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This is not about coaxial cables, antennas and satellite frequencies,
here we have a look at a normal audio, mic or microphone cable.
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Cable and treble damping treble cut filter (low pass) Simplified principle C = Cspec · d |
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A frequently asked question is at which cutoff frequency fc the microphone cable has a (−) 3 dB loss of treble (Treble frequencies are attenuated)? Which length d a cable can have to reach this damping? How are high frequencies damped by the length of the cable? Cables are characterized by the capacity of the cable between the conductors, the resistance, inductance of the cable along the conductor, and the current crowding, which boost the resistance at high frequencies. The microphone cable usually has a capacitance from wire to wire of about Cspec = 100 pF per meter. The line resistance, and the inductance is usually negligible in practice. Each wire has an unavoidable cable capacitance, that leads to the damping of high frequencies (cable loss). Because the input resistance (load) is large against the small output impedance (source), the input impedance (load) can be neglected. |
Calculating the treble cutoff frequency of a cable
| Zout = Output impedance of microphone, source impedance |
| Cspec = Specific capacitance of the cable in pF per m cable lenghth |
| d = Length of the cable in m C = Cspec · d |
Formula for the cutoff frequency of the treble damping:
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Another frequently asked question is of how long can the microphone cable be,
without having treble loss?
Cable length
Calculation of the length of a cable at 3 dB treble attenuation
Formula for the length of the cable d and a given cutoff frequency fc: |
| fc = Cutoff frequency at (−)3 dB treble loss |
| Zout = Output impedance of microphone, source impedance |
| Cspec = Capacitance in pF per m cable lenghth C = Cspec · d |
| If you want to know how long the cable can be and permit a level damping of only 1 dB, you need to enter the double value of the cutoff frequency. |
| Cable characteristic impedance is a cable characteristics which is only valid for high frequency signals. Multimeters use DC current for resistance measurements, so you cannot measure the cable impedance using your multimeter or other simple measurement equipments. |
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