User:Interferometrist/AudioFilter

What a filter does (or something)
An equalization filter is classified as a linear filter specified by a particular (but usually adjustable) frequency response. Unlike a non-linear audio processing device (such as a fuzz box), such a filter introduces no harmonic distortion, that is, the only frequencies present in the output are ones present in the input. Unlike an adaptive filter or dynamic processor (such as a Dolby decoder) the frequency response does not vary in response to the program material. Rather, every frequency component in the input is passed to the output, but its amplitude (loudness) is multiplied by an amount set by the filter's response (usually specified in decibels) for that frequency, as we shall now illustrate.

In the figure at the right (not present) is the power spectrum of a 10 second segment of a song which can also be listened to using the player control (not present). This shows the net frequency content of the sounds contained within that short tract. This music is passed through the shelving filter whose frequency response is shown in the following figure (blue curve) which boosts the treble frequencies by up to 9 dB. The spectrum of the resulting audio signal is next shown (not present), and it can be seen that the level of each frequency component has been adjusted in magnitude according to the filter's frequency response. Being a linear filter, no new frequencies have been created, but the amplitude of each frequency in the original sound has been multiplied by an amount specified by the filter. Bass and midrange frequencies are multiplied by one, thus unaffected, whereas higher frequencies are multiplied by a larger number (dependent on the specific frequency) to create a sound which might be described as "crisper" as can be appreciated by playing the second audio file.

Now one might find the modified sound played through a hi-fi system somewhat unpleasant or "tinny" as its frequency content is now unbalanced. On the other hand, playing that very sound through an inexpensive full-range speaker (not containing a tweeter) might very well help compensate for that speaker's limited treble response thus equalizing the system's frequency response in a coarse sense. This is a typical use for the treble control in a home stereo system, and the frequency response shown above is typical of a treble control turned to its maximum.