The curious sound of 864.0500 (SD City TRS)

[GroundLoop]

LCN #522 at 864.050 MHz on the San Diego City TRS has a sound unlike the other frequencies.

In the background of every call assigned to this channel there is a strong whistling/warbling signal. I've heard it on my Uniden, and on Mot radios as well.

I thought it might be interference, but there is a predictable 'data' sound to the whistle that changes near the end of the call. I could be fooling myself.

The sound is not subtle, and the same radios on different channels don't exhibit it.

Any ideas what this is from?

[GroundLoop]

Hmm.. I should probably just start by asking: "Does anyone else hear this?"

If not, it's something about my locale or listening equipment.

[inigo88]

I monitored it last night and it sounded the same as all the other voice frequencies, so it may be local interference.

[GroundLoop]

Good info! Thanks inigo88.. I'll do more sniffing around. I'm starting to think the 'data' sound comes from the subaudible tones at the end of the voice transmission somehow mixing with my local interference.

[WayneH]

There's always subaudible data ("lowspeed") during a trunked analog call in this situation. If it disappears the call will drop. Then there's the specific disconnect tone/word at the end.

[GroundLoop]

I did some exploring and found that this whistle is somehow caused by an NVidia graphics card, even a hundred feet away. It's entirely local.

My guess is that the graphics card is putting out a CW at close to this frequency, and it 'beats' with the signal in such a way as to make the subaudible data audible. You can see that RF is voodoo to me, and this is clearly outside my ken, but at least I know the source.

[GroundLoop]

I like threads that come to closure, so I thought I'd correct my own misinformation here.

I borrowed a fine Anritsu spectrum analyzer and went hunting for the source and nature of this signal a bit more. I was wrong to blame the NVidia card.. the source of my particular spur is (with certainty), a Plantronics Audio 630M USB conferencing headset. This little bugger has a small ferrite choke near the USB connector, but puts out an incredibly strong unmodulated (22 Hz wide) signal that starts around 910MHz at power-up and drifts down to 864.053400 MHz as it warms up. It's temperature dependent.

I read about -40dBm off just a Radio Shack scanner antenna.

Disabling the (Windows) device driver does not stop the signal.

Just goes to show, you never know what gadget brings in a hidden splinter for radio listening.