SDRPlay / FM SCA decoding

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radioguy73
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SDRPlay / FM SCA decoding

Postby radioguy73 » Mon Sep 05, 2016 10:01 pm

I am an old hand at 'radio' but SDRs are new to me so bear with me on this please!

I have a 'local' FM that uses a 67 kHz SCA and tried to decode that using SDRPlay and the 'included' CubicSDR (OSX) and it appears there is not sufficient 'separation' from the main FM modulation to allow the decode to work.

I tried turning off the AGC and playing with the levels manually (What exactly is the difference between LNAT -- and what the heck is that!?) and 'tuner' gain?) but alas, no joy. I also played with the IF options and none appeared to make a difference.

Am I doing something wrong, or is this Beta iteration not 'good enough' to separate the SCA from the main modulation and in need of 'updating'?

And why oh WHY is it so hard to get software with all the appropriate 'hooks' installed so it 'just works'? I was VERY pleased with how easy it was to get the SDRPlay to work with CubicSDR on MacOSX (kudos to whoever did this! You 'get' it!) but I'd like to try other software packages too. I mean, they're out there, but (for example) I can't get ''gqrx' to recognize the SDRplay is attached to the USB port, and I gather I need to know how to compile things to do this, because the software doesn't actually have all the 'bits' needed to 'address' the USB port, or something like that. Hey, I am a radio geek not a computer geek! I have similar issues with DREam software too. It is like the computer zoomies want to make us feel inadequate! :)

Thanks for whatever help you can offer! 73.
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radioguy73
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Re: SDRPlay / FM SCA decoding

Postby radioguy73 » Tue Sep 06, 2016 4:39 pm

Update -- I can SEE the SCA signal, and the problem seems to be that the main carrier modulation 'overwhelms' the sub carrier. Is there a way to 'deemphasize' the off channel modulation (a 'band pass filter' essentially) or at least modify the receiver so that it will DEemphasize off channel signals?

Like I said, playing with the gain doesn't do the trick. During silent periods of the main programme, there IS audio decoded, but that is only briefly!

Ideas?
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dsalomon
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Re: SDRPlay / FM SCA decoding

Postby dsalomon » Wed Sep 07, 2016 1:46 pm

Look at this webpage: http://www.blackcatsystems.com/radio/sca.html

It contains all the information you need to learn how to receive SCA transmissions.
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KM4JOJ
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Re: SDRPlay / FM SCA decoding

Postby KM4JOJ » Wed Sep 07, 2016 11:35 pm

You cannot pickup an SCA by the method I think you're doing; which is using a NFM on the main signal. This is not how the system works.

SCA's are a narrow-FM modulated signal within the baseband of the FM; yes...you can somewhat see it and decode it during times of silence on the main program; but that's just a side effect of the fact it's modulated within the carrier...or something. All of the sub-carriers are basically pre-modulated; then modulated within the main program. The first 15khz of an FM transmission is sum/mono audio, 19khz is a pilot carrier, centered at 38khz is a DSB supressed carrier signal that contains the difference audio (for stereo; 19khz pilot was originally used to generate the carrier to decode this), 57khz is RDS, then you have SCA options at 67 and 92khz. You can actually generate all of it in software and modulate a basic FM transmitter and get stereo.

The only way you're going to decode an SCA is with the use of something like HDSDR and some kind of virtural audio cable option in Windows. You have to tune the FM station in HDSDR, then make it's outputting to the virtual audio cable at 192khz; you must have 192khz support in your virtual cable. You then input this audio stream in to another SDR program...SDRUno will probably work for this; select a narrow FM filter...then tune to 67khz.

I haven't done this live as I don't have VAC or anything installed; but my audio editor can pull from WASAPI and I have done this using recorded demodulated output from HDSDR.
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radioguy73
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Re: SDRPlay / FM SCA decoding

Postby radioguy73 » Thu Sep 08, 2016 4:01 pm

Thank you Loren, but I'm still confused. You are correct in what I am trying to do -- just decode that SDR narrow band FM signal directly -- but from my understanding of how SCA is encoded, that should work.

I don't understand how what you are suggesting different from just decoding from the baseband signal? Isn't that the POINT of a zero IF ('direct conversion') in the first place? How would decoding the signal twice do anything different?

My understanding of things is that the SCA signal isn't encoded "on top of' the analog signal. The analog signal is encoded from 0-15 kHz relative to the nominal carrier for the L+R and from 23-53 kHz for the L-R, and the 'stereo' decoder (triggered by the presence of the 19kHz pilot and using a sub-carrier at 38kHz) does the algebraic combinations to come up with a Left and Right channel to send on to the audio amps. SCA (and RDS) look 'further out' on the bandpass and decode either the digital data (RDS) or the analog audio in the SCA channel(s) SCA uses subcarriers at 67 and/or 92 kHz which are completely independent of the other carriers... In other words, NONE of these things is dependent on (or related to) the carrier on the nominal channel except for the R+L signal.

A picture will help:
The spectrum display in the upper right shows the placement of the elements plainly, but the display in the bottom of this screenshot (where the 'SDR' is 'looking' to decode things) has them all jumbled together despite the lack of AGC....
Screen Shot 2016-09-08 at 11.54.32 AM.jpg
Screen Shot 2016-09-08 at 11.54.32 AM.jpg (168.8 KiB) Viewed 2732 times


As I said, it appears in the waveform shown on CubicSDR that the main modulation is 'overwhelming' the SCA (And the RDS too, but that is not what I'm trying to decode as it is digital rather than analog data) in essence (I THINK!) because it swamps the AGC -- even when the radio is supposedly in 'manual gain' rather than AGC mode. Is that a flaw in the implementation of CubicSDR, an issue with the SDRPlay itself, or something else I'm overlooking? I tried reducing the 'bandwidth' down as low as it would go, and putting that bandwidth OUTSIDE the bandwidth of the WBFM signal, but that did nothing useful either.

As I said, I'm a radio geek not a computer geek, so I *get* the implementation of FM signals, I just don't "do" Windoze.... :)

And perhaps a simpler question that will help me understand things better: what exactly IS "LNAT" (can anyone even just tell me what the letters stand for!) and how is it different than "Tuner" gain if that is not obvious once I understand what the acronym is?

Thanks!
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KM4JOJ
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Re: SDRPlay / FM SCA decoding

Postby KM4JOJ » Thu Sep 08, 2016 10:45 pm

radioguy73 wrote:but from my understanding of how SCA is encoded, that should work.


All I can simply say is your understanding of SCA is incorrect. Let me point out a few things.

SCA uses subcarriers at 67 and/or 92 kHz which are completely independent of the other carriers... In other words, NONE of these things is dependent on (or related to) the carrier on the nominal channel except for the R+L signal.


It sounds like you think SCA is an entirely separate transmission. It is not. FM is multiplexed; this means that ALL of the signals are combined in to one signal that then modulates the carrier using frequency modulation. Your main analog audio program is combined with the 19khz pilot, stereo difference, RDS, and SCA signals...which all modulate the main carrier *at the same time*. That's the idea behind multiplex; you're combining multiple signals in to one for transmission.

Take digital TV for an example; you can have multiple streams on the same "channel", but since they're multiplexed; you have to decode EVERYTHING before you can get the parts you want.

FM is no different; everything is mixed in to one signal, which is then used to modulate the carrier using FM. It might seem...strange to think about recovering RF signals from audio; but SDR is essentially doing just that.

has them all jumbled together despite the lack of AGC....


Because they are. That is EXACTLY what is happening, it has nothing to do with AGC...or software...or hardware...the very nature of FM Multiplex is everything is jumbled together in to a baseband signal before that baseband is used to modulate the carrier. The only reason you are able to somewhat decode an SCA during quiet times is becuase the carrier is essentially not moving...therefore you are able to recover some of the signal. But as soon as your audio content comes back; the modulation index is so high it causes what you're noticing. You need to remember...the modulation index on WFM is huge...and there is by far MUCH more power put in to your main program than any of the sub-carriers.

For the record, I have put narrow filtering on a standard FM tuner and tried to directly pick up an SCA signal; you have less success than you do with SDR.

Isn't that the POINT of a zero IF ('direct conversion') in the first place? How would decoding the signal twice do anything different?


As far as it relates to an SDR....the zero IF just relates to how the RF gets to the ADC; in a zero IF, you are directly sampling the tuner vs a lowIF in which you're using a superhetrodyne effect. It has nothing to do in relation to decoding indvidual signals within your baseband.

But even so...if this was a standard FM tuner doing direct conversion...it doesn't change the fact that ALL of your subcarriers are multiplexed in with the main audio; so you have to demodulate the FM signal in order to get the "baseband" that was used to modulate the carrier. I'm not talking about the baseband you're getting from the tuner IC to the ADC; I'm talking about the individual baseband used for one particular FM signal.

I actually started doing work on this a little over a year ago when I was playing around with RTL-SDR stuff. There wasn't a lot of information available; but I looked at several SCA hardware receivers. They all rely on getting a raw MPX baseband from the FM demodulator, and then they further "tune" to the 67khz NFM carrier within.

The thing you need to remember is the signals are multiplexed; they are not separate. You *HAVE* to decode the entire FM signal in order to demodulate the multiplexed components.

Is that a flaw in the implementation of CubicSDR, an issue with the SDRPlay itself, or something else I'm overlooking?


It's not an issue with the software or the SDRPlay; it's your lack of understanding exactly how multiplex works on FM. One can have a good grasp of how FM modulation itself works without any understanding of how multiplex works.

I covered this in a video I did last year (https://www.youtube.com/watch?v=hF6nCtayKek), using basically the exact method I used.

Again...ZeroIF/LowIF/direct conversion has *nothing* to do with decoding SCAs....for the purposes of SDR...that only has to do with getting from the tuner front-end in to the ADC. In regards to wideband FM broadcasting; you cannot direct-convert a signal that is multiplexed *with others*. If it was AM....yes...you could. I can pull the signals out of a VOA radiogram without demodulating it; but FM and AM are two entirely different beasts.

You have to demodulate the entire FM signal in order to be able to extract/demodulate the sub-carriers. The sub-carriers are contained within the baseband...which modulates the carrier. It's all mixed together before it even hits the exciter. I have 192khz files created in software that if I play them directly in to an FM exciter; I will get FM Stereo with RDS.

The issue is; you have to find software that *isn't* doing usual wideband FM de-emphasis/filtering or otherwise modifying the 100khz baseband it's demodulating from the actual FM signal. But you'll have to pipe that stream in to another SDR program to further demodulate the 67khz NFM. Most "WFM" modes in most software makes little to no provision for this; HDSDR and the MPX Output for SDR# are the only two I've found...at least on Windows.

Hardware receivers have to demodulate the whole signal before demodulating SCA...it's just how it works. If you find some other way of doing it...then you'd be the first.

73,
-Jay
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