[Audacity-nyquist] RE: Audacity-nyquist digest, Vol 1 #73 - 2 msgs

[Roger D. <rb...@cs...>]

David,
	DC bias is the average value of the signal. Some reasons to get rid
of DC bias are that people cannot hear DC and audio systems cannot reproduce
it. In some cases, audio systems will try to reproduce it resulting in
running current through bass speaker coils, generating heat and also pushing
the speaker cone from the "neutral" position, which might limit the
speaker's ability to produce normal audio. When a signal with DC bias is
started, the onset of the DC bias is essentially a "thump" that's audible
and annoying.
	Signals are not ordinarily symmetric about zero, so if you offset
the signal so that positive and negative peaks are equal, you will probably
*introduce* DC bias rather than remove it.
	In practice, what you really want to get rid of (in my opinion) is
very low, inaudible frequencies, not just the "zero Hertz" DC bias. E.g. if
there were a sizeable component at 0.1 Hz, your DC bias could be zero, but
the signal would alternate between positive bias and negative bias every 10
seconds. I'm not sure how this would happen in real recordings, but I've
seen it happen in digital audio effects in Nyquist. The high-pass filter is
by definition the way to get rid of the low frequency terms.

	-Roger