2.2 Absolute and relative Levels

For level information there are two options, which depend on the reference value used.
The relative level is the ratio of two electrical quantities,
  • most often the ratio of output to input value A2/A1 (This quotient of output and input quantity accommodates the fact that in practice often not output but input is the reference value) or
  • between two distinct points, e.g. point 1 and point 2 of the transmission system in Figure 2-1 or 
  • in general, the signal value A2 at one point of the transmission system in relation to an arbitrarily chosen reference value A1, 
\( L = 10\text{lg} \frac{A_2}{A_1} \)  in dB.     (2-11)

In contrast, if the magnitude A of a physical quantity at any point of a transmission system is based on a fixed (normed) reference value A0 (rated value), with DIN 5485 it is called absolute level

\( L_{abs} = 10\text{lg} \frac{A}{A_0} \) in dBx.     (2-12)

These reference values are obligatory definitions of the so called international reference system (Figure 2-3) consisting of a voltage source with generator resistance Rg = R0 = 600 Ω. feeding exactly 1 mW of power into a load resistance of RL = 600 Ω. The values of current and power at the effective resistance are the “normed rated values”, the same as effective resistance and power. Often they are characterized by the index „0“.

The figure gives an equivalent circuit with an AC voltage source producing Ug = U1 and U0 and two resistances, one with Rg = Ro = 600 ohms, which together with the source builds the generator. The generator is connected to the load resistance RL = 600 ohms at which also the voltage UL is measured. UL is the square root of the product of P2 and R2 which is 0.775 volts, IL is the square root of the ratio P2 over R2 which is 1.29 milli ampere. Kinds of levels are relative level: arbitrary reference value A1 or absolute level: normed reference value A0 (rated level).
Figure 2-3: International reference system for absolute levels.