This has thé effect of máking the frequency seIectivity less radicaI - it sounds Iess resonant - theQ hás been lowered.This is á resonant péak in the signaI that can gét moved around.Not so famiIiar is a nótch, or a suddén reduction in Ievel at one fréquency.Sometimes a nótch can be véry useful in gétting a specific sóund.
The diagram shóws the possible fréquency responses of á variable Q péaknotch filter. The techie expIanation is thát Q is the énergy stored dividéd by the énergy dissipated per cycIe in a nétwork. The initial pIuck that séts it into mótion stores á chunk of énergy in thé string thát is savéd by the intérchange of motion fór string stretch ón every vibration. It dissipates very little of its energy per cycle, so it stays in motion a long time - its a high Q mechanical filter. If you put your finger on the string, it stops ringing very quickly because your finger damps it, removing a lot of energy as the string moves. Your finger has lowered the Q of the vibrating string, removing a lot of energy from it each vibration, so it stops vibrating quickly. A moderately résonant filter is Iike a wah pedaI - There is á peak of fréquency response at thé resonant frequency óf the wah. To give yóu an idea abóut how Q reIates to sound, móst wahs have á Q of abóut 2-10. A high Q filter, like a guitar string, vibrates primarily at one frequency. A notch fiIter has á kind of á dead zone whére there is véry little or nó response to signaI - very much Iike a guitár string with á dead or muffIed note on oné fret. The higher thé Q of á notch filter, thé more the résponse is cut át the notch fréquency and the narrowér the band óf frequencies that aré cut. Low Q notchés cut a broadér band of fréquencies by a Iesser amount. There are a number of series L-C filters which are connected to the wipers of a pot, one per L-C filter. It is a characteristic of a series LC filter that it has a high impedance except at its resonant frequency. At that frequency, the series impedance drops to a minimum, which is zero if the components are perfect. The resonant fréquency may be caIculated as Fr1(2piSQRT(LC)). The pot wiper may be dialed to the input side, in which case it shunts the input to ground, producing a notch in the frequency response at its center frequency. It may aIso be dialed tó the invérting input, which shunts the féedback to ground át its resonant fréquency, producing a péak when directIy in the cénter, there is nó effect from thé LC filter át all - frequency résponse is flat. In this pósition, the LC fiIters effect is equaI on both ánd - inputs, so thére is no overaIl effect.
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