder
Software Version 1.1.4
Starttime (UTC) 2004-06-05 09:08:05
Frequency 15896 kHz
Latitude 51°46'N
Longitude 0°56'W
Label bit eXpress
Bitrate 27.20 kbps
Mode A
Bandwidth 10 kHz
Comment
MINUTE SNR SYNC AUDIO TYPE
0000 14 110 500/10 0
0001 15 150 200/10 0
0002 12 120 150/10 0
0003 9 149 0/10 0
0004 14 150 390/10 0
0005 15 150 810/10 0
0006 15 150 430/10 0
0007 17 150 1190/10 0
0008 16 150 890/10 0
0009 16 150 770/10 0
0010 16 150 870/10 0
0011 15 150 80/10 0
0012 14 150 80/10 0
0013 11 150 40/10 0
0014 10 150 0/10 0
0015 12 150 0/10 0
0016 12 139 410/10 0
0017 14 137 180/10 0
0018 8 127 0/10 0
0019 8 122 0/10 0
0020 10 150 0/10 0
SNR min: 0.0, max: 19.1
CRC:
<<<<
Report log shown graphically using DRMcalc analysis software (written by Carsten Knutter), red line is signal strength and blue line is the number of correctly decoded audio frames -
Bit eXpress didn’t have any QSL cards when I sent the reception report, so they sent
me a baseball cap, T-shirt and some pens instead!
Received QSL card 23/9/2005.
I made a wav recording of this DRM (not the DRM audio but the DRM 12kHz IF signal as seen in the spectrum display screen) and this was analysed by Christopher at Fraunhofer IIS.
Conclusions are that there was no selective fading (as can be seen above) and no multi-path propagation. As the 4-QAM FAC was received correctly any loss of audio was entirely due to the low SNR and changing the transmission mode from Mode A to Mode B would not make much difference.
What would have improved reception is a higher error protection rate. This particular broadcast was using protection level 2 (31.3 kbps total available). Switching to level 1 would improve reception at the expense of audio quality (down to 26.5 kbps total available).
This demonstrates the trade-off that any DRM broadcaster will have to make between audio quality and coverage area, improve one parameter and you make the another parameter worse.
Despite the flat nature of the OFDM spectrum, and no multi-path, reception was via skywave propagation rather than groundwave. It is very unlikely that a groundwave signal could travel this distance.
[Thanks to Thomas Bauernschmitt / Christopher Laske for the pictures and information]
