Signal and information transmission circuits operating at high frequency need impedance matching to reduce
Needless to say, standards requiring high speed data communications, such as (Ethernet) LAN, USB, IEEE1394, and SCSI, defines the following to secure quality of impedance matching:
However, impedance matching is not considered in low frequency signal transmission lines, such as those used in analog audio.
Why can we ignore characteristic impedance in cables used in analog audio transmission cables ?
This is the question.
Note 1 - Characteristic impedance of cables
Impedance in electrical engineering is the ratio of voltage to current (voltage/current). In electromagnetic wave transmission lines, there are many traveling and reflected waves, and the characteristic impedance is defined as (voltage/current) of a single traveling (or reflected) wave. The impedance of the entire set of electromagnetic waves depends on the situation, thus waves with different frequency and direction must be considered separately.
The physical entities of voltage and current are electric and magnetic fields, respectively. The impedance becomes (electric field/magnetic field) for electromagnetic waves traveling in space, and in vacuum,
Z0 = sqrt(μ0 / ε0)
= 4 * π * c * 1e-7
= 376.7... Ω
Here,
Z0 = Wave impedance (Ω)
μ0 = Vacuum permeability (H/m)
ε0 = Vacuum permittivity (F/m)
c = Speed of light in vacuum (299 792 458 m/s) .. Defined value
Note that (characteristic) impedance indicates the ratio of energies in
electric field to magnetic field of energy in electromagnetic fields.
The symbol Z is used for impedance because Oliver Heaviside, the electrical
engineering genius who invented this concept, chose Z.
50 Ω and 75 Ω are commonly used in unbalanced coaxial cables. The attenuation of cables is proportional to the conductor resistance and inversely proportional to characteristic impedance. When the characteristic impedance is increased without changing the outer diameter, the internal conductor becomes thinner and conductor resistance increases. If the inner conductor is to be made fatter to increase conductor resistance, the characteristic impedance decreases. Therefore, there is a characteristic impedance that minimizes attenuation; assuming that the conductivity of the internal and external conductors are the same, the optimum impedance is about 50 Ω for polyethylene insulation and approximately 75 Ω for air insulation. Modern cutting-edge high frequency equipment typically uses 50 Ω, however the 75 Ω for air insulation used in the distant past is still used in video and audio equipment because of historical reasons.
Balanced cables generally use 90 Ω, 100 Ω, or 110 Ω because the characteristic impedance of parallel conductor lines in opposite directions are around this value. External shields decrease the characteristic impedance, thus small values are usually chosen in standards requiring shielding. There are two methods to regulate characteristic impedance: one is simply direct definition of the tolerance of characteristic impedance, and the other indirect method regulates return loss.
Note 2 - Digital audio cable standards
Digital audio standards such as AES, EBU, and ITU-T regulate 110 Ω balanced cables and 75 Ω coaxial cables.