Observation on mix_offset and frequency setting.

[DickvanNoort]

Hi Jeroen, Last week I tried and managed to decode the 18 channels from 25E using a RPI4B running the Dragon OS.

The goal was to use as few resources as possible by using as small as possible bandwidth and position the signal in the middle of it. So for the 600 bps signals I used filter_bandwidth=2000. For 1200 bps I used filter_bandwidth=3000 and for the 10500 ones I used filter_bandwidth=15000.

For this RTL-SDR V3 dongle I needed to set mix_offset=3500 to position the 600 bps signal in the middle of the bandwidth. However this made the 1200 and more so the 10500 signals fall outside the bandwidth. For the 1200 signal to end up in the middle I needed to use frequency=1545124500 for 1545.1250 (- .005) For the 10500 signals to end up in the middle I needed to lower the frequency by values between .006 and .007 KHz, like:
frequency=1546006000 for 1546.0130 (- .007) frequency=1546021000 for 1546.0270 (- .006) frequency=1546036000 for 1546.0430 (- .007) frequency=1546086000 for 1546.0920 (- .006) frequency=1546101000 for 1546.1080 (- .007) frequency=1546116000 for 1546.1200 (- .006)

Does this make sense and/or can you explain why this works like this?

Cheers and thanks for this great program, Dick

[jeroenbeijer]

Hi Dick!

For the regular data channnels, 600, 1200 and 10500 I am not sure using the audio filter really makes any difference, I have turned them off by setting filter_bandwidth to 0. This will save CPU compared to having them on. I do use them for the audio channels though to prevent the adjacent call channels getting picked up in each jaero.

The mix offset really just moves the whole spectrum up or down. It basically has the the same effect as using the little arrow buttons next to the center frequency I suppose. So instead of using mix offset you can just raise or lower the center frequency in the ini file and get the same result. Each dongle and LNA are a little different by some (K)hz. I think I included this to just move all VFO's a little instead of changing each VFO. But you can also just change the center frequency.

Hope that explains it a little :-)

I have been thinking to move to raspi but I am not sure I can run all 25E on one device. Do you think it is possible to run all 25E data and voice channels on one raspi?

Groeten

Jeroen

[DickvanNoort]

Hoi Jeroen,

But you can also just change the center frequency. Are you sure? It seems that center_frequency together with sample_rate defines the band that is covered and has no effect on the VFO frequency.

Interesting that NOT using the audio filter will save CPU. The reason I used it was to reduce the audio bandwidth in an attempt to reduce the data that needs to be processed by ZeroMQ and passed over the network to other devices running Jaero. Never thought/realized that reducing bandwidth could have a negative effect on the CPU usage. Currently away on vacation, but when back home I will try with filter_bandwidth = 0 and see if I can determine what is best.

As I did/do use filter_bandwidth to reduce the audio bandwidth the signal needs to be within that defined filter. Using mix_offset=3500 this worked fine for all the 600 channels, but the 1200 channel and even more so the 10500 channels needed their VFO frequency to be changed just a little bit. The 1200 channel needed 1545.1250 - .0050 = 1545124500 ( => signal centered at 1518 Hz ) to fall within a filter_bandwidth=3000. The 10500 channels needed 1546.0430 - .0070 = 1546036000 ( => signal centered at 7526 Hz ) to fall within a filter_bandwidth= 15000. And that puzzled me.

PS. It would be useful if a VFO window would show a frequency value just like the main window does. So you know what you are looking at without need to refer to the .ini settings. :-)

Do you think it is possible to run all 25E data and voice channels on one raspi? On my RPi4 with the Dragon OS I currently run SDRReceiver with 18 data and 3 voice channels from 25E and 18 instances of Jaero decoding the data channels. See attached screenshot. This RPi4 was first used for Iridium, hence the Dragon OS. The 3 voice channels are decoded by Jaero on another PRi (3) and/or a win10 laptop just to test and play with it.

As new RPI4’s are not available I bought an OdroidN2+ which currently runs dumphfdl. Already managed to compile SDRReceiver and Jaero but so far failed to get VNC running which I need as the OdriodN2+ is headless and both SDRReceiver and Jaero seem to need a screen.

Gegroet, Dick 25E_VFO-a4 .txt