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NFM and DV measurement requirement


David J Taylor
 

Background: It would be a useful function if SDRangel could be used for measuring some signal parameters - for example NFM deviation and the digital voice amplitude relative to full-scale. There are a number of amateur radio stations unaware of whether they are running the appropriate FM deviation for the channelling in use (12.5 or 25 kHz), and in particular for digital voice (DMR) where the can be a huge variation in audio level with no ability for users to check just what audio they are sending, just relying on reports from others of "too loud" or "too quiet". At least with SSB you can watch the output power!

Is there any way in SDRangel to measure the level of the recovered audio? Calibrated. Perhaps there is and I've missed it, but if not, how easy might it be to add? Although the NFM deviation may be the easier to implement, digital voice would be more useful, and should be easier to calibrate as dBFS or whatever. Is anyone interested in adding this?

I could do Pascal/Delphi for Windows, but not C/C++ and not Linux.

Thanks,
David GM8ARV
--
SatSignal Software - Quality software for you
Web: http://www.satsignal.eu
Email: david-taylor@...
Twitter: @gm8arv


Edouard Griffiths
 

Hi David,

it would not be too difficult to report the maximum FM deviation and could be an interesting indication on the general shape of the signal. For digital this would be based on whatever the codec sends at its output and does not depend so much on what is sent from the mic on the other side so I am not sure to get how useful it would be. Moreover it is significantly more complex because what you have internally is either a software decoder with mbelib and dsdcc or a hardware AMBE dongle. In the end it would not be really different from showing you the audio level in general. It is much more important I think to control the audio input on the Tx side but DV transmission is not covered yet.

Brgds, Edouard.


David J Taylor
 

Hi David,

it would not be too difficult to report the maximum FM deviation and could be an interesting indication on the general shape of the signal. For digital this would be based on whatever the codec sends at its output and does not depend so much on what is sent from the mic on the other side so I am not sure to get how useful it would be. Moreover it is significantly more complex because what you have internally is either a software decoder with mbelib and dsdcc or a hardware AMBE dongle. In the end it would not be really different from showing you the audio level in general. It is much more important I think to control the audio input on the Tx side but DV transmission is not covered yet.

Brgds, Edouard.
==============================

Thanks for your comments, Edouard.

In the first instance I am interested just reception, not transmission. Without prior knowledge, I am guessing that the output from a decoder would be a digital signal rather than an analog one, and hence displaying dBFS would not be an issue. Of course, if the decoder is hardware and includes a D=>A then it's a different problem. My perception is that a good transmission tries to maximise the SNR, and so would work close to 0 dBFS, but without overloading.

With the DSD decoder, seeing the constellation diagram has shown how useful the program could be as a /calibrated/ device, either for FM deviation or for dBFS measurement in the digital domain.

So it's not that different from controlling the transmission level. Does one aim for some sort of AGC or voice compression so that the transmitted SSB/deviation/dBFS is near to the maximum? In any case, you need some way of measuring it. I've not yet played with transmission, but I may need that in the near future if I use my LimeSDR Mini or Pluto for SSB up to QO-100.

Cheers,
David
--
SatSignal Software - Quality software for you
Web: http://www.satsignal.eu
Email: david-taylor@...
Twitter: @gm8arv


Edouard Griffiths
 

Hi David,

I am still not sure of getting the point about the audio in digital. The audio has absolutely no incidence on the signal. If you overdrive the codec (AMBE in that case) then you will get bad quality audio but if your FM modulation is correct the signal will be decoded perfectly although the audio will sound crap. In that sense I don't think any measurement related to the audio on the Rx side will tell you if things are correct on the Tx side. Only your ears (and brain) will tell. 

As mentioned you already have the means with the (pseudo) constellation display to calibrate the FM demodulator to get the best out of the signal so that it will decode correctly. You can also tell that if this diagram is blurred too much then it will be hard for the decoder to manage.

Brgds, Edouard.


David J Taylor
 

Hi David,

I am still not sure of getting the point about the audio in digital. The audio has absolutely no incidence on the signal. If you overdrive the codec (AMBE in that case) then you will get bad quality audio but if your FM modulation is correct the signal will be decoded perfectly although the audio will sound crap. In that sense I don't think any measurement related to the audio on the Rx side will tell you if things are correct on the Tx side. Only your ears (and brain) will tell.

As mentioned you already have the means with the (pseudo) constellation display to calibrate the FM demodulator to get the best out of the signal so that it will decode correctly. You can also tell that if this diagram is blurred too much then it will be hard for the decoder to manage.

Brgds, Edouard.
=====================================

Edouard,

I appreciate what you're saying. Until we have the tools, we won't know what to look for, but the typical problem is too low a level of audio rather than too high a level. I suspect that a combination of an audio waveform display and a dBFS indication would be a big help. As you say, the constellation display provides an excellent way of checking other aspects of the transmitted signal.

The complication is one reason why I thought that starting with a simple FM deviation meter (ideally couple with a waveform display) would be a good starting point.

Some ideas for future developments in any case.

73,
David GM8ARV
--
SatSignal Software - Quality software for you
Web: http://www.satsignal.eu
Email: david-taylor@...
Twitter: @gm8arv


Edouard Griffiths
 

Hi David,

I am not sure you realize that FM deviation and audio level have strictly no relation in digital. Therefore it is useless trying to relate one to the other. The correct audio level can be measured on the Tx side only.

Brgds, Edouard.


David J Taylor
 

Hi David,

I am not sure you realize that FM deviation and audio level have strictly no relation in digital. Therefore it is useless trying to relate one to the other. The correct audio level can be measured on the Tx side only.

Brgds, Edouard.
=================================

Edouard,

Yes, of course. My apologies that this was not clear. I'm talking two completely different things:

- using the NFM demodulator, provide some sort of deviation measurement - peak or whatever - to be able to confirm that the signal has the correct maximum deviation for a specified channel spacing. This alone would be useful.

- using the DSD demodulator, provide an indication of the recovered audio level from the digital demodulation. A view of the audio waveform might allow too high an audio level at the transmitter to be detected (and with the ears too!), and a meter display would allow an indication of the dB below full scale to be determined. At least some digital rigs have a mic gain adjustable in e.g. 3 dB steps.

Adding either would be helpful, and perhaps some sort of waveform display and level meter might form part of both?

73,
David GM8ARV
--
SatSignal Software - Quality software for you
Web: http://www.satsignal.eu
Email: david-taylor@...
Twitter: @gm8arv