A.M. Modulation Index & Bandwidth

[pauldbnut]

Just curious...

I sometimes see broadcast station reception reports with comments along the lines of "very low modulation", "over-modulated" or "excessive bandwidth".
Are these properties that SDRC could calculate (and display on demand), and would any more sane people find that interesting/useful?
Thanks.

[Simon G4ELI]

Could measure - I assume so. Going to do so soon - unlikely.

[pauldbnut]

Hehe thanks, Maestro.

[jdow]

Just noodling here. My AM detector is the canonical "filter the thing to the desired audio bandwidth" then apply the mag function to the complex data out of the filter, sqrt( re^2 + im^2). This gives a DC term and the audio term. They can be compared neatly. Of course, using frequency domain FFT data may make this less clean and straight forward. I'd display it with a tiny LED in the corner of the RX window in the matrix. Red-orange for audio peak larger than long term DC term. Green for audio peak less than DC term and larger than 0.25 the DC term. Blue would be lower modulation than this. It would flicker a lot so some filtering might be appropriate. The problem I'd have with it is stated simply, "Do *I* feel it is worth doing?" (Quiet reasoned requests repeated gently but periodically months apart can sometimes remind the author who is into that code already that this might be easy to add leading to good results. But, ask politely. Insistence breeds resistance.)

{o.o}

[pauldbnut]

"filter the thing to the desired audio bandwidth" then apply the mag function to the complex data out of the filter, sqrt( re^2 + im^2).

OK, Joanne, just waking the brain up a tad...
Does that just mean: within one RX BW, for every bin (except zero/DC taken separately) do |x|, then take the average?
I'd better go back to basics because that seems to imply a single tone at very high amplitude is not over-modulation.
Maybe it's |sum(x)| ?
Better still, I'll just go back to sleep. Simon doesn't need more crap from me here.

[jdow]

In the frequency domain it is something like that. If you are in the time domain you have the nice handy complex number that encompasses all the energy in that sample that has gotten through all preceding processing including filters. Take the magnitude of that number and you have the (relative) power of the signal. You can trace that back through any artificial level changes in the previous processing as well as front end gain to get input power within that bandwidth to a modest level of accuracy. If you have the radio calibrated from a precision input signal then you know the level to a high degree of accuracy, of course.

(At the moment my brain is stuck on groking the time domain aspects of it all. That has led to the possibility that I now have a slightly better phase lock loop for demodulating stereo FM with RDS. I committed a sin and thought analog and a VERY good PLL for synthesizers I used back in the day. I converted it to digital and it works a treat. I don't think it would work really well for demodulating FM. It probably could be twisted into working for SAM and its cousins, though.)

{^_^}