Tone control circuits

Tone control circuits are electronic circuits used to modify an audio signal before it is fed to speakers, headphones or recording devices by way of an amplifier. They are akin to and sometimes called filters. In their most basic form, they elevate or attenuate the high and low frequencies of the signal. This is called treble or bass boost or cut, respectively, in the vernacular of audio engineering. More elaborate tone control circuits can elevate or attenuate the middle range of frequencies too. Really elaborate tone controls provide elevation or attenuation in 1/3 octave bands spanning from approximately 30 Hz to 18 kHz. These units are called graphic equalizers because their controls are in the form of sliders that are arranged so as to graphically display the amount of boost or cut being applied at any frequency of interest. The most elaborate units can control not only the boost and cut but also the frequency at which the boost and cut takes place and the bandwidth of the control's action. These are called parametric equalizers because they control all of the equalization parameters.

Tone control circuits exist for many reasons. One is to allow listeners of recorded or broadcast music to adjust the sound to their liking. The adjustment may be necessary as a result of recording deficiencies, hearing deficiencies, listening room acoustics or playback equipment deficiencies. For example, some people prefer an attenuation (reduction) of the "strength" (amplitude) of the higher pitched tones, while many older people, who may have a hearing problem, prefer an emphasis of them. Another is to make adjustments to a musical signal prior to recording, e.g., when the acoustics of the recording site absorb some frequencies more than others.

Historically, all of this has been done via analog electronics and most tone control circuits produced today still use the analog process. However, digital electronics approaches to this function are increasingly being implemented through the use of digital signal processing.

The simplest circuits utilize only resistors and capacitors or inductors. They rely on the property of capacitive reactance or inductive reactance to inhibit or enhance an AC signal, in a frequency-dependent manner. This is relevant because sound (audio) is reproduced via signals consisting of alternating voltages or currents. Capacitors "let through" more of the higher frequency tones, while inductors reduce their amplitude. For example, a simple RC circuit (one capacitor and one resistor) could reduce or increase (depending on how they are connected), the amplitude of a signal at the rate of 20 decibels per decade, which is equivalent to 6 decibels per octave. Also, in order to boost or cut frequencies in the middle ranges, use is made of the resonance at the frequency at which the reactance of a capacitor equals the reactance of an inductor (coil). More elaborate circuits also utilize amplifiers to enhance their characteristics. The most modern analog units use operational amplifiers, resistors and capacitors, abandoning inductors because of their size and sensitivity to ubiquitous electromagnetic interference. Advanced versions of this type were extensively used in a preamplifier design published in Audio Amateur Issue 1/1992 (USA) by Williamson and Watling.

See also

*Audio amplifier
*Electronic filter
*Audio crossover

External links

* [http://www.schmarder.com/radios/tech/tone.htm The James-Baxandall Passive Tone-Control Network]
* [http://www.st-andrews.ac.uk/~jcgl/Scots_Guide/audio/part8/Page2.html Amplifiers and tone controls]
* [http://www.ecircuitcenter.com/Circuits/op_tone1/op_tone1.htm Audio tone controls]
* [http://home.comcast.net/~stphkeri/NegativeFeedbackTone.pdf Negative-Feedback Tone Control - Independent Variation of Bass and Treble Without Switches by P. J. Baxandall -pdf]

NB. The above mentioned file should now be downloaded from [http://www.thermionic.info/] Also, an article referenced by it entitled "Simple Tone Control Circuit: Bass and Treble, Cut and Lift, by E.J.James" may be downloaded from there as well.
* [http://www.duncanamps.com/technical/baxandall.html]