Um, there's an "Audio Help" forum...
Anyway, noise in audio comes from a lot of places. Some of it comes from the recording device; this can be compensated for by running denoise filters on the audio (most audio editing systems have such a feature).
However, another significant source of noise that is much harder to reduce comes from the hardware itself, and the nature of analog->digital conversion. The hardware noise is due to a number of factors, such as electromagnetic radiation from other components in your system that interfere with the sound board. (This is why a lot of people like external sound units.)
As far as the analog->digital conversion goes: Computers are digital machines, and sound is an analog phenomenon. Mathematically, what you're trying to do when you do A->D conversion is trying to use a finite number of digits (and hence limited precision) to represent an infinite number of digits (infinite precision). This happens all the time with computers: even something as simple as representing 1/10th of a dollar (i.e. $.10) is impossible to do completely. (See what happens when you try to represent 0.10 in binary.) But I digress.
This is quantization, and quantization with low precision introduces noise. For example, if you quantize a continuous sine wave
with a very rough quantization function (nearest integer)
you get nasty waveforms. When played back, these produce "clinking" sounds. This is an exaggerated case, obviously, but it shows you another source of noise.
The only thing that you can really do cheaply to denoise audio is software processing. Anything else requires better hardware.
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