What is Causing a Cyclic 1kHz Noise Signal in Audio Recording? I dug my old Behringer UCA202 out of my shed and plugged it into a computer. I did some tests and it appears to be working fine... apart from some noise signals on the line input channels. I'm not sure where this type of noise could be coming from; weather it's the computer, the powered USB lead, or the device itself. What components could cause this type of interference?

###Spectrum Analysis and Statistics

###8 Second Spectrogram 750Hz - 5kHz (showing ~3.3Hz Period)

###1 Second 0-10Hz Low-Freq Noise Signal(related? i.e above ~3.3Hz cycle)


I have unplugged everything else from the computer (except power lead and VGA monitor).
I have tried plugging the device into a powered hub. The DC input on the hub states 5v 2.5A, but I'm using a 5v 2.0A power source. I don't know if this matters.
This is the recording:
https://soundcloud.com/n00dlz/uca202-testdevices-unplugged-monitor-off/s-H9zQsQzqArY
Only the monitor, mains lead and UCA202 are plugged into computer, and I unplugged the monitor from the mains while recording. There is nothing plugged into the UCA202.
"The UCA202 uses the ubiquitous TI PCM2902 integrated USB DAC chip"

So this a "Full speed" USB device, and the USB data is being sent at 12 MHz, but it doesn't send data continuously; it sends it in short bursts every 1 ms, which means it has a frame rate of 1 kHz.  This 1 kHz noise finds its way into everything and it's really annoying when you're designing USB audio products. 

Sometimes it becomes much more audible because of a ground loop and you can improve it by lifting a ground somewhere, like unplugging your laptop from the wall, or connecting to a balanced input on your monitor instead of an unbalanced one, or using an audio transformer to isolate one thing from another.


  There is nothing plugged into the UCA202.


What if you short out the inputs instead of leaving them floating?  They're RCA inputs so they might not have normals that short themselves when not connected to anything.