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PostPosted: Sat Jan 21, 2023 7:22 pm 

Joined: Tue Dec 20, 2022 7:42 pm
Posts: 9
Hello!
First of all, I want to mention that I'm running the demo version of microMPX (downloaded the latest one from the website today).
I'm attempting to do a 2 way SFN Network, and I have completed most of the steps, but I'm running into issues when it comes to GPS Lock.
On my Stereo Tool PC I have a GPS Lock but on the two local machines I have micrompx installed on, for testing purposes I have everything ok except the GPS Lock. This message shows up:

<span class="message">2023/01/21 20:20:48 No GPS lock. NMEA: true, pulses: true, pulses are reliable: true, pulses match nmea data: true, pulse level: true, stream has gps: true</span>

Is there something that I'm missing?
I am running 2 behringer umc202hd's for my outputs and I have both 1pps signals & loopback signals connected successfully.
For my GPS Receivers I'm using some generic ones I found on aliexpress (this is the link https://www.aliexpress.com/item/3304976 ... Yu8tWdrhv8)

Thanks!
Markos


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PostPosted: Sun Jan 22, 2023 12:25 am 
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Hello Markos,

All the data that's logged looks fine. But what does the resample factor show? (You might need to press the > to make more settings visible).

How this is supposed to work: Once the GPS signal is available on both sides (and according to that log message, it is), the audio needs to speed up or slow down to reach the target delay. Note that the target delay may be negative, in which case you'll never reach it. The incoming stream display should will show if that's the case; you want the green line to be far enough away from the lighter gray line. If it's not, you need to increase the delay (you typically need to set it to at least 2 seconds when using GPS). A screenshot of what you're seeing might help us to analyze what's happening.


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PostPosted: Sun Jan 22, 2023 2:06 am 

Joined: Tue Dec 20, 2022 7:42 pm
Posts: 9
Quote:
Hello Markos,

All the data that's logged looks fine. But what does the resample factor show? (You might need to press the > to make more settings visible).

How this is supposed to work: Once the GPS signal is available on both sides (and according to that log message, it is), the audio needs to speed up or slow down to reach the target delay. Note that the target delay may be negative, in which case you'll never reach it. The incoming stream display should will show if that's the case; you want the green line to be far enough away from the lighter gray line. If it's not, you need to increase the delay (you typically need to set it to at least 2 seconds when using GPS). A screenshot of what you're seeing might help us to analyze what's happening.
This is what I see, I installed an older version I had handy to see if the issue still persists (it does). Although I do notice that in this version GPS Lock would light up for a fraction of a second and then turn off again. Is there something like a log I can upload, because I couldnt find anything on the web interface.
Markos

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PostPosted: Tue Jan 24, 2023 1:24 am 
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It looks like there's a problem with the decoder sound card; it doesn't run at the correct sample rate.

The measured timing between the 1PPS pulses is off, which means that the sound card isn't running at the correct speed. This value should be very close to 192000 samples between pulses, but on the decoder I see 191970.5 samples. That's a difference of 29.5 samples, which is a lot. On the encoder it's showing 191997.6, a difference of 2.4 samples - that's in the normal range.

The pilot frequency, according to FM specifications, must be at 19000 Hz +/- 2 Hz - beyond that RDS and stereo cannot be decoded properly. So we allow at most a 2 Hz offset at 19 kHz, which is 20.2 Hz at 192000 Hz. We could theoretically compensate for bigger differences, but since it's hard to tell whether an offset is caused by a constantly wrong offset (in which case compensating would be good) and an audio glitch that causes some lost samples (in which case compensating could kill stereo/rds), we don't. Also, offsets from the correct sample rate that are this large are very rare.

The cause must be that the sound card isn't running at the correct speed - the GPS clock is always accurate, so the sound card must be running at 191970.5 Hz instead of 192000 Hz. OR it could be that there are audio glitches and short bits of audio are missing. If the number is pretty much constant at 191970.5, the speed is wrong; if it moves around a lot it's probably audio glitches.

Do you have another sound card that you could try?

By the way, older versions took a lot longer to get a lock (30 minutes a few years ago vs 1-2 minutes now), so you'll probably get the best results with a recent version.


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PostPosted: Tue Jan 24, 2023 6:02 am 

Joined: Tue Dec 20, 2022 7:42 pm
Posts: 9
Yeah it appears to be the same latency on both instances running the same audio interface (umc202hd). Unfortunately I don't have another 192khz audio card handy right now so I'm looking to pick one up to try it out.
Tried a few different drivers on both machines with the exact same results (latency is stuck at 191970 and it is rock solid).


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PostPosted: Tue Jan 24, 2023 1:40 pm 

Joined: Tue Dec 20, 2022 7:42 pm
Posts: 9
Just got my hands on two focusrite Scarlett 2x2. Will do some testing at the end of the week and come back with the results.


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PostPosted: Fri Jan 27, 2023 6:13 pm 

Joined: Tue Dec 20, 2022 7:42 pm
Posts: 9
Quick update, managed to get a gps lock using the focusrite Scarletts. My only problem now is that my exciters are different models (rvr Tex and rvr TEX-lcd) and I cannot get the audio to sync on air. Will probably try to get two identical transmitters to solve this, since no matter the delay adjustments I'm doing in uMPX nothing seems to make them sync up.


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PostPosted: Fri Jan 27, 2023 9:46 pm 
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Quote:
Quick update, managed to get a gps lock using the focusrite Scarletts. My only problem now is that my exciters are different models (rvr Tex and rvr TEX-lcd) and I cannot get the audio to sync on air. Will probably try to get two identical transmitters to solve this, since no matter the delay adjustments I'm doing in uMPX nothing seems to make them sync up.
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.


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PostPosted: Fri Jan 27, 2023 10:29 pm 

Joined: Tue Dec 20, 2022 7:42 pm
Posts: 9
Quote:
Quote:
Quick update, managed to get a gps lock using the focusrite Scarletts. My only problem now is that my exciters are different models (rvr Tex and rvr TEX-lcd) and I cannot get the audio to sync on air. Will probably try to get two identical transmitters to solve this, since no matter the delay adjustments I'm doing in uMPX nothing seems to make them sync up.
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.


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PostPosted: Mon Jan 30, 2023 11:13 am 
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Quote:
Quote:
Quote:
Quick update, managed to get a gps lock using the focusrite Scarletts. My only problem now is that my exciters are different models (rvr Tex and rvr TEX-lcd) and I cannot get the audio to sync on air. Will probably try to get two identical transmitters to solve this, since no matter the delay adjustments I'm doing in uMPX nothing seems to make them sync up.
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.


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