Analog and digital system block diagrams; linear and non-linear distortion

Analog communication system

Source -> Input  -> Modulator/ -> Channel
          transd.   Transmitter      |  <- noise
                                     v
Dest.  <- Output <- Demodulator/ <---+
          transd.   Receiver

Digital communication system

Source -> Format -> Source -> Channel -> Modulator
          (A/D)     encoder   encoder        |
                                          Channel
                                             | <- noise
Sink  <- Format <- Source <- Channel <- Demodulator
          (D/A)    decoder   decoder

Linear and non-linear distortion

• Linear distortion: occurs in a linear system whose transfer function is not ideal. It does not create new frequencies; it only changes the amplitude and/or phase of existing components.
  • Amplitude distortion: $|H(f)|$ not constant over the signal band, e.g. a cable attenuating high frequencies more.
  • Phase (delay) distortion: phase not linear, so different components are delayed differently; causes pulse spreading and ISI.
  • Corrected by equalizers.
• Non-linear distortion: occurs when the system's input–output relation is non-linear, e.g. $y = a_1x + a_2x^2 + a_3x^3$ in an overdriven amplifier. It creates new frequencies: harmonics ($2f_1$, $3f_1$) and intermodulation products ($f_1 \pm f_2$, $2f_1 - f_2$), which cause crosstalk and interference. Reduced by operating devices in their linear range, negative feedback, or pre-distortion/companding.