ABSTRACT
Telephony, landline and mobile phones, and digital TV are digital in form. The analogue signals are sampled and then digitized prior to transmission as binary digits (bits). The digitization occurs within an analogue to digital converter (ADC) and the resulting data stream is commonly referred to as pulse code modulation (PCM). Baseband signalling cannot be used for radio broadcasting because the bandwidth of the signals is simply too great and only one transmission is possible. Demodulation is simply a matter of mixing the amplitude shift keying (ASK) down to the baseband. It is possible to combine ASK with phase shift keying (PSK) to give quadrature amplitude modulation (QAM). Coding is used to increase the reliability of transmission by adding a certain number of redundant bits. It can be very simple as in parity checking, or more complex as in Reed–Solomon (RS) coding. In convolutional coding, the input data is gated with itself to give an additional code containing extra bits.
