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Entries found for search: filter

anti-imaging filter : See reconstruction filter.

anti-aliasing filter : Before a signal is subjected to the process of A/D conversion, it must be passed through a lowpass brick-wall filter to remove any components that are higher than the Nyquist frequency. This is because it requires at least two samples per cycle to determine the existence and strength of a frequency component or the A/D process will create aliased signals. See reconstruction filter, decimation, FIR, IIR.

all-pass filter : See all-pass network.

bandpass filter : A filter which has both a high-frequency and low-frequency rolloff, and only frequencies in between are allowed to pass. When applied to sound synthesis, a bandpass filter makes the waveform sound like it is coming down a phone line as telephone lines cannot reproduce lows or highs. The opposite of a band-reject filter.

band-reject filter : A filter that discriminates against signals in a specific frequency band. The most common band-reject filters reject a vary narrow frequency band, and they are usually called notch filters. The opposite of a bandpass filter.

bell filter : A type of filter that allows the boost or attenuation of a specified set of frequencies around a center frequency. Bell filters often allow users to adjust the center frequency, Q, and the amount of boost or cut. Bell filters are sometimes also known as haystack filters.

brick-wall filter : A very sharp filter which masks any frequency outside the passband, for example, the lowpass filter at the input of an A/D, used to prevent frequencies above the Nyquist frequency from being encoded by the converter. See aliasing, reconstruction filter, anti-aliasing filter, decimation, FIR, IIR.

comb filter : A type of notch filter which produces a series of very deep notches, or dips, in its frequency response. The spacing of the notches along the frequency axis is at multiples of the lowest frequency notch. A comb filter is produced when a signal is time-delayed and added to itself. Frequencies where the time delay is one-half the period and multiples of these frequencies are cancelled when the signals are combined because they have opposite polarity, usually used to filter out 60Hz hum and its associated harmonics. If the signals are of equal strength, the cancellation is perfect and the notches are infinitely deep on a decibel scale. See common mode. Also called timbral interference cues..

elliptical filter : A multiple-element, lowpass or bandpass filter which has the steepest possible rolloff slope and a small amount of ripple in the passband, with one or more notch filters added to it. Elliptical filters are used as anti-aliasing filters in digital audio devices.

filter : (1) A type of equalizing device for subtractively eliminating (subtractively) selected frequencies from the sound spectrum of a signal and perhaps, in the case of a resonant filter, increasing the level of other frequencies. See VCF. For example, a lowpass filter passes lower frequencies and removes the higher frequencies. By raising or lowering the filter rolloff frequency parameter, a sound will be made brighter or darker. (2) A device or MIDI software filter that eliminates selected messages from the MIDI data stream, usually called MIDI filtering by data thinning. See also running status.

filter resonance : The greater the resonance on a filter, the greater the effect of the filter: as resonance control is turned up, a little peak appears at the rolloff frequency. The harmonics that fall within that peak are accentuated. The greater the resonance, the higher the peak and the more pronounced is this effect. The effect of the swept resonant peak does not occur in real instruments. See also Q."

filter scaling : See keyboard scaling.

haystack filter : See bell filter.

highpass filter : A filter that attenuates the frequencies below its rolloff frequency.

lowpass filter : A filter that attenuates the frequencies above its rolloff frequency.

MIDI filter : See filter.

multimode filter : A type of filter which has a switch that allows a choice among lowpass, highpass, and bandpass modes.

scrape-flutter filter : In tape transports, a smooth or low-friction, non-magnetic, low-mass flywheel installed in the tape path in the order to minimize the pressure with which the tape meets guides, rollers, or other potential sources of scrape-flutter.

rolloff filter : A filter which has a reduced output as the frequency is increased is called a rolloff filter. A tone control is a rolloff filter when turned down. Sometimes the attenuated portion of the frequency content of the signal itself is called rolloff. A circuit that attenuates a signal that is above (lowpass) or below (highpass) at specified frequencies. For example, microphones usually have a bass rolloff filter to remove wind noise and/or excessive breath pops.

resonant filter : Nearly all musical instruments are a type of resonant filter, i.e., they pass certain particular frequencies and attenuate others, producing the tuning of the instrument. Each instrument, from a simple electrical resonator to a complex resonant system such as a guitar or organ pipe take an initial impulse and impress upon it the particular transient profile which gives the resonator its unique timbre and pitch.

reconstruction filter : In a digital audio system, in order to recover the analog signal from the digital words, a D/A converter is used. The output of the converter is a stair-step waveform which contains a great deal of high-frequency artifacts called images. To reconstruct a smooth replica of the original signal, the stair-step is passed through a steep lowpass filter, also called an anti-imaging filter. It is similar, or even identical, to the anti-aliasing filter at the input of the A/D converter, but its purpose is very different. Also called an anti-imaging filter. See quantization, decimation, FIR, IIR.

pop filter : A device that is used to reduce the "popping", distortion caused by overmodulation of a microphone, usually as a result of placing it too close to the sound source. It commonly occurs with the vocal pop screen, windscreen, or windshield.

notch filter See band-reject filter.

noise filter Either a narrowband or notch filter used to eliminate pitched noise, or a broadband filter used to attenuate the entire high or low frequency range.

swept filter : A type of filter where the frequency of the filter can be controlled electronically, rather than being manipulated by the user. An example of a swept filter is where a LFO could be used to sweep the filter frequency up and down, or an envelope could be generated to shape the filter sweep.

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