[App_rpt] an attempt at P25 dual mode RX

[Don Russell]

I need a little help or ideas on what to try next.

I had what I thought was a good idea of using an old astro saber for
the receiver of my repeater. It is pretty sensitive (.13 uV) and dual mode
P25 IMBE. Just wanted to see if it would work as a couple of us have
them around here and it would be a crude attempt to integrate asterisk and P25.
Granted the transmiter it still FM, but you do gain a little on the RX in the fringe
area with P25.  I had a spare mic (old coil tube), a battery eliminator, 
and the proper coax adapter. I spliced into the speaker/mic cable and took the
speaker audio into the usb fob. No COR or PL detect available(it may be
on one of the pins on the back but I have no manuals). It is speaker audio not 
discriminator and filtered of PL/DPL. I tried using DSP but it will keep the tx keyed
and shows rxsquelch level of 1000 even with the squelch closed. I tried opening 
the squelch and then locking it open while adjusting the audio and started getting the rxsqlech 
level to drop. Once I got it under say 700, No amount of adjusting levels and tweaking 
of the rxvoice and rxnoise levels by hand our auto adjust would give me "good" audio 
its all HIGHLY distorted and is either noisy or clipping as well. If I just ignore the squelch
and let the transmitter stay keyed, I could get some great audio, but the rxsquelch level
is at 1000 in DSP and doesn't close the software squelch. I tried VOX mode, but it stays 
constantly keyed, or will start kerchunking itself off and then back on. In adjusting the 
levels I can get good audio but can't get the DSP or VOX to react properly. 
I assume they are trying to calculate S/N ratio but there is no good compromise between level it likes for squelch 
noise and then the voice level that is then associated with it. I did try playing with the usbradio.conf and changing the
rxdemod=no, then speaker, then flat. no option worked. I got best audio with it on speaker, but the transmitter stays constantly keyed.

I tried a cap to eliminate any DC and a 1:1 transformer on the speaker mic audio into the usb fob. I can detect no 60hz groundloops
and I used a hound sniffer and then hooked up some headphones to the output of the receiver and don't hear any hiss.
I would like to make this work but its not looking good so far. 

Don
W9DRR



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