Software P25 Transmitter

[rjschilder]

Hi folks,

Just thought i'd throw this out there since i've been working on this for a little while now. I saw a youtube video of a guy using OP25 to transmit P25 audio using a usrp. I was also able to accomplish this task but without the usrp!

I used a Yaesu FT-7800R, a radiotone full duplex repeater controller, a custom cable connected to the 7800R, a baofeng uv-5ra (for squelch...this will change), a Uniden BCD396XT (to decode the p25 audio), Gnuradio 3.2.2 and the latest version of op25 (release 328 I believe).

So, I made a virtual machine of ubuntu 10.04, updated all the packages, installed gnuradio 3.2.2 from source, and installed op25 via the instructions at op25.osmocom.org.

I took the audio output from my laptop (3.5 mm audio cable) and wired it into the 6-pin mini din jack on the back of the 7800R. (Tip of audio cable to Data-In, ring is unused, sleeve to ground).

I then connected the PTT pin on the 7800R's 6 pin mini din port to the appropriate pin on the Radio-Tone's TX port. The Packet Speed must be set to 9600bps on the 7800R.

I connected the baofeng UV-5RA to the input of the radio tone (a makeshift carrier detect (...for now).

I used my laptop's built in microphone as the input source and ran the following command to start the op25 transmission via soundcard:

./op25_audio_tx.py -G 0.5 -n 1 -O pulse -v

This runs op25 audio transmit, sets the output gain at 0.5, 1 channel, outputs using pulse audio, and is verbose (shows levels every few seconds).

The baofeng uv-5ra connected to the radio-tone controller is set to receive on 449.6. To actually key up on the FT-7800R (with p25 audio) you have to use another radio and transmit on 449.6 (kind of goofy, but it's all I could figure out at 11pm...).

So when you key up on the second radio using 449.6 the baofeng receives the signal. This causes the Radio-Tone to key up the FT-7800R and transmit the p25 audio from op25 (laptop audio out).

I had to adjust the output levels a little bit (maybe 20 seconds to find the right levels) but it works great!!! The Uniden BCD396XT decodes the p25 audio perfectly.

Sorry if this is a bit scatter-brained...it's late. I plan on making a mini youtube video to show the setup and demonstrate the working audio.

73
-Ryan KC9GMY

 

[KA1RBI]

Ryan

Many thanks for this report ! We've been looking for a success report like this one for a long time!

One big problem historically with transmitting using an audio patch such as this has been group delay distortion or other linearity maladies. The fact that the 7800R has a data-in port is probably key here.

One other issue that you must have dealt with at some point would have been establishing the proper deviation of the transmitted signal.....

Anyway, great to hear this

73

Max

 

[MattSR]

I once tried this with an Yaesu FT-8900 and had no luck at all. I purchased a USRP around the same time and didn't bother trying again as I had sold the Yaesu.

This is great news! Max has jammed plenty of software goodness into OP25 for everyone to play around with

 

[rjschilder]

Hi Again,

Sorry I forgot to post the video I took. It's not the best video since i was trying to record it on my cellphone. I'll try to post it on youtube in the next couple of days.

Max - the great thing about setting up op25 on a virtual machine is that i can clone it quickly and have a second working setup to check the deviation (as you mentioned). To check the proper deviation I piped the output from the "transmitting" op25 vm to another "receiving" op25 vm. I ran op25 on the receiving vm and adjusted the output levels on the transmitting vm to get the proper +3 +1 -1 -3.

-Ryan
KC9GMY

 

[rjschilder]

@Max (or anyone that can answer this)

Is there a way to change the NAC on the transmit side of op25. I'm sure it's in there somewhere but i haven't found it yet.

Thanks!
-Ryan KC9GMY

 

[KA1RBI]

Is there a way to change the NAC on the transmit side of op25.

Not at present - it's hardcoded at 0x293. We're starting to retool all the apps to conform to the new GR3.7 API, and the TX code hasn't yet been fully converted. I'll try to add an ability to change the transmitted NAC as part of this changeset...

73

Max

 

[rjschilder]

ok sounds good. I have a few p25 repeaters in my area and i'd like to try to use them with op25. They all use 293 so I think I'm good for now., but changing the NAC may be useful anyway

Thanks!!
Ryan KC9GMY