Filter

Any of various electric, electronic, acoustic, or optical devices used to reject signals, vibrations, or radiation of certain frequencies while passing others. Think sieve: pass what you want, reject all else.

Audio, Electronics, Signal Processing

For audio use the most common electronic filter is a bandpass filter, characterized by three parameters: centre frequency, amplitude (or magnitude), and bandwidth. Bandpass filters form the heart of audio graphic equalizers and parametric equalizers.

Some of the types of filter are:

1. Low Pass attenuates high frequencies.
2. High Pass attenuates low frequencies.
3. Band Pass attenuates both high and low frequencies.
4. Active includes amplifier.
5. Passive utilizes only capacitors, inductors and resistors.
6. Digital algorithm executed on digitized data.
7. Tracking Filter A narrow bandpass filter whose center frequency follows an external synchronizing signal.

Some of the terminology associated with filters:

• Bandwidth for band pass filters this is the difference in frequency between the -3dB attenuation points.
• Cut-Off Frequency the frequency at which the response is -3dB with respect to the maximum response.
• Delay Time time for a speific point on the input signal to pass through the filter.
• Effective Noise Bandwidth of a filter is defined as the width of an ideal filter which, with an identical reference amplitude gain, would transmit the same power from a white noise source.
• Fall Time time for the output to fall from 90% to 10% of the initial magnitude.
• Frequency Response the signal amplitude versus frequency for the filter.
• Insertion Loss the attenuation in the pass band of the filter.
• Order the number of reactive components in the filter.
• Overshoot initial increase of output amplitude above final steady state level for a step input.
• Phase Shift shift in phase between the input and output signals.
• Q-Factor for bandpass filters this is the ratio of bandwidth at -3dB to centre frequency.
• Rise Time time for the output to rise from 10% to 90% of the final magnitude.
• Roll-Off the terminal slope of the attenuating part of the frequency response expressed as x dB/octave.
• Shape Factor for bandpass filters this is the ratioof the bandwidth at -60dB to the bandwidth at -3dB.
• Skirt the terminal slope of the attenuating part of the frequency response expressed as x dB/octave.
• Transient Response The output of the filter for a step input.

Mechanical

A device to remove dirt and other impurities from air, oil, or water.

Optical Filter

A device used to reject certain frequencies of light.

See also: Band Stop Filter, Bandpass Filter, Constant Percentage Bandwidth Filter, Convolution, Infinite Impulse Response Filter, Kalman Filter, Optical Filter, Recursive Filter, Roll Off, Tracking Filter.