[linux-audio-user] Noise removal, what filter is best?

[Anahata]

On Mon, Aug 25, 2003 at 10:17:43PM +0100, Tim Beauregard wrote:
 
> I'm looking for a filter/software to remove noise from a sound file.
> I'm not an audio expert so will try to describe the noise in a way that
> you out there might be able to put a name to it and recommend a suitable
> filter/software to remove it.
> 
> It is a short sharp blip, fairly high frequency, sounds like a compass
> point stabbing glass.  It occurs as often as every minute or so in the
> two hour sound file.

Noise removal tools like that in audacity are for removng a constant
background hiss or hum and would not be suitable for an occasional loud
sqeak. I'm surprised that gramofile does not do anything for it, but I
think your best bet in this case may be to use a sound editing tool like
audacity and manually cut the noise blips. If they are much higher
amplitude than the normal sound they should show up on the display;
otherwise you'll just have to locate them by ear. When you find one you
can either cut the noise out (shortening the recording by the length of
the section you've cut out) or use the amplitude envelope tool to make a
rapid fade out/fade in over the noise. The first method will cause a
time discontinuity: the second may cause an audible droput in the sound.
It depends on the nature of the recorded material and the length of the
noise burst and you'll have to experiment to see which is the least
objectionable.

If the noise is at exactly one frequency, it may be possible to select
a short section containing the noise and apply a filter with audacity,
with a deep notch at the frequency of the noise. I wouldn't advise
applying such a filter all through the recording because it would
probably not sound very nice, and it's a pity to spoil a minute or good
sound just to reduce the effect of a fraction of a second of noise.
Again you'd have to do this for each noise burst, but with a bit of luck
(i.e. if the frequency is the same each time) the same filter setting will 
work on each one.

-- 
Anahata
anahata at treewind.co.uk       Tel: 01638 720444
http://www.treewind.co.uk    Mob: 07976 263827