Horizontal spikes in waterfall = distorted audio

[Peter Fox]

Hello,

I'm having trouble with distorted audio.

I'm running 2 RTL-SDR at 1.8Mhz bandwidth on a 800Mhz P25 Phase I/II System. I am within earshot for two of the three sites on my county's radio system.

If I listen to both sites, within an hour I'll have horizontal spikes on my waterfall. The attached picture is an example, it's a very minor example all I have right now.

If I listen to one site, it happens occasionally. (meaning it will happen every few minutes in the middle of a transmission, which pretty much ruins the remainder of that transmission)

- I've tried not setting a preferred tuner in sdr trunk, as well as setting one. (Control channels are near each other for both sites so they always end up on the same dongle).

- When this happens, It seems to only affect the dongle receiving the most action. What is puzzling is even if I'm listening to one site, it still picks up all the rf from the other site, it's just not decoding it. If I switch preferred tuners, problem goes away for an hour.

- Have a fan blowing on my dongles, that keeps them cool to touch.

- I've adjusted the bandwidth down from 2.4 mhz to 1.8, no noted changes.

- Played extensively with gain settings, have improved it but not fixed.

- SDR dongles plugged directly into back of my PC. Mouse and keyboard i moved to front, no change.

- Turned on and off CPU vector calibrations.

- Reinstalled Java and SDR Trunk

- Swapped antennas and dongles

Here's my next options, hope to get your input:

1: buy a USB power meter, make sure dongles are getting sufficient power supply from PC motherboard.

2: If power supply is good, how do I know if my USB is the problem? Looks like I could buy an expansion USB card on Amazon for $30-50.

3: Other RF interference (FM, etc), buy a filter. I know I can receive FM using SDR#, is there a way to know what "to much" signal strength would be?

4: buy an Airspy R2 or mini.

I much prefer diagnosing the problem before throwing in parts. Thanks for your help.

[Peter Fox]

Well, I found This post. I've implemented it, and will see how I get along!

[charley....@gmail.com]

The post you found deals with assigning individual sdrs to fixed slices of spectrum.  Your issue as described sounds more like a hardware or signal issue.  We need some data points:

Describe your hardware more completely;

What SDRs are you using?;

What antenna are you using?;

Identify what systems and sites you are trying to monitor; 

What is your CPU loading?  Does your audio degrade if the spectrum monitor is disabled?

What does "within earshot" actually mean?  500 feet, one, five, or ten miles from the nearest tower site?

Note that my 0.5 B5 install uses two Airspys and monitors two systems: one 700 system (Mini) and one 800 system (R2) - it runs 24/7 and never degrades.  However, it uses almost all available cpu cycles on all 4 cores of an i5-6600T CPU, but significantly less cycles if I turn the spectrum monitor off - Java is a pig.

[Peter Fox]

2 SDRs - both RTL-SDR v3 (genuine not knock off)

Included Dipoles (Tuned to frequency)

Harrisonburg/Rockingham P25 (Harris system)

Valley and Mountain Sites

To be honest I havent even looked where my site towers are- my gain I have configured manually. LNA = 222, Mixer = 14, VGA 196 (or 210 i forget right now).

I know there is one tower about 1.5 miles as the crow flies.

But actually implementing those phantom channels to "lock" each SDR to a specific range has worked flawlessly for me. Ever since I did that (3 days ago), I have been monitoring the two sites with near perfect audio. Every now again one traffic call will get garbled and be illegible.

CPU use stays below 20% unless decoding more than 2-3 calls. have 8GB ram that stays low also.

Thank you for the quick reply, sorry took me so long, but at this point my problem is resolved!