Stereo Tool

With the 'FM Calibration' menu and its settings now deprecated in recent releases of ST, is there any method in software to compensate for a non-flat response across the MPX output into the transmitter, ie. increase the amplitude on a slope as the output frequency increases? I have already done the RC calibration with a square wave which improved the deviation control but the resulting stereo double sideband is uneven.

Although the real solution is to acquire or build better equipment, do we have any method of making this up in software as a workaround?

Is ‘rules made to be broken’ under composite clipper settings on? If this is on, it can cause uneven L-R. Even tho you made it flat this can still happen and it’s normal. If you don’t want this feature you can turn it off and it will be flat.

Which audio card you are using? If this setting doesn’t change anything you might need to find a new soundcard

Also how are you displaying your MPX? Is it via SDR or some software? There might be some sligh miss calculations on this side also.

You can try all of these things and inform us.

Some small tip: I see you RDS level might be a little low. Check it out with some tool and if you need any help let me know.

Yes, L-R Asymmetry will boost Left L-R side, turn that OFF and it will be same as Right L-R side.
My recommendation for that filter is 3-5.

Also, if calibration is deprecated that does not mean you should not use it. I ALWAYS use it for calibration for every transmitter i set. Same for tilt.
Idk why Hans set it as "deprecated", it actually should be re-done with proper PEQs.

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Okay, so if the settings are "greyed out" in the user interface they are still mostly usable? Is there a toggle to reveal the deprecated features and allow them to be used normally like in the past?

The idea is that I should be able to hit EVERY Bessel carrier null frequency. Of course with this tilt, if I set it perfectly for 31185Hz it will be off at 13587Hz and so on. I think a calibration routine that could take input from maybe nulls 1-6 or so, then determine the correction factor would be an idea.

The DAC is a WM8740 driven by CM6631A. Obviously all budget pieces here, but works asides from this weird slope. Comparison is against a separate standalone appliance fed with white noise for this test which shows equal sidebands and proper RBDS level without having to manually bump the injection percentage. So I would think the SDR IF rendering is not skewing the results.

No composite clipper asymmetry settings are enabled. I also disabled Multipath Stereo on that same page, in case that was affecting something as well.

FM TILT calibration is mostly used because low freq sounds, you need scope for that, because, there is biggest impact on overall modulation (soundcard) output. Also no metter how perfect soundcard is, analog MPX input on transmitter is not ideal too, so you always need to check for tilt calibration using some low freq sound, i usually use lowest freq that can be easily show on my oscoloscope, around 30Hz (squere) You have those test tones in ST.

For everything else you use, FM Output > Calibrate, ofcourse FM output need to be enabled. Best would be to reset fully ST and just use FM Output and do calibration, later you load your whatever preset.
Me personally is not fan of that calibration in ST, that's why i said we need PEQs, but that's what we have now so that we'll use.

And last, I don't know what you use to calibrate FM transmitter from air, i use MPX Tool, so it's very easy to calibrate/see everything. It's important that you trust your measurements tools.

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I'm using an IFR 2975 to do the RC/Tilt and deviation calibration with its demod scope, no filter, and 200kHz IF BW, but I can try to get a second or third opinion from another device if needed.

I was wondering the other day, how much can someone trust mpx tool or should I say the SDR that’s connected into mpx tool. Do they need to be calibrated? To my knowledge mpxtool doesn’t have a setting to calibrate an Rf source. I have an RTL SDR laying arround and I was wondering how flat could it be or if I could trust it.

I wondered same, it does not need calibration since afaik it works with "ID", idk what exactly that means, but i checked with 4 different units, and its very, very accurate.

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