OP25 OP25 Default Audio Issue

[boatbod]

"tunable: true" devices can be re-tuned by the app to point to whatever voice channel is needed at the time, but the limitation is they can only be used by one channel at a time.

"tunable: false" devices can be shared by multiple channels, but their frequency coverage is dictated by the "frequency:" parameter in the devices definition along with the bandwidth/sample rate of the device. Generic rtl devices are generally good for 2.0-2.4MHz coverage, whereas Airspy are either 3, 6, or 10Mhz. There are other compatible devices with different bandwidths, so it all depends what you need.

If you are using a non-tunable config, you need to find the lowest and highest frequencies that you want to be able to monitor, check that the bandwidth of the device is sufficient, then set the device frequency to the mid-point of the range. That way, op25 can then select the relevant portion of the received spectrum and filter out the specific frequencies you are interested in. If the desired frequency exists outside of the device bandwidth, you will generally see an error message saying "Unable to tune %s to frequency %f".

 

[DC31]

okay, I feel like I am getting closer.

i am a little confused by the airspy example. One sdr stick, three channels. The three channels all feed to the same udp port. The audio section has one instance listening to that single udp port. So, after we separate the talkgroups in the Channels section, then we recombine them to the single udp port? Seems to defeat the purpose of separating them.

i am not attempting to do a liquidsoap stream. I only want the three talkgroups to stream separately to pulseaudio.

attached is my rtl.json file

what am I missing?

 

[wgbecks]

Unfortunately, a single RTL SDR does not have enough bandwidth to support the configuration of the three Voice Channels.

To determine the correct choice for an SDR, you must first account for all of the RF frequencies (channels) in use on the intended site you
wish to receive. Once all of the frequencies have been identified, determine the total bandwidth the SDR must be capable of by subtracting the lowest from the highest channel frequency.

It's also important to keep in mind that for given SDR's maximum sample rate, that you'll only be able to utilize approximately 85% as it's
total bandwidth. This reduction in apparent (useful) bandwidth is due to a roll off in the frequency response or sensitivity as you approach
each end of the available sampled spectrum.

With the appropriate SDR selected for the application, you then divide the calculated total bandwidth by a factor of two and then add that
to the lowest channel frequency. This new value becomes the center frequency that you must park the SDR on for non-tunable configuration such as yours that it can ingest the control and service channels into the effective bandwidth of the selected device.

I will caution that some systems have site channel assignments encompassing spectrum in both the 700 and 800 MHz Public Safety bands
that will preclude the use of a single SDR. Cost wise, you might be better off configuring multiple instances of rx.py using the lower cost
RTL-SDR type devices.