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HIGH PERFORMANCE MICROPHONE CABLE
In the previous column, we talked about the three factors to choose in analog microphone cable: ruggedness, low noise, and performance. We had enough space to knock of the first two, and that leaves performance.
What is "high performance" in a microphone cable? This simple question is clouded by a lot of pseudo-science and mis-information. Exotic materials, tightly twisted pairs, silver conductors are sometimes mentioned. And many parameters are cited as "important". But the actual list of "important measurements" is really quite short. Table 1 shows a list of measurements and parameters of cable that are often cited as "important" with a note of how important each one actually is.
PARAMETER
MEASURED IN
EFFECT ON SIGNAL
Resistance
Ohms (Ω)
This concerns the size of the wire and the material it is made of. While resistance can affect distance, based on IR loss, it affects all frequencies equally so does not alter the signal. The effective signal distance is much more related to following parameters. Larger gage (AWG) wires might be chosen to increase the pull strength or ruggedness of a cable.
Capacitance
Picofarads (pF)
Stores energy between the two conductors in any cable. Since it affects high frequencies more than low, the effect is not linear, not the same at all frequencies. This gives cable the ‘slope' or ‘attenuation' that increases with high frequencies. A key parameter.
Inductance
Microhenries (μH)
Stores energy between the two conductors in any cable. However, the effect is opposite to capacitance and the two effects (called "capacitive reactance" and "inductive reactance") cancel each other out. Since the inductive reactance in a cable is much smaller than the capacitive reactance, capacitance always win. For that reason capacitance is listed in a catalog or data sheet, where inductance is not even mentioned.
Impedance
Ohms (Ω)
Impedance is the total effect of resistance, capacitance and inductance on a cable. However, it is only important when the length of the cable approached a quarter of a wavelength at the frequency being carried. For analog audios highest frequency, 20,000 Hz (20 kHz), the wavelength is over 9 miles, and the quarter-wavelength over 2 miles. Since we never run cables that far, the impedance of the cable is not even considered, and usually not listed in a catalog or data sheet.
Skin Effect
Sometimes mentioned in audio, this effect moves the signal from traveling down the whole conductor to the outside of the conductor at high frequencies. While measurable at audio frequencies, it is insignificant until the signal is 100 kHz or more, and generally not considered until 1 MHz or more, way beyond analog audio.
Table 1
So, you will note that there is really only one parameter to consider, and that is capacitance. And the lower the capacitance, the better the performance of the cable. This can also be shown in a different way.
In Table 1 it says that you can't go far enough to make the impedance of the cable important. However, this is not true of the impedance of the equipment sending and receiving the signal. In fact, you can calculate how far you can go on a cable by knowing the capacitance of the cable (in picofarads-per-meter, pF/m.) and the source impedance of the sending device. (If this is a microphone cable, then you want to know the source impedance of the microphone.)
Table 2 below shows the results of these calculations for various source impedances and cable capacitances. The distances shown are where a signal of 20 kHz would be attenuated by 1 dB, a very slight amount.
Source impedance
49 pF/m
66 pF/m
98 pF/m
164 pF/m
50 Ω
1648m
1236m
824m
495m
150 Ω
571m
412m
275m
165m
600 Ω
138m
103m
69m
41m
1000 Ω
83m
62m
41m
25m
10 k Ω
8m
6m
4m
2m
Table 2
Table 2 explains why high impedance microphones (10kΩ), sometimes used with consumer equipment, only come with short cables. Even the lowest capacitance is effective to around 8 metres until the signal is affected.
You will also note that professional microphones, typically around 150 Ω impedance, can go hundreds of meters, even with cheap cable. And with good cable, with low capacitance, over half a kilometer. This is also why some of those super-expensive microphones have very low output impedance, such as 50 Ω. (And some of those ancient condenser microphones allowed you to change their output impedance.) Look at the distance those signals can go. With a really low source impedance, the cable, in effect, disappears and has no effect on the signal. That first column applies to Category 5e, 6 and other computer data cables, such as the single pair Category 5e (Belden 1353A) mentioned in the previous issue.
And there are cables even lower capacitance than that, such as Belden 1800F, with a capacitance of 43 pF/m. This was intended to be digital audio patch cable. As such, it must have very low capacitance for those digital signals. But that means you can take advantage of that requirement for digital (low capacitance) and use it for analog audio.
This also means that all those single pair and multipair digital audio cables, make fabulous analog audio cables, because of their low capacitance. And that also means that, if you even move from analog to digital in the future, you won't have to pull those cables and re-wire that installation. It's already wired for digital ! You're "future-proofed" !
And if your customer balks at using a cable clearly marked ‘digital' for an analog application, just use some rubbing alcohol and rub off the word "digital". After that, the cable will work perfectly for analog. In fact, it will give you the best analog performance you have ever heard!
About the Author
Steve Lampen has been with Belden for 19 years and currently is Multimedia Technology Manager and Product Line Manager for Entertainment Products. He has an FCC Lifetime General License (formerly an FCC First Phone License) and is a BICSI Registered Communications Distribution Designer. His latest book "The Audio-Video Cable Installer's Pocket Guide" is published by McGraw-Hill.
Visit our web site www.beldenapac.com or email us info@beldenapac.com
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