Michael - NA7Q wrote:
Interesting. I don't know what chipset is in that dongle. Many
chipsets have volume switches that are built into the audio lines.
When grounded, they trigger the switch to go down or up. It sounds
like you may have triggered this switch by the grounding when you're
transmitting. I had this issue on a USB sound card a while back when I
was playing with a CM108. Which is one USB card that has played better
with RF than any other.
What does your wiring look like?
Yeah, this is looking more and more like the issue. The wiring is
simply a pin-to-pin connection, speaker to mike, and mike to speaker.
Common ground for everything. It's definitely triggered by RFI, since
transmitting into a dummy load, or into an antenna some distance away,
will run just fine. I can notice the volume control move with even a
single packet, when the antenna (twin-lead J-pole) is in the shack.
So how is the volume actually controlled? The wiring is sort of
shielded, but not very well. Do I need to isolate the Pi and radio with
some in-line DC-blocking caps, or does it need a complete isolation
transformer? I'm guessing it's the mike input to the USB dongle where
this is happening, as that's the not-so-shielded wire. The TRRS pinout
is frankly bizarre. Might try to squeeze a cap in there, at the dongle end.
I'm guessing that somewhere in the /var/run/udev files is where the
volume controls are kept. Based on the rsync tests, those are the only
files that seem to change across test runs.
Thanks,
Greg KO6TH
