Hello all. I have just joined the Forum to offer, I hope, the solution to that annoying high pitched sound when recording. This bugged me for months and a notch filter is a really clumsy fix. The problem is similar to the (low frequency) hum we used to get with earth loops in the old Hi-Fi days. I record mainly Vinyl via my Hi-Fi amp. I also use and iMic external A-D but that is not the issue. The noise happens as soon as the amp is plugged in. Total and 100% fix is simple: I purchased as Audio isolator from Ebay (around £10.00). There are various connector options on offer, I used 3.3mm jack in/out. Hope this helps. Please note this noise has nothing to do with any OS or other applications or software, it is a hardware issue.
The fact that the isolator fixes it indicates a ground-loop problem.
A high-pitch whine usually originates from a switching power supply or just general data/switching from the computer getting into the ground and the analog circuitry.
Noise is ALWAYS an analog issue (acoustic or electrical). Except with 8-bit audio you get quantization noise, or dropouts/glitches/interrupts in the digital stream can create clicks & pops.
Another common problem is that USB power tends to be noisy, which is no problem for the digital audio data. But with USB-powered interfaces that noise sometimes gets into the analog electronics of the interface. II’s more common with microphone inputs (or phono inputs if your interface has phono inputs) because the preamp amplifies any noise along with the signal.
Hi Doug, bit bemused by this and why does ANALog have to be upper-case? I just wanted to post an easy non-technical answer to a problem that seems to have been going on for years (Google). Of course it is a hardware issue, I said so. I once measured the frequency (I think it as 3KHz but it was a few years ago) and I made a twin T notch filter which worked fine but had a bad effect upon the audio. Hi-Fi ground loops were always earthing issues. As a retired Analog/Digital/Hardware/Software design engineer of some forty years I thought an easy explanation best. Your additional details whilst totally correct don’t help anybody who just wants to make decent recordings. BTW the iMic (at least the version I use) is 16bit but that is irrelevant. Anyway, thanks for your input (without any whine).
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