I am very happy that I contributed to finding a solution for that kind of problem. It's true that I searched for a long, long time for a solution. I first thought of audio delay and phase reversal back in 2003, when I came across the problem when we had to use the same frequency for 2 transmitters having a large overlapping area.
It's true that the exciters should be in perfect phase accordance in RF with the appropriate phase delay to have modulation wavefronts arrive with extreme sync precision at the target area.
It's also true that multipath distortion can ruin all efforts regardless the cost of the system, when energy reflected off the mountains reaches at the target area in levels greater than -20db of that of the carriers.
In general, since we remain on the analog domain there are no perfect remedies to the problem. Even if multipath is miraculously absent in the target area, the optimised stripe can have a maximum width of 7 km's and for a single spot, ie overlapping areas at other angles could have the interference effect worsen rather than optimised. In proper design, one would try to allocate those areas in rural areas, like mountaintops or forests.
The SFN implementation in Stereo Tool is the cheapest way to achieve SFN modulation correction, but the proper adjust of modulation indicies in analog STL's and analog input transmitters is mandatory and should not be overlooked.
Best practice includes
>Stereo tool instance controlled remotely from target area
>finding a spot in the target area where audio is muffled mainly by main transmitter interference rather by multipath (I always do it in my car, I move around till I can see both transmitters, then I am sure that the signal I gather belongs to main front and not to reflections off hills around). I turn one of the two modulations completely silent and scan around for a location that has both signals at the same level, so I can get audio muffled by the unmodulated carrier.
> Enable the 2nd modulation and set proper delay as calculated by the distances (see youtube video descriptions)
Opt for "invert" if I have STL's and exciters that are of different models and observe the effect in audio. There is a concept of positive modulation (positive modulation voltages cause increase in carrier frequency) and negative modulation (positive modulation voltages cause reduction in carrier frequency). That is not standardised by broadcast standards, so each manufacturer could have either positive or negative modulation even on different models.
Listen for "chirps" that are acceptable in non-DDS exciters -( they are due to differences in carrier frequency that are in the order of 3 ΚΗz or less. Can be mended a bit by aligning both exciter's pll at exactly the center frequency. most pll's have a stability of less than 500Hz as per broadcast standards, so if the temperature is constant at the transmitter site if you align pll crystal reference once that should remain w. less than 200 hz/year. you'll definately need a good frequency counter if you opt for the procedure, preferably an oven-stabilised one w. 1Hz precision at 200MHz.
I haven't performed that procedure yet on the RS of the video. )
If one can get the "chirps", then the right phase btw. 0 and 180 degrees is selected, (invert on or off accordingly)
Then enabling a test tone of ~440HZ and adjusting both modulation indicies and trim delay +-2 sample around the sample delay value calculated (To cope for propagation delay at the transmitter site and all that transistor phase reversing inside the transmitters), to have the tone as clear as possible while maintaining proper modulation levels in order to be legal.
Then double the frequency and repeat in more fine increments. And once again.
Revert to normal program and observe the effect with and without SFN correction.
Note the levels for both transmitters down to paper, disable modulation on one transmitter and seek for a spot where the other can be cleary heard. (The unmodulated carrier of the interference is below 20 db of the useful signal) Measure modulation index for that transmitter. Then reverse silence on the transmitters, seek for the other transmitter to be heard clear and measure the other transmitter modulation, they must match and be at legal maximum. If not, adjust them as per the noted value at 1st SFN alignment by adding or removing fraction's of dB's as calculated.
Then for a final time, Return to that spot you found first that has the worst signal overlap, and do some check with and without SFN corrections (enabling and disabling the feature. It should have a big improvement on sound at that spot.
When I'm done with all these, I perform a celebrational victory lap around the overlap area to observe and enjoy the effects of rectification and to congratulate me of some good work done, and maybe capture a video to show the results and upload it to youtube for my fellow stereo tool users to check!