next up previous contents index
Next: Equalizing filters Up: Taxonomy of filters Previous: Low-pass and high-pass filters   Contents   Index


Band-pass and stop-band filters

A band-pass filter admits frequencies within a given band, rejecting frequencies below it and above it. Much the same terminology as before may be used to describe a band-pass filter, as shown in figure 8.3. A stop-band filter does the reverse, rejecting frequencies withing the band and letting through frequencies outside it.

Figure 8.3: Terminology for describing the frequency response of band-pass and stop-band filters. The horizontal axis is frequency and the vertical axis is gain. A band-pass filter is shown; a stop-band filter would have a contiguous stopband surrounded by two passbands.
\begin{figure}\psfig{file=figs/fig08.03.ps}\end{figure}

In practice, a simpler language is often used for describing bandpass filters, as shown in Figure 8.4. Here there are only two controls: a center frequency and a bandwidth. The passband is considered to be the region where the filter's output has at least half the power ($\sqrt(1/2)$ times the gain) of its peak. The bandwidth is the width, in frequency units, of the passband. The center frequency is the point of maximum gain, which is approximately the midpoint of the passband.

Figure 8.4: A simplified view of a band-pass filter, showing bandwidth and center frequency.
\begin{figure}\psfig{file=figs/fig08.04.ps}\end{figure}


next up previous contents index
Next: Equalizing filters Up: Taxonomy of filters Previous: Low-pass and high-pass filters   Contents   Index
Miller Puckette 2005-04-01