Thursday, November 30, 2006

Signal Processors, Equalizers, and feedback Control

Signal processors fall into three main categories based on which property of the audio signal they affect: equalizers affect frequency response, dynamic controllers affect amplitude, and delays affect time properties such as phase. Each of these can be useful in the operation of microphones but the equalizer are of particular interest because of their potential use in feedback control.

Feedback is a very frequency dependent phenomenon because it occurs first at peaks in the overall sound system frequency response. Equalization of the response may significantly affect the onset of feedback. Equalizers are frequency dependent filters that fall into several categories based on the characteristics of the filters and their adjustment. Hi-cut and Lo-cut filters progressively attenuate, or reduce all frequency. That is, the attenuation increases with frequency further above or below the cut off frequency. The cutoff frequency may also be adjustable from a minimum of 6db as to steep as 24db. Hi cut and low cut filters are used to reduce the bandwidth or frequency range of the signal to remove unwanted high frequency or low frequency sounds such as hiss or rumble. Shelving equalizers allow low frequencies to be cut or to be boosted. The cut or boost is not progressive: it is the same as all frequencies below or above the filter frequency. The response curve looks somewhat like a shelf above or below the filter frequency. The amount of cut or boost is adjustable typically up to plus or minus 15db/ the filter frequency is usually fixed: about 250 Hz and blow for the low frequencies, about 8000Hz and above for the high frequencies. Shelving equalizers are used for general response shaping at low or high frequencies. They are the type of filter used as bass or treble tone controls. Bandpass equalizers allow frequencies within a certain band or range to be cut or boosted. They are classified according to their bandwidth and/or according to the number of filters employed. Bandwidth is usually given as a fraction of an octave. For example a midrange tone control is a single bandpass filter with a 1 octave bandwidth designed to affect the frequency range between a bass control and a treble control, typically 500Hx-1000Hx. Again the range of cut or boost is typically up to 15 db. Bandpass filters have fixed frequency and fixed bandwidth.