Understanding Audio and Video Wire and Cable
Contributed by: Ram Electronics
Before we get into cables themselves, you need to know what a conductor is.
What is a Conductor?An Electrical conductor is an element (remember the periodic table in chemistry?) which conducts electricity, as opposed to an insulator, which does not, or a semiconductor which allows some electricity to pass. There are also alloys that have different electrical characteristics, and platings or other element to element contacts such as in connectors which also have electrical characteristics as well as chemical characteristics in their applications. Simply stated - it is not just a matter of connect metal to metal and you have a good connection. The best connectivity designs take into account the source connector materials, the receiving device connector materials, the cable connector materials, the conductor materials and the electrical and chemical ways in which they interact with one another.
Meet the Conductors - Silver, Copper, Gold, Tin, Nickel, Steel
Silver is the best conductor, with a very slight edge over copper. Silver also has the benefit of having oxidation that conducts as well as unoxidized silver.
Copper is the next best conductor, with about 1.05 times the resistance of silver, and due to its lower cost is the most commonly used conductor for audio and video cables. Unfortunately copper oxidation is a semi conductor and should be avoided because of the "skin effect" which causes high frequencies to use the outside of the conductor at high frequencies. If the outside of the conductor is oxidized, the performance at very high frequencies will suffer. Note: This is does not have any significant effect in the audio frequency range.
Gold has about 1.4 times the resistance of copper and does not oxidize making it a popular plating for audio and video connectors.
Tin is a poor conductor, with about 8.5 times the resistance of copper, but has good resistance to oxidation and the oxide has good conductivity. Tin is quite often used to protect copper from corrosion.
Nickel has about 4.5 times the resistance of copper, good resistance to oxidization and good oxide conductivity. Nickel is a very common connector plating.
Steel has about 7 times the resistance of copper, lousy resistance to oxidization and lousy oxide conductivity. Steel is generally used only on high frequency cables plated with copper that need very high strength.
Connectors, conductors materials and corrosion
Connectors do add the possibility of being more impervious to conductor and connection corrosion and oxidation. If done wrong they can can actually increase the possibility.
OFC - OFC Copper is annealed in an Oxygen free atmosphere. While this should produce a slightly purer form of copper which should have slightly better conductivity and reduced skin effect, the overall production techniques involved as well as the insulation applied can have significant impact on whether or not the wire or cable will remain "oxygen free". A poor insulation, or poorly applied insulation will oxidize "oxygen free" wire or cable just like non oxygen free wire or cable.
Types of wiring and the "birth" of the audio/video cable
Discreet wiring - Discreet wiring is where separate conductors are run in some fashion (wires or circuit board traces) to the circuit components or input/output connections to which they need to go to. In some circuits, especially non signal related connections the path or interaction of these types of wires to one another is not particularly important as long as they are separated from one another to a reasonable extent. The closer they get, the more likely they are to possibly interact. This all goes out the window when a signal or "change" (whether they are due to ac power, analog signals or digital signals - basically any type of current or voltage change) becomes a part of the mix.
Example: Two wires are close together while another two wires are farther apart. The closer wires act more like a capacitor (have more capacitance) than the second two wires. If the circuit is in a quiescent state - "at rest" (no changes are going on) , there is potential energy storage due to capacitance, but without change, it has no effect on any signal, since there is no change. If you then create a change, or signal that causes electrical current flow, the two sets of wires may then have an effect on those changes, due to capacitance, inductance, etc, and the changes created may be different depending on the frequency of the signal involved and the characteristics of the wires and the overall circuit.
Example: A wire that has current flowing through it has a magnetic field. Two wires close together with current flowing through them both have magnetic fields. The two magnetic Fields will interact with one another if close enough and this interaction will effect the currents flowing through the wires more or less, depending on the rate of the change. If the wires are "send and return" signal wires the current flow will be opposite. If the current flow is opposite, the magnetic fields are in opposite directions. Electromagnetic noise acting on these opposite fields are canceled out (dependent on the frequency and distance between conductors) - thus is born the improvement for signal transmission called the cable.
On an interesting side note - our best friend, the Sun, and other interstellar objects create electromagnetic noise of various frequencies. While a black hole thousands of light years away is not going to cause you any trouble with your cables, the Sun is a measurable cause of noise on earth.
Types of Cables
Paired WiresWires which are in a configuration keeping them somewhat closely spaced together are paired wires. Paired wires can resist some noise due to cancellation of the signals created on the wires by the electromagnetic noise going in the same direction, and are canceled due to the opposite direction of current flow on the wires as seen by the receiving device. Er, kind of hard to picture, and very hard to explain. Maybe this helps - If the noise could create the opposite effect, it would reinforce the current flowing through both conductors and be an additive signal. The closer the wires are together, the greater the noise cancellation. The closer two wires of a given size are together, the greater their Capacitance and the lower their Inductance.
Twisted pairs are wires which are closely spaced as well as twisted around each other. Twisting the wires together makes it easier to keep the wires close together, and in addition greatly increases the noise rejection due to the twisting itself creating more or less equal exposure to the noise signal dependent on the twist ratio and frequency of the noise generated field. A mild twist will work for lower frequencies, whereas tighter and tighter twists are necessary for higher and higher frequencies. The consistency of the twist is very important for proper noise cancellation as well as constant spacing for consistent capacitance and inductance.
Capacitance - The closer the wires of given size, the greater the capacitance. The larger the gauge of two wires at the same distance the greater the capacitance. The higher the dielectric constant of the insulator, the higher the capacitance.
Inductance - The closer the wires of given size, the lower the inductance. The greater the size of the wires at a given distance the lower the Inductance.
Resistance - Dependant on wire gauge and material.
Impedance - Depends on wire spacing, wire material, dielectric constant of insulation and frequency. At higher frequencies, twisted pairs have what is called "characteristic Impedance". *See the coax cable section for more on this.
EMI (electro-magnetic-interference) - Twisted pair cables have electro-magnetic interference (EMI) cancellation. This cancellation is increasingly effective at lower frequencies and unlike coax, twisted pairs have EMI noise protection at frequencies under 1kHz.
Twisted Pair with Shield (Balanced Cables)The shielded twisted pair is a twisted pair with a shield (braid or foil, etc) surrounding the twisted pair. Basically used in balanced audio, you have a twisted pair with a common ground which is electrically between the two. Opposite polarity signals are applied between each wire and the shield. Using a transformer or electrical circuitry the signals are combined back together re inverting their polarities so they are additive, and the noise on them is subtractive. Balanced lines like this therefore have even greater noise immunity than twisted pairs and are excellent at running signals very long distances with very low noise. There are frequency limitations, since the twist can only be so tight and the greater degree of consistency becomes harder and harder to achieve.
The area where Coaxial cables spank twisted pairs, and therefore are so worthwhile for so many applications is in maintaining a constant impedance which becomes very important at higher frequencies.
Center Conductor, dielectric, shield, drain - A coaxial cable looks like a circle with concentric rings of different materials which do the dirty work. There are numerous configurations, some have multiple shields of different or similar types, and some have a "drain wire" - a wire running along with the shield. The very outer layer is the cable jacket, an insulator and protector for the shield(s). Next is the shield layer(s). The shield is one of the two conductors along with the center conductor. The "signal" is carried by both the shield and center conductor. Inside the shield, between it and the center conductor is the insulator which is sometimes referred to as the dielectric. The material and size has profound effects on the performance of the cable, particularly at higher frequencies.
Impedance - Impedance is the ability of a cable to impede electrical signal flow. Impedance changes over frequency, because it is determined by the Inductance and capacitance of the cable, which, in a coax cable is determined by the ratio of diameter of the center conductor, the diameter of the shield and the dielectric constant of the insulator in between them. The common spec you will see advertised for coax cables is the "characteristic impedance". For audio signals which are relatively low in frequency, the characteristic impedance is meaningless, since the wavelengths of the highest frequency signals are usually thousands of times longer than the cables carrying them. Because of this, reflections are not a factor. As frequency increases and the wavelength's actual length traveling through copper wire approaches the cable length, characteristic impedance becomes very important. This is very generally in the MHz range, and why characteristic impedance is so important for video and digital audio signals. The Characteristic Impedance is the impedance at witch, with matching input and output impedances there will be no reflections on the cable. The output device and input device should match the characteristic impedance of the cable to minimize reflections and signal loss. Of course, in the real world, no cable is perfectly uniform in size and shape, so small imperfections in the manufacturing process can have a real impact on cable performance, especially as frequencies increase.
Capacitance - Capacitance is again decreased with distance between conductors and size of conductors as with all conductors. Capacitance is increased with higher dielectric constant of insulator between the conductors.
Inductance Inductance usually is a small factor in Coaxial cables since they are straight rather than twisted and generally used in high frequency rather than low frequency applications. Once again, the amount of Inductance has to do with the sizes of conductors and their proximity. Closer and larger means less Inductance.
Velocity of Propagation and Dielectric Performance - The Velocity of propagation (VP) is the percentage of the speed of light in a vacuum that the signal can travel through the cable. It is directly related to the dielectric constant of the insulation between the center and outside conductors. PVC has a relatively low velocity of propagation compared to polyethylene (PE), polypropylene (PP) or the Teflon dielectrics - ethylene-propylene (FEP) or tetrafluoroethylene (TFE). Air or other gases are often used to "foam" dielectrics - adding air, which has a close to 100 VP reduces the overall VP of PE, PP or the Teflon dielectrics to around 78%.
Conductors, Dielectrics and Migration - Insulations between conductors can be harder or softer, and conductors can move from their designed distance from one another by twisting or bending the cable. This will change capacitance, inductance and characteristic impedance. Put one good kink in a cable and you now have strong reflections, signal loss and distortion. Care must be used when foaming dielectrics, since it softens the insulation and allows the center conductor to move off center when the cable is flexed, especially with solid conductors. Hard cell "high density" foaming and gas injection foaming can alleviate this problem to a large degree.
Shielding - Types of shielding and properties:
Braids, Served Shields and Foil - A braid is as it sounds a "braiding" of conductor material that in this case surrounds the inner insulator and center conductor. Served shields are a layer of individual strands of wire which are laid one next to the other with a spiraling twist around the insulator and conductor. A foil is pretty much what it sounds like - an extremely thin, solid (not stranded) foil like shield surrounding the insulation and inner conductor and often inside and/or outside of another shield.
Coverage and frequency - A served shield does quite nicely for coverage at lower frequencies, at least until flexed, which will make them more noise susceptible. A braid is great for coverage and dual braids reach up to around 95% coverage. The tighter the braid the smaller the "holes" and the higher the frequencies need to be to penetrate the shield. Obviously, dual braids would end up with smaller holes, and a foil braid can provide up to 100% coverage. So why not just use foil? Isn't 100% perfect? No, because foils just don't give a stable, and consistent impedance over the length of the cable, durability, or consistency when flexed. Many extremely cheap audio and video cables are made with a simple foil shield - they break very easily and generally perform very poorly.
Triboelectric effects, microphonic effects (handling noise) - Flexing, twisting, or transient impacts on cables with the floor, etc, while in use will cause "snaps", "crackles", "pops" and other noises in the signal due to rapid changes of capacitance between conductors. This is generally regarded an audio problem, normally effecting microphone cords and guitar cords. Served shields are ideal for reducing triboelectric effects. The geometry is good at resisting large capacitive changes while flexed or impacted. Braids are generally not nearly as good as served shields for this, again due to their design. Movement, and compression will cause noise. Foils are the worst for this - they are easily deformed causing large changes in capacitance and large "pops" coming from PA systems. Any live sound PA system engineer or studio engineer Needs to be deeply knowledgeable about these effects and how to avoid them at any cost.
*Note - Steel Coat Hangers as Cable
While the typical crazies on forums suggest that coat hangers will operate just fine as an audio cable (and poorly designed tests concur) there are definitive reasons why it is not a remotely good choice. First of all, Steel is a very poor conductor with over 7 times the resistance of copper wire. That means lots of wasted power. Second, steel oxidizes easily and the oxidization is a poor conductor. Third, steel is rather hard to make into a twisted pair, allowing for EMI and RFI resistance. Without making the steel into a twisted pair, Noise and Hum may become a larger than desired portion of the sound!