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A single tone signal carries only one piece of information - its frequency. It goes on at the

same level for ever.

To transfer any more information than that, you have to modulate the amplitude, phase or

frequency of this 'tone'. As soon as you vary some aspect of the tone, you can call it a

carrier wave - it is carrying information and will then occupy more spectrum space

(bandwidth) and other frequencies will be introduced.

An audio filteris a frequency dependent amplifier circuit, working in the audio frequency range, 0 Hz to

beyond 20 kHz. Many types of filters exist for applications including graphic

equalizers,synthesizers,sound effects,CDplayers andvirtual realitysystems.

Being a frequency dependent amplifier, in its most basic form, anaudiofilter is designed to amplify, pass

or attenuate (negative amplification) some frequency ranges. Common types include low-pass, which

pass through frequencies below their cutoff frequencies, and progressively attenuates frequencies above

the cutoff frequency. Ahigh-pass filterdoes the opposite, passing high frequencies above the cutoff

frequency, and progressively attenuating frequencies below the cutoff frequency. A bandpass filter

passes frequencies between its two cutoff frequencies, while attenuating those outside the range. A

band-reject filter, attenuates frequencies between its two cutoff frequencies, while passing those outside

the 'reject' range.

Anall-pass filter, passes all frequencies, but affects the phase of any given sinusoidal component

according to its frequency.

In some applications, such as in the design ofgraphic equalizersor CD players, the filters are designed

according to a set of objective criteria such aspass band, pass band attenuation, stop band, and stop

band attenuation, where the pass bands are thefrequencyranges for which audio is attenuated less than

a specified maximum, and the stop bands are the frequency ranges for which the audio must be

attenuated by a specified minimum.

In more complex cases, an audio filter can provide a feedbackloop, which introducesresonance(ringing)alongside attenuation. Audio filters can also be designed to provide gain (boost) as well as attenuation.

In other applications, such as with synthesizers or sound effects, the aesthetic of the filter must be

evaluated subjectively.

Audio filters can be implemented in analog circuitry asanalog filtersor inDSPcode or

computersoftwareasdigital filters.

Generically, the term 'audio filter' can be applied to mean anything which changes the timbre,

orharmoniccontent of an audio signal.

Audio-frequency signal generators generate signals in the audio-frequency range and above. An early

example was theHP200AAudio Oscillator, the first product sold by theHewlett-Packard Companyin1939. Applications include checking frequency response of audio equipment, and many uses in the

electronic laboratory.

Equipmentdistortioncan be measured using a very-low-distortion audio generator as the signal source,

with appropriate equipment to measure output distortion harmonic-by-harmonic with awave analyser, or

simplytotal harmonic distortion. A distortion of 0.0001% can be achieved by an audio signal generator

with a relatively simple circuit.[1]

http://en.wikipedia.org/wiki/Equalization_filterhttp://en.wikipedia.org/wiki/Equalization_filterhttp://en.wikipedia.org/wiki/Equalization_filterhttp://en.wikipedia.org/wiki/Equalization_filterhttp://en.wikipedia.org/wiki/Synthesizerhttp://en.wikipedia.org/wiki/Synthesizerhttp://en.wikipedia.org/wiki/Synthesizerhttp://en.wikipedia.org/wiki/Audio_signal_processinghttp://en.wikipedia.org/wiki/Audio_signal_processinghttp://en.wikipedia.org/wiki/Audio_signal_processinghttp://en.wikipedia.org/wiki/Compact_dischttp://en.wikipedia.org/wiki/Compact_dischttp://en.wikipedia.org/wiki/Compact_dischttp://en.wikipedia.org/wiki/Virtual_realityhttp://en.wikipedia.org/wiki/Virtual_realityhttp://en.wikipedia.org/wiki/Virtual_realityhttp://en.wikipedia.org/wiki/Audio_frequencyhttp://en.wikipedia.org/wiki/Audio_frequencyhttp://en.wikipedia.org/wiki/Audio_frequencyhttp://en.wikipedia.org/wiki/High-pass_filterhttp://en.wikipedia.org/wiki/High-pass_filterhttp://en.wikipedia.org/wiki/High-pass_filterhttp://en.wikipedia.org/wiki/All-pass_filterhttp://en.wikipedia.org/wiki/All-pass_filterhttp://en.wikipedia.org/wiki/All-pass_filterhttp://en.wikipedia.org/wiki/Equalization_filterhttp://en.wikipedia.org/wiki/Equalization_filterhttp://en.wikipedia.org/wiki/Equalization_filterhttp://en.wikipedia.org/wiki/Pass_bandhttp://en.wikipedia.org/wiki/Pass_bandhttp://en.wikipedia.org/wiki/Pass_bandhttp://en.wikipedia.org/wiki/Frequencyhttp://en.wikipedia.org/wiki/Frequencyhttp://en.wikipedia.org/wiki/Frequencyhttp://en.wikipedia.org/wiki/Audio_feedbackhttp://en.wikipedia.org/wiki/Audio_feedbackhttp://en.wikipedia.org/wiki/Audio_feedbackhttp://en.wikipedia.org/wiki/Resonancehttp://en.wikipedia.org/wiki/Resonancehttp://en.wikipedia.org/wiki/Resonancehttp://en.wikipedia.org/wiki/Analog_filterhttp://en.wikipedia.org/wiki/Analog_filterhttp://en.wikipedia.org/wiki/Analog_filterhttp://en.wikipedia.org/wiki/Digital_signal_processinghttp://en.wikipedia.org/wiki/Digital_signal_processinghttp://en.wikipedia.org/wiki/Digital_signal_processinghttp://en.wikipedia.org/wiki/Softwarehttp://en.wikipedia.org/wiki/Softwarehttp://en.wikipedia.org/wiki/Softwarehttp://en.wikipedia.org/wiki/Digital_filterhttp://en.wikipedia.org/wiki/Digital_filterhttp://en.wikipedia.org/wiki/Digital_filterhttp://en.wikipedia.org/wiki/Timbrehttp://en.wikipedia.org/wiki/Timbrehttp://en.wikipedia.org/wiki/Timbrehttp://en.wikipedia.org/wiki/Harmonichttp://en.wikipedia.org/wiki/Harmonichttp://en.wikipedia.org/wiki/Harmonichttp://en.wikipedia.org/wiki/HP200Ahttp://en.wikipedia.org/wiki/HP200Ahttp://en.wikipedia.org/wiki/HP200Ahttp://en.wikipedia.org/wiki/Hewlett-Packardhttp://en.wikipedia.org/wiki/Hewlett-Packardhttp://en.wikipedia.org/wiki/Hewlett-Packardhttp://en.wikipedia.org/wiki/Distortionhttp://en.wikipedia.org/wiki/Distortionhttp://en.wikipedia.org/wiki/Distortionhttp://en.wikipedia.org/wiki/Spectrum_analyserhttp://en.wikipedia.org/wiki/Spectrum_analyserhttp://en.wikipedia.org/wiki/Spectrum_analyserhttp://en.wikipedia.org/wiki/Total_harmonic_distortionhttp://en.wikipedia.org/wiki/Total_harmonic_distortionhttp://en.wikipedia.org/wiki/Total_harmonic_distortionhttp://en.wikipedia.org/wiki/Signal_generator#cite_note-0http://en.wikipedia.org/wiki/Signal_generator#cite_note-0http://en.wikipedia.org/wiki/Signal_generator#cite_note-0http://en.wikipedia.org/wiki/Signal_generator#cite_note-0http://en.wikipedia.org/wiki/Total_harmonic_distortionhttp://en.wikipedia.org/wiki/Spectrum_analyserhttp://en.wikipedia.org/wiki/Distortionhttp://en.wikipedia.org/wiki/Hewlett-Packardhttp://en.wikipedia.org/wiki/HP200Ahttp://en.wikipedia.org/wiki/Harmonichttp://en.wikipedia.org/wiki/Timbrehttp://en.wikipedia.org/wiki/Digital_filterhttp://en.wikipedia.org/wiki/Softwarehttp://en.wikipedia.org/wiki/Digital_signal_processinghttp://en.wikipedia.org/wiki/Analog_filterhttp://en.wikipedia.org/wiki/Resonancehttp://en.wikipedia.org/wiki/Audio_feedbackhttp://en.wikipedia.org/wiki/Frequencyhttp://en.wikipedia.org/wiki/Pass_bandhttp://en.wikipedia.org/wiki/Equalization_filterhttp://en.wikipedia.org/wiki/All-pass_filterhttp://en.wikipedia.org/wiki/High-pass_filterhttp://en.wikipedia.org/wiki/Audio_frequencyhttp://en.wikipedia.org/wiki/Virtual_realityhttp://en.wikipedia.org/wiki/Compact_dischttp://en.wikipedia.org/wiki/Audio_signal_processinghttp://en.wikipedia.org/wiki/Synthesizerhttp://en.wikipedia.org/wiki/Equalization_filterhttp://en.wikipedia.org/wiki/Equalization_filter

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Using TFM Audio Filter to produce high pass, low pass and band pass filter effect and to save them.

To do this you will need a microcontroller with a A/D and D/A converters and some memory.

If you log data from analog port at a rate of, say 8000/s (twice the highest frequency as per

Nyquist cratirion quality of a telephone) and store all logged data in a memory.

The playback through D/A converter has to be done at the same rate as recording.

http://neural.cs.nthu.edu.tw/jang/books/audiosignalprocessing/matlab4waveWrite.asp?title=4-

5%20Writing%20Audio%20Files

http://www.audiocheck.net/audiofrequencysignalgenerator_index.php

http://neural.cs.nthu.edu.tw/jang/books/audiosignalprocessing/matlab4waveWrite.asp?title=4-5%20Writing%20Audio%20Fileshttp://neural.cs.nthu.edu.tw/jang/books/audiosignalprocessing/matlab4waveWrite.asp?title=4-5%20Writing%20Audio%20Fileshttp://neural.cs.nthu.edu.tw/jang/books/audiosignalprocessing/matlab4waveWrite.asp?title=4-5%20Writing%20Audio%20Fileshttp://www.audiocheck.net/audiofrequencysignalgenerator_index.phphttp://www.audiocheck.net/audiofrequencysignalgenerator_index.phphttp://www.audiocheck.net/audiofrequencysignalgenerator_index.phphttp://neural.cs.nthu.edu.tw/jang/books/audiosignalprocessing/matlab4waveWrite.asp?title=4-5%20Writing%20Audio%20Fileshttp://neural.cs.nthu.edu.tw/jang/books/audiosignalprocessing/matlab4waveWrite.asp?title=4-5%20Writing%20Audio%20Files