Discriminator Audio

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wiredwrx

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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, (https://michaelhq54.tonidoid.com/urlzdb35x) 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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rjschilder

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Hey I know this may sound dumb/obvious but did you adjust the line in recording volume? It might need to be lowered. Just a thought as I ran into something similar in the past.

-Ryan KC9GMY
 

wiredwrx

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Thanks. Ya, with discriminator out, the volume/line input level actually reduces when it receives a signal. It is certainly confounding me.
 

737mech

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This might help, I tapped my bct-15 and the instructons I followed required a resistor (5-10k) and a cap (10uF) inline.
 

rjschilder

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sorry it didn't help out :( the reason the signal levels appear to drop is because of the following:

The tapped audio has white noise whenever no signal is detected, and those levels will be higher than when an actual signal is received. That was a mini-issue I ran into immediately when I started developing my repeater program using tapped audio.

--Ryan KC9GMY
 
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