Sunday, January 08, 2006

Adaptive Audio Forensic Filters

A point of confusion shared by many people is exactly what is meant by the term adaptive filter. A further point of confusion is why adaptive filters are needed at all. Adaptive filters, used in this context, are filters that automatically reduce noise(s) by changing their own 'settings'. This is in contrast to fixed filters that are set once and stay that way until the user changes them.

For an adaptive filter to change its own settings, it must be given some sort of rule to follow so that it will know which sounds to throw away and which to keep. The rule may be as simple as 'mute when the sound level goes very low to remove any left over hiss' or ' turn the gain down if the signal gets too loud and may clip'. At the other extreme, the rule can be as complex as 'figure out when no one is speaking, assume what is left is all noise, and then change your own settings to remove the noise even when the speech comes back'.

At this point, you might can see some of the potential pitfalls in using adaptive filters - if they make wrong judgements then they can accidentally remove desired speech along with the undesired noise. Or they can make the speech sound very sterile, like it was recorded in an anechoic chamber. Or they can go hay-wire and create static and distortion. For these reasons, forensic examiners need to follow the central principle of the physicians' Hippocratic Oath when using these powerful filters - first, do no harm.

If they can be so dangerous, then why use adaptive filters at all? The reason is that many common noises can not be removed completely, or even at all, using fixed filters. Take echo, for instance. Real-world echoes change constantly. Every time the speaker turns his head or a door opens, the echo paths in a room shift around. What it all boils down to is if the character of the noise changes, then a filter that changes itself is often required to remove it. If you want to be an expert audio forensic technician or examiner, mastery of adaptive filters is a must.

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