I happen to have several UHF GP300's sitting around. I thought I might be able to turn one into a scanner to monitor the local PD P25 radios using the GP300 and DSD.
I already have DSD+ up and running perfectly with my SDR. I then began searching for, and finally found, a schematic for the GP300. While searching the schematic, I found what I believed to be adequate discriminator audio output. In the linked pdf, (http://ift.tt/PGzZop) one can see PI, pin 7 (and 6 for some reason) appear to have disc. audio out. I soldered an audio cable to pads 7 and 8 (8 is ground) I connected it to my sound card line in.
I was able to listen to the audio out through my computer, and can tell when it receives a p25 signal (in addition to the flashing red receive light on the radio)
In the windows control panel, I can see and manipulate the audio in stream. I fired up DSD, and chose the "i" option for the line in of my sound card (I actually tried both option 2 and 3 listed as audio in and line in of my sound card) I also tried routing the disc audio from the GP300 through VB cable to DSD+, but I do not get any decode. When choosing the line in option in DSD+, when no signal is being received, the dsd "lvl" reading is 99%. When the radio receivs a p25 signal, the "lvl" drop to about 60%.
I am thinking I might need a variable resister, or perhaps an RC circuit, or perhaps, I just don't have discriminator audio. Then again, it might be something else.
Can anyone advise?
I already have DSD+ up and running perfectly with my SDR. I then began searching for, and finally found, a schematic for the GP300. While searching the schematic, I found what I believed to be adequate discriminator audio output. In the linked pdf, (http://ift.tt/PGzZop) one can see PI, pin 7 (and 6 for some reason) appear to have disc. audio out. I soldered an audio cable to pads 7 and 8 (8 is ground) I connected it to my sound card line in.
I was able to listen to the audio out through my computer, and can tell when it receives a p25 signal (in addition to the flashing red receive light on the radio)
In the windows control panel, I can see and manipulate the audio in stream. I fired up DSD, and chose the "i" option for the line in of my sound card (I actually tried both option 2 and 3 listed as audio in and line in of my sound card) I also tried routing the disc audio from the GP300 through VB cable to DSD+, but I do not get any decode. When choosing the line in option in DSD+, when no signal is being received, the dsd "lvl" reading is 99%. When the radio receivs a p25 signal, the "lvl" drop to about 60%.
I am thinking I might need a variable resister, or perhaps an RC circuit, or perhaps, I just don't have discriminator audio. Then again, it might be something else.
Can anyone advise?
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