Chapter 2. From Analog Sound to Digital Audio

2.4. Cables

Why do I care about cables?

Connecting the microphone to whatever comes next in the signal chain is an audio cable. An audio cable is made of conducting material surrounded by shielding material. Its role at this stage is to carry the electrical current generated by the microphone to the preamp. Because electrical current is subject to interferences by other electrical devices or magnetic devices, and because one can generate the other, shielding is needed so that the electrical current (carrying all the information of the recorded sound!) can arrive at its destination safe and sound.

Cable impedance effects are generally negligible because they come into effect only when the highest frequency present in the signal corresponds to about 10 times the cable length. For example, a 20 kHz frequency is equivalent to a wavelength of about 10 km: Equation 2 applies, but here, speed is the speed of light, that is the speed at which electromagnetic waves travel through the medium; because there is resistance (impedance) to the progression of those waves in a cable, the waves do not travel at the full speed of light of around 300’000’000 m/s but at a fraction of its speed: that fraction is called the velocity factor VF and it depends, without any surprise, on the capacitance C and the inductance L of the material used for the cable.

Balanced and unbalanced cables: what are they?

There are two types of cables: unbalanced and balanced. Unbalanced cables use only two conductors: a central conductor, carrying the changing current, and a shield connected to ground. Examples of unbalanced cables are ¼ inch guitar cables or older RCA connectors. Balanced cables use three conductors, two in the middle to carry the changing current, and the last one for shielding purposes.

The advantage of balanced cables is that because there are two conductors, they can be designed so that interferences cancel out. Examples of balanced cables are XLR connectors and TRS connectors. While TRS does stand for Tip Ring Sleeve, XLR does not stand for Ground (X) Left Right as many people think it does; for the truth, read this: it has to do with how cables were once built.

Most images found on the web being legally protected against copying, simply type “balanced unbalanced connectors” into any search engine, select “Images” et voilà: all sorts of nicely drawn connectors, from RCA to TS to TRS to XLR.

It is important to know the difference because unbalanced cables should only be connected to other unbalanced cables to avoid ground loop issues. Cables can be recognized by their type to be balanced or unbalanced (see above); in case of doubt, device manufacturers usually indicate which type of connector you are dealing with.

To read a detailed discussion of cables, please see reference [1]; balanced vs. unbalanced cables are discussed here.


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