It is very common for people to pay little attention to cable quality when building electronic systems. Cables play a huge role in your system potentials, as they are the medium for transmitting data or power. If the cables are inefficient as a transmitting medium, the performance of the system will suffer as there is a high level of signal losses.
Before we continue with this topic, we must first recognise the difference between a wire and a cable. In simplified terms, a wire is a conductor (e.g. copper) whereas a cable is made up of multiple conductors. For instance, a speakon connector cable will usually have a pair of conductors in order to transmit amplified signals to the respective speaker cabinet.
As a result, good quality cables are definitely important when it comes to building a system. In this post, I would like to address a few considerations for any practitioners building an audio system. All audio cables are screened (shielded), meaning that an outer conductor wraps around the others conductors to shield them from electromagnetic interference. The outer conductor may be made of various materials such as wire braid or metal foil.
In most cases, the outer conductor is connected to ground so that any induced currents (due to interference) will flow directly to ground rather than being mixed with the audio signal. However, this is not a perfect solution, which is why balanced signals were introduced.
In an unbalanced cable, there is a single inner core (conductor) that carries the audio signal while the outer screen act as the ground cable. The screen also doubles as the signal return path, hence any interference induced will cause the audio signals to be mixed with the interfering currents. These unwanted interferences are more prominent in long cable distances or where there are nearby sources of strong interference (e.g. dimming circuits).
Even in long distances, balanced cables are resistant against such interferences. As compared to its counterpart, a balanced cable has two inner conductors, commonly known as positive and negative. Similar to an unbalanced cable, the screen conductor is grounded but no signal will run through this path. Balanced audio transmission relies on Common-Mode Rejection Ratio (CMRR) to cancel out any interference picked up in the cable. A differentiating process will be implemented at the input stage of the receiving end to cancel out any noise. If any interference makes it through the screen conductor, it is likely to have the same influence on both the positive and negative conductors, thus the CMRR will reject any interference common in both conductors (positive and negative).
As mentioned, CMRR is based on the premise that unwanted external interference is being induced into both signal conductors equally, but is there any way to minimise the susceptibility of interference pickups to maximise the benefits of CMRR? Decreasing the distance between the two conductors by twisting them together helps to equalise the coupling effect of the interference with the audio signal.
A "loop antenna" is formed when two conductors have a spaced gap between them. Therefore, the farther apart the two conductors are, the large the antenna will be, thus picking up more interference along the line of transmission. Minimising the area between the two conductors helps to reduce unwanted hum and buzz from this type of interference, which the cable shield is almost ineffective against. The distance between the twists is called the lay of a pair. Shortening the lay by increasing the number of twists will improve not only the CMRR but also cable flexibility. Do take note that by shortening the lay distance, it would require more material (wire) and machine time, thus increasing hardware cost.
Understanding the characteristics of a good quality cable is vital in system building, as it determines the signal integrity carried through the medium. As a rule of thumb, if there is no option for a balanced output, I will usually not have any unbalanced cable of more than 5m, any more than that would require a Direct Injection (DI) box to transform the unbalanced audio to balanced. The above characteristics have been proven to help sustain signal integrity in real world practices and investing in a bunch of good quality cables is certainly wise.