Hey Joe...how are you making out with your Zello? Just my two cents...I've used Zello for many years. I've always found 'it's in the levels' to get it working correctly. I usually use the audio out (not line) from my scanners. I also have just a 'mic in' on my computers (I just run really old gear). Keep the volume out from the scanner quite low so as not to overdrive things. I usually end up with my Zello Activation Threshold at about 27. Deactivation between 40 & 50. I keep my attack at 100, and relax at 600 - 700. Those numbers are a general guide. Of my 6 distinct Zello feeds on different computers, there is a bit of variability, but those should be somewhere close to those numbers that work. It can get a little frustrating getting the app input to grab the audio consistently, yet staying quiet when I should. I end up playing with it until I get it working.
I can understand that you might get some drop-offs at -40 if your scanner audio output is a bit low. I do encourage you to play with it, I do like the Zello functionality, especially when on a mobile device for listening. Satisfying when you can get it working.
Good luck...
cheers
