I just bought a QYT KT-8900D mobile radio. One of the first things I've noticed is that I cannot use my mixer to completely lower the volume of the radio's 3.5mm TRRS output. When the fader is all the way down, there's still a good amount of audio coming through.
After closer inspection by my friend on his identical radio, an audio signal is being sent over just the ground pin somehow. Tested it on 3 radios with the same result.
Does anyone know why this is happening and how to fix it?
More info: the mixer takes 2 stereo unbalanced 3.5mm inputs. maybe the extra ring on the radio's output jack is actually for stereo? Some weird behavior I experienced with the mixer: When both faders for radio and bluetooth audio receiver are at negative infinity, there is no sound coming out. However, when I fade up only the BT audio receiver's level while keeping the radio at negative infinity, it increases the level of the radio's audio, and I can hear it in the background. It's not as loud as it would be if its fader were up, but it's definitely noticeable and only happens w/ radio.
Aha!
So, you've got cross-talk between the radio input and the other channels :)
Cross-talk between audio channels at a level this high is typically not because audio-frequency signals somehow couple into other conductors, but because higher-frequency components couple more easily.
My guess is that your radio might be leaking some IF signal into its audio outputs, which get transported along wires towards your mixer, influence some non-linear circuitry inside, and thus get mixed down to baseband involuntarily by your mixer acting like a radio receiver.
If you have a ferrite clamp lying around (old computer cables?), that might be something you can try adding to your audio cable.
Audio isn't send just over the ground pin, or the other pin. It's sent over both. If both weren't necessary, then why not just make audio connectors with one pin and audio cables with one wire?
The audio signal is represented by a voltage at the input. And the source of the signal manipulates that voltage by adding or removing electrons, what we usually call "current". And since electrons aren't just magically created, but must come from somewhere, where are they obtained?
The answer is the signal source doesn't create or destroy electrons: if it wants to increase the voltage then it takes electrons from the "audio" pin and deposits electrons in the ground pin. And to decrease the voltage it does the opposite.
So naturally, if you try to measure "just" the ground pin, you're going to see something. Only what you see is going to be highly dependent on how you've connected the test equipment.
The more likely reason your mixer can't totally eliminate the signal is probably just because mixers don't work like that. The faders only attenuate the signal, perhaps to the point where it's no longer audible. Or perhaps not, depending on how much amplification follows the mixer output, how good your ears are, and how quiet the room is.
