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Friday, September 17, 2010

DC Offset

Okay, so today I'm feeling like a bit of a maroon. I think I've sussed what my "problem" is when recording my acoustic via Reaper.

Last night I recorded a section of heavy air (ok, that was a bad Grateful Dead reference)... actually, I recorded a few seconds with the microphone turned off. Even though I wasn't actually recording anything, I still saw what looked like noise on the waveform. I zoomed in. It wasn't noise, it was a constant zero offset. Recording for real I got some hiss but it was superimposed over the offset. What I'd been doing to remove the hiss was to heavy-handedly cut with filters to remove both the actual noise and the offset. A bit of googling came up with the phenomenon of "DC offset". I should have recognised this from my physics O-level rather than assuming I was seeing noise. By the looks of it, Goldwave automatically removes DC offset, which is why it doesn't show on the waveform. Now I need to either figure out how to remove DC offset within Reaper (I'm 99% sure there'll be a way) or I need to just open each track in Goldwave, remove the offset then save back to Reaper. Just a few clicks and bingo.

DC offset to one side (ha-ha), I obviously still get some hiss when mic-ing, which is mostly an issue with a low signal to noise ratio. That's where ReaFIR comes in.

ReaFIR is a Reaper VST plug-in which does a number of things, one of which is noise reduction. It's pretty simple to use. You record a few seconds of "silence" (hiss) before or after playing. You then use ReaFIR in "subtract" mode to sample the noise and remove those specific frequencies. Obviously you're likely to kill some of the good signal, too, but you can choose how much noise reduction you want to get the right balance of cleanup vs sonic joy. Goldwave has a similar "clipboard noise print" noise reduction feature, but ReaFIR's a bit more controllable.

So, I think I now have the know-how to record clean signals!

1 comment:

  1. :-S - see I think I'm a bright guy but that is all gobbledygook (to quote my Dad). That is why I stick to a simple little 8 track and live with it's limitations :-)