DRM without modification
It's possible to demodulate DRM (Digital Radio Mondiale - the new digital short wave standard) without modifications on the receiver. I used the compiled version of the free DReaM-Software
which is available at http://www.g7ltt.com/drm/software/DReaM.zip.
My 400 MHz-Pentium II did it though this is below the announced requirements.
I set my AOR AR5000 to USB and the bandwidth to 30 khz and tuned in to strong
"RTL 104.6" (the Berlin programme) that is transmitted on 6095 kHz (center) with 10K bandwidth
via Luxemburg. Theoretically the 15K-filter should work better, but it
reduces the upper frequencies too much.
The software automatically detects the offset. It worked when I put the receiver to 6089 khZ,
which resulted in a DRM-signal-range from 1 to 11 kHz. Other values worked too, like 6083 with
7-17 K DRM signal. Unfortunately there were some non DRM-signals occupying the rest of the AF-range.
They probably affect demodulation very much.
If you try it, don't forget to mute your line-bypass! The input signal had to be strong. Best was when the
shifted-PSD spectrum didn't differ more than 10dB over the whole range. I tried to adjust that with a
simple analogue 10-band equalizer from my home stereo.
The software easily detects the station-ID, bitrate, programme type etc. I only got audio
for some seconds. SNR should be more than 25 dB. Next I found the 4-channel Fraunhofer-radio
on 5965, but decoding didn't work. Deutsche Welle on 6180 was the 3rd try, this worked better
than RTL. Wiener and Linear Interpolation didn't make a difference. AGC should be fast. By the way the sound quality was a bit disappointing, far
away from FM or even CD. Date, time and programme info are often wrong (probably
a transmitter error): Deutsche Welle in English is labelled as German, the Fraunhofer Radio shows
weird time & date and so on.
After retrying this experiment with my new AMD2500+ it didn't work better. After all you probably still need a good IF filter or at least a good NF/DSP-pre-filtering.