Nasty audio static from T2i using USB connection...

imqqmi wrote:

It sounds like a ground loop to me. The desktop pc is usually the culprit of such noise. It is caused by the power supply and power converters on the mainboard. It can also be caused by electromagnetic interference caused by switching power suplies like those of a laptop adapter. Make sure you keep those as far away from any audio device as possible. Also keep cabling apart.

The usb lead carries the noise through the ground and +5V and data leads. An usb mike is powered and amplified by this power source. The filtering is not always sufficient in such devices. I've experienced something similar with the alesis IO2 record and playback usb device while recording using a shure pg48 mike.

The solution is to use an usb isolator, powered by an old transformer based power adapter. Don't use a switching one as that will introduce harmonics, high pitched whines in the audio signal.

I use the cheap Olimex usb isolator, about 40 euros, it works well. The audio sounds very clean.

My laptop is also much better and don't need the usb isolator but it's nice to use the desktop sometimes for voice overs.

Thanks so much for your detailed response, Imqqmi, I'm only just now seeing it (thought I turned on the email updates, but I guess not). I did finally determine that unplugging the computer before recording "solved" the problem (creating another one, of course: the computer battery going dead).

Unfortunately, I am now hitting this problem again, but from a less dispensable source: the AC power adapter that I bought for the camera. The battery there was dying pretty quickly as well, so this highly-rated power adapter seemed like a perfect solution, but it's also causing the same noise issue (no longer sounds like a helicopter, but still completely disruptive).

I don't 100% understand everything in your response, but I gather that the Olimex device ($45 on Amazon) will probably take care of the problem using the old (camera battery powered) setup. Given that the on-camera microphone works just fine when recording with the AC adapter plugged in, do you think it's safe to say that the noise will disappear if I install the USB isolator with both the camera and the computer plugged into AC power?