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PostPosted: Mon Jan 30, 2023 1:49 pm 

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I doubt that that's the transmitters. Have you also connected a loopback so MicroMPX can measure the exact delay that the OS and sound card introduce?

It might help (but it's not really necessary) to use identical hardware on all the decoders. (Same PC, same OS, same sound card). But the loopback connection should take care of this.
Yes everything is connected properly, loopbacks are all ok and everything. The only difference in the computers is the cpu. The rest is all the same(motherboard etc).
At this point the only thing that is not the same is the transmitters, so I concluded that it would be my problem. Is there something else I can try? Should slightly different volume levels play a part in this somehow? Because both transmitters have analog dials I can't get them to match exactly.
When you say you cannot get them to sync on air, what exactly do you mean? Do you hear a difference in timing? Or do you mean that the combined signal doesn't receive properly?

We have seen before that there can be *very* small (but constant) differences in timing, of maybe 2-3 microseconds, which you would need to compensate for manually. You can't hear it (it's less than 0.1 sample at 48 kHz), so to see these very small differences you would need to measure it. But these differences should be small enough to not interfere with reception in the center between the two transmitters (it could move the center by a few hundred meters though). More importantly, if the overlap area between the transmitters is not in the center, you need to compensate for the speed of light (I think it's 3.336 microseconds per kilometer). It's easiest to first verify that the decoders are in sync in one location (I can't think of any reason why they wouldn't be, if the loopback is connected and used, but it's always good to verify things), then move them to the target locations and adjust for the distance.

For the transmitters: For optimal results, the modulation should be exactly the same. So, ideally, the transmitters themselves should be identical, and also locked to GPS. But even without that, just syncing the MPX signal should already improve the reception.
After messing around with the the precise delay sliders I managed to get some pretty good results (stereo/rds working) but there are still some differences to the audio (audio appears to be perfectly in sync, but where the two transmitters overlap the audio has some noticeable quality reduction; turning either of the transmitters off fixes the issue. I think it's just a matter of both transmitters not being in sync at this point, since no matter how much I mess with the delay adjustment I cannot get a perfect match.


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PostPosted: Mon Jan 30, 2023 6:10 pm 
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That's about as good as I would expect it to get without syncing the transmitters as well (and you might need to use identical ones too.)

By the way, even if the 2 transmitters are perfectly in sync, there will still be places where they cancel out each other. For the RDS (the highest part of the MPX spectrum), at 60 kHz the distance of 1/4th of a waveform is 300000000 / 60000 / 4 = 1250 meter (so from the center point the waveforms will pretty much match without TOO much difference to about 600 meters in both directions). The FM carrier frequency is much higher though, around 100 MHz, so that distance to get perfect anti-phase (a half waveform apart) is 300000000 / 100000000 / 2 = 1.5 m. So every 3 meters there will be a point where the signals cancel each other out, in between the signal will be stronger or weaker depending on where you are. But, if your FM transmitters are not synchronized these points will move around, with a speed of 3 meters per second per 1 Hz of offset in frequency. And you can easily be a few hundred Hz off without knowing (most modulation meters won't show this).

What this comes down to is that your receiver will see huge AM noise in the signal (moving possbily hundreds of times per second between no signal at all and the combined signal of both transmitters), and that makes it difficult to properly demodulate it.

How much correction did you need to use? (Or is the point where you're checking this not in the middle?)


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PostPosted: Mon Jan 30, 2023 6:40 pm 

Joined: Tue Dec 20, 2022 7:42 pm
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That's about as good as I would expect it to get without syncing the transmitters as well (and you might need to use identical ones too.)

By the way, even if the 2 transmitters are perfectly in sync, there will still be places where they cancel out each other. For the RDS (the highest part of the MPX spectrum), at 60 kHz the distance of 1/4th of a waveform is 300000000 / 60000 / 4 = 1250 meter (so from the center point the waveforms will pretty much match without TOO much difference to about 600 meters in both directions). The FM carrier frequency is much higher though, around 100 MHz, so that distance to get perfect anti-phase (a half waveform apart) is 300000000 / 100000000 / 2 = 1.5 m. So every 3 meters there will be a point where the signals cancel each other out, in between the signal will be stronger or weaker depending on where you are. But, if your FM transmitters are not synchronized these points will move around, with a speed of 3 meters per second per 1 Hz of offset in frequency. And you can easily be a few hundred Hz off without knowing (most modulation meters won't show this).

What this comes down to is that your receiver will see huge AM noise in the signal (moving possbily hundreds of times per second between no signal at all and the combined signal of both transmitters), and that makes it difficult to properly demodulate it.

How much correction did you need to use? (Or is the point where you're checking this not in the middle?)
Thank you for the insight. The test I did was using the secondary transmitter as a gap filler because of poor reception of the main transmitter site. The main site is about 15km away from the gap filler and the test I did was roughly in the middle where the two signals interfere, I used -42.5μs for the main site and 3.7μs for my gap filler. Again, the values I used were purely by ear but they do check out compared to the maths you gave me. I'm looking into getting a pair of fm transmitters with external clock input to continue further with my testing..

On a side note, I find adjusting the precise delay a little difficult because I'm using a laptop with my car running around and I remote in the TX network; I'm getting some stutters here and there (even when holding down the mouse for even more precise) when I'm connecting to the interface remotely, or using AnyDesk to control the interface connected to a computer at the TX site. Perhaps some arrows to the side of the slider would improve the usability, or some sort of zoom function to the delay bar, once you get your rough adjustment in.

Markos


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PostPosted: Tue Jan 31, 2023 10:06 pm 
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I don't know how big the effect of GPS syncing the FM transmitters will be. It should definitely help, but *any* difference between the two setups will have some impact.

One thing you could try now already: The audio levels must match exactly. So if you configured both to hit 75 kHz for example, one might be at 74.7 and the other at 75.3. You can try if adjusting the level of one of the two VERY slightly has some impact.


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