onship between the input and output voltage is known as the transfer characteristic. The gain therefore varies with the Distortion of signals 10/9 instantaneous magnitude of the input signal and non-linear distor- transmission link is called the delay. At a reference frequency the tion occurs. delay a signal experiences is referred to as the absolute delay. 10.7.1.1 Attenuation distortion 10.7.1.3 Phase distortion If all frequencies involved in the transmission were subject to the With the propagation time being different for different frequencies, same gain and the same loss then attenuation distortion would not the wave front of one frequency will arrive before the wave front of occur. However, in all transmission media some frequencies are another frequency, thus the phase has been shifted or distorted. The attenuated more than others, therefore attenuation distortion results result is that the relative phases of the harmonic components of a from imperfect amplitude frequency response. signal are not maintained. Attenuation distortion across the voice channel is measured against a CCITT reference frequency of 800Hz. In North America 1000Hz is commonly used as the reference frequency. If a signal at 10.7.1.4 Harmonic distortion 10dBm is placed on the input of a channel, then assuming no gain or loss the output would be 10dBm at 800Hz, whereas the output at With non-linear distortion the application of a sinusoidal input 2500Hz could be 11.9dBm and at 1100Hz it could be 9dBm. The voltage results in a periodic output waveform that is non-sinusoidal. attenuation distortion for a voice channel is illustrated in Figure Fourier analysis shows that spurious harmonics are present, the 10.12. result being known as harmonic distortion. Total harmonic distor- tion is measured as the root of the sum of the squares of the r.m.s. voltages of the individual harmonics divided by the r.m.s. of the total output signal, V, where VH2, VH3 are the r.m.s values of the harmonic components. Equation 10.24 gives the total harmonic dist