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View Full Version : Modded my Icom 725, but no DRM audio decoding.


Damon
26-11-2006, 22:02
Greetings.

I purchased the 9.0115 MHz to 12 KHz converter from I5XWW via eBay. Crispino was excellent and had the converter posted from Italy to my doorstep in the US in only 4 days. The unit included good general instructions, and after some trial and error, I have the converter working in my old Icom IC-725. I tapped the IF at the empty slot for the CW filter.

I fired up DReaM on 9800 KHz, and immediately got about a 13dB SNR. The RCI banner shows up, and all lights are green, except for MSC CRC, which I have never seen go green. It stays red. I get NO AUDIO no matter what I do.

I have made no changes to the default DReaM config other than select "Flip Input Spectrum" per I5XWW.

How do I begin troubleshooting my lack of audio? My computer is a Latitude D820 with the SigmaTel sound chipset.

Here is a screenshot of DReaM running on my machine:

http://static.flickr.com/107/307027836_1a94ef3a1e_o.jpg

I would appreciate any advice you can offer.

Damon Cassell
Salem, Massachusetts

simone
26-11-2006, 22:17
Hi Damon,
the SNR is too low to get audio decoding on this transmission, so maybe nothing is wrong with your setup, you just need a better signal. What kind of antenna are you using? And can you please post a screenshot of the MER plot? Do you run the laptop on batteries, if not the problem could be interference from the power supply.
Simone

Damon
26-11-2006, 22:38
Hi Simone, thanks for the quick reply.

My antenna is an outdoor 40m dipole at about 15m height. I get very good signals here. For example, 9800 KHz will be S9+20. 7370 KHz is also very strong, about S9+40.

I do not run the laptop on batteries. It is plugged into the AC mains. I tried using just the batteries but there was no difference.

Can you tell me how to generate a MER plot? What is the minimum SNR required to decode audio?

zfyoung
27-11-2006, 02:03
Observing the signal strength from your input PSD, I think the SNR should not be so low if the channel is clean. Don't forget to insert a ferret coil between your down-converter and your computer lest the noise from your PC interferers with your receiver. Otherwise, the phase noise of LO is too high? I did't use ICOM 725 before, so you have to check it out yourself.


PS:SNR spectrum in the chart selector can show if the channel is clean or not.

simone
27-11-2006, 04:07
Hi Simone, thanks for the quick reply.
can you tell me how to generate a MER plot? What is the minimum SNR required to decode audio?
Hi Damon,
MER plot, thatīs called SNR spectrum in Dream, please try again with batteries to see if there is a difference, you would need about 20dB SNR on this transmission, Impulse Response would be interesting too.
Does using the attenuator change anything and using manual gain instead of AGC?
Simone

Damon
27-11-2006, 10:32
Thanks for the suggestions. I will install ferrites on the IF lead, and I will also post the requested plots here. Strong DRM transmissions here are infrequent, so it may take a few days.

Damon
27-11-2006, 22:50
Tonight I cleaned up my work a bit, replacing an unshielded IF tap with some shielded cable, shortened up some of the internal connections and also added several ferrites to the lead going out to the computer. I was then able to successfully decode the 7370 KHz DRM signal. Decode had many drop outs while I adjusted levels. I still have some cleanup work to do inside the radio, so hopefully things will get better from here. But overall, the modification of my Icom 725 for DRM seems successful. I will post some more details in a few days about my modification, with pictures, if anyone wants to modify an Icom 725 like I did.

Here is the plot for my reception of 7370 tonight (11/27):

CT4RK
29-11-2006, 13:31
Hi Damon1
Are you using the correct FI bandwide? SSB or narrow band AM filters, dont work proplly! IF you dont have at less filters with 10 KHz bandwide, on AM mode, try use the FM filters, by change mode for FM!

Damon
29-11-2006, 13:47
I am using the CW-N (narrow) mode because my IF mixer is tapped in where the CW narrow filter normally would be (the filter itself is absent).

Over the last several nights, I've had pretty good reception of DRM on 9800 and 7370 KHz. On 9800 KHz (CBC), I've averaged about 28+ dB SNR, which I understand is quite good, with peaks up to 33 to 34 dB.

I think the biggest problem at this point is that the few hours of DRM transmissions to North America that I can tune happen when propagation becomes somewhat unstable. 9800 KHz really starts to die around 0000Z, but is good up until then. The 7370 KHz CRI transmission is reasonably good, but only lasts one hour. The 6080 KHz transmission is heavily interfered with by other strong signals. The rest of the North America transmissions happen when I'm at work and thus unable to listen.

I will post some plots later.

Damon
29-11-2006, 22:50
Here is a plot of Vatican Radio 7370 KHz this evening on my Icom 725. You can see it looks pretty good.

fibber
30-11-2006, 15:36
Sackville added some hours, some in English:

(Format is UTC time, Days of the week, Frequency, Power, Azimuth, Type
of antenna, Language)

15:05-16:05 7 DAYS 9800 70 268 HR 2/1/0.5 ENGLISH
16:05-17:05 MON-FRI 9800 70 268 HR 2/1/0.5 RUSSIAN
16:05-16:35 SAT&SUN 9800 70 268 HR 2/1/0.5 RUSSIAN
16:35-17:05 SAT&SUN 9800 70 268 HR 2/1/0.5 UKRAINIAN
17:05-19:05 7 DAYS 9800 70 268 HR 2/1/0.5 ENGLISH
19:05-20:05 7 DAYS 9800 70 268 HR 2/1/0.5 FRENCH

Join our North American DRM group if you haven't already. We'll keep you up with the "breaking news" for monitoring DRM in the USA :)

Damon
30-11-2006, 16:16
Thanks for the info on Sackville. I hope they continue to add hours. I joined the DRM-NA group a while ago; I should probably post an introduction there.