Stereo Tool

Hello Everyone,

I think this might be an interesting topic for most of processing freaks Today while I was scanning USW (VHF FM) band via HDSDR I've noticed that one radio station was performing SSBSC transmission tests. This is the first time I've seen it live. Below you can see how the MPX spectrum looked like. It seems like they used a very steep filter to get rid of the USB part.



I think we are all familiar with theory, right?
http://www.radioworld.com/article/a-met ... tereo/4000




Below, the recorded MPX sample (reception quality was not perfect with my USB dongle)

https://drive.google.com/file/d/0B1oKe7 ... sp=sharing

Regards,
Slawomir B.

UPDATE:

A video sample from today (November 10, 2014):

https://drive.google.com/file/d/0B1oKe7 ... sp=sharing (here I switch between DSBSC and SSBSC transmission of the very same station but at different frequencies - you can also hear they're synchronized, but the sound texture changes slightly on transition so it's like A/B comparison).

For those of you who happen to be from Poland - it can be received around limited area in Upper Silesia. Freq. is 104.3 MHz.
For some reason their RDS injection seems a bit low.
Transmitter localization is the chimney of the thermal-electric power station in Zabrze (EC-Zabrze) - so quite exposed.


UPDATE 2:

Raw MPX recording: SSB vs DSB switching https://drive.google.com/file/d/0B1oKe7 ... sp=sharing (SSB had weaker signal so expect significantly more noise in this one).

Off Topic question: Can you read modulation level with the USB dongle and the software that are using?

Yes, I can read modulation level with USB dongle and HDSDR. Not directly, though.

For this you may use VAC - just route signal from HDSDR audio-out into VAC and then into MPX tool. Please note - set FM-BW to 150 kHz in HDSDR as well as volume to the maximum and turn NR off. You may still need to calibrate the system as your dongle (or software) may do something to audio after it was received. I also have a modified tuner (I mean real external tuner) that has MPX output instead of line out and I can connect it into PC line-in (this also needs calibration). I got similar reults using both dongle and tuner. But correctly calibrated pc and external tuner might be more accurate.

As a guideline - most of the stations use pilot injection in the range of 8,5-10%. If your system shows something like that, you can use it for rough estimation. Not necessarily all radio stations will use 100% of modulation (+/-75 kHz). For example, this could result from a serious overlap of many radiostations in one area. So lower deviation can be a win sometimes.

In theory if your USB dongle is not enhancing pure MPX in any way with volume set to 100% in HDSDR, it should be accurate, but you never know.

I obtained these using my dongle:
http://i.imgur.com/9RBTJiH.jpg
http://i.imgur.com/SM8tpok.jpg

As you see one used 100% another 70% sharp with similar pilot levels.

This one used about 80 something with 9% pilot injection:
http://i.imgur.com/XUkhMxk.jpg

Perhaps others (like mpex2006km, who also uses dongles) could tell you more about it.

In fact, mpex2006km invented his own plugin for gnuradio, strictly with MPX monitoring in mind. It's in this thread: viewtopic.php?f=4&t=5686

Regards,
Slawomir

Hi. What i have to say is that general purpose (like the one in sdr# and in HDSDR) dsp based fm demodulator are not designed for the best absolute deviation/"voltage" ratio. That radio changes with parameters like how strong a station is and what kind of other station are around it (frequency interference plays an important role). In fact what had actually happened to me at once is that a mono station with narrower deviation was showing as if it was deviating more that a stereo station with a much higher degree of deviation.... (I let you be the judge)

My flow graph in gnu radio is designed for mpx monitoring as it shows MPX Spectrum , LR Phase , LR Spectrum , RDS Spectrum, RDS Constellation and RDS Decoding. But there is something missing as you can see... No absolute fm deviation measurement.... What i have tried to do was to look a the MPX and calculate deviation but that was a disaster when measuring weak stations (basically static noise would set the deviation much higher) and when two station are within 75Khz of each other.(one on 90.0 and 90.05 for example) .... I tried to solve the sensitivity problem by implementing offset tuning , but it only helped and not fixed the problem....

What I have in mind for an absolute deviation measurement is to lock a pll on the carrier and look at the error signal... but not even done as a test yet.....

Thanks A Lot For Your Time

_________________
Very proud user of Stereo Tool since 2010. Thanks Hans.

My radio station
http://amradio.ddns.net

The problems that you are mentioning (weak signal/noise level and stations at very close frequencies) are also a problem for expensive measurement devices. Because there's just no way to get around that.

Thanks Hans ... I haven't tested these conditions on expensive mod monitors and i didn't knew that they have the same problems... But i still think there is room for improvement.... (like pll locking to a specific carrier and very steep filtering)...

Thank you for the info.

_________________
Very proud user of Stereo Tool since 2010. Thanks Hans.

My radio station
http://amradio.ddns.net

I'd go the other way - if you already can see RF spectrum you actually CAN roughly judge what is happening to the signal and whether it is over modulated or not. mpex2006km, You already showed it to me using example of one Greek station around you

Kind regards,
Slawomir

What i think i will do is re-modulate a carrier (0hz) and show the spectrum with an peak-holding mask.... (something that it is easily done in gnu radio because i have direct access to the MPX). Let me know what you think ...

Back to the topic SSBSC haven't helped me. I haven't seen any difference in sound neither in the mutlipath distortion. I only use the "Steep" filter setting in Composite Clipper because i can use the frequencies above 16Khz (i usually use filter in the 17.5Khz to 18Khz region).

_________________
Very proud user of Stereo Tool since 2010. Thanks Hans.

My radio station
http://amradio.ddns.net

To tell the truth I didn't hear any improvement either. Two days ago I took a ride just to listen to that particular signal in a car. The route I took goes directly from my town into the vicinity of the transmitter site with numerous bridges and little bumps (+/-10 m) here and there in the way, sound absorbing barriers, as well as tall buildings nearby, tunnels too.
http://upload.wikimedia.org/wikipedia/c ... nel_02.JPG
http://d.naszemiasto.pl/k/r/bb/67/4e514 ... g?16771189

How it sounded like? Well, my car radio started doing some weird things at some point and actually no improvement (multipath-wise) was audible whatsoever. Just to mention, this particular Philips receiver I have blends into mono very easily everytime there is some disturbance in the signal.

Maybe next time I will take my laptop with me and I'll try to record the raw mpx signal while driving. I also have a car camera

@mpex2006km, why 18 kHz? It sounds a little bit too high for me. Personally I don't think I would ever go higher than 16 kHz, just to be safe with pilot. I got across some older tuners, especially those without PLL (like one Grunding, something like mid 80's - don't remember the name now) and also some cheap newer ones (mainly car radios), that are very prone to loosing stereo or even distort if anything is not "right" enough.

Yes its a bit in the high side but i never had a problem even with a very old Philips a completely transistor based stereo car radio.

_________________
Very proud user of Stereo Tool since 2010. Thanks Hans.

My radio station
http://amradio.ddns.net