Today I got my new Sennheiser PC 8 USB headset which - to my understanding - basically is a headphone + microphone connected to an integrated soundcard. So to the PC this is just another soundcard. According to some reviews I expected a decent sound quality, especially from the microphone.
I plugged it into the USB port on my "das Keyboard" (connected via a KVM switch) and did a quick recording to check sound quality. I was quite disappointed that the quality was extremely poor: There was lot of noise and the voice was hard to understand.
To be on the safe side I tried to directly connect the headset to one of the USB ports on my computer. To my surprise the result was completely different there: Excellent voice quality and no noise at all!
This left me baffled, as there's only digital signals on USB. How can there be any noise on one USB port but not on the other?
For completeness, here's the two setups:
1) PC(USB) --- Aten CS682 KVM --- das keyboard --- PC 8 headset 2) PC(USB) --- PC 8 headset
- 1) Lot of noise when recording
- 2) Perfect sound, no noise
- When I directly connect the keyboard to the computer, there's no static
- I found the whole setup with KVM switch is extremely picky in regard to the USB ports: Sometimes after changing connections (like when testing above) and going back to my initial configuration the USB hub on the keyboard does not work at all anymore. Then even powering off everything and restarting does not help. I have to reconnect the USB port plug on the KVM a couple of times to make it suddenly work again. It will then keep working even when restarting everything (power-off).
- I've also seen the USB port to suddenly stop working when I touch the KVM. Then again restarting/power-off does not help and I have to play around a lot to make it work again.
Somehow grounding issue sounds reasonable to me. The big question remains: What to do about it? All components are connected via regular power cords (German standard).
How to make everything properly grounded?
Here's an example recording of the noise: https://instaud.io/1Tdq
I tried to analyse the frequency spectrum with spectrum view in audacity. I may be wrong but to me this does not look like typical 50 Hz hum. It rather looks like 125 Hz (and multiples at 250, 500, 750, ...) and some other frequency at 875 Hz.