ith a signalling rate of only 1200 bauds (CCITT Recommendations V.26 and V.27). Six- teen state QAM is used to provide 2400bit/s data transmission over switched telephone circuits with a signalling rate of only 600 bauds (CCITT Recommendation V.22 bis) and 9.6kbit/s over private cir- cuits with a signalling rate of only 2400 bauds (CCITT Recommen- dation V.32). Pulse code modulation Pulse code modulation (PCM) can be produced by applying a train of amplitude modulated pulses to an analogue to digital (A/D) converter, as shown in Figure 19.14. Each analogue sample is thus converted to a group of on/off pulses which represents its voltage in a binary code. At the receiving terminal, a digital to analogue (D/A) Figure 19.10 Signal-space diagram for eight state converter performs the decoding process. The combination of a amplituded modulation coder and decoder at a PCM terminal is often referred to as a codec. The group of bits (i.e. binary pulses) representing one sample is 19/12 Digital modulation Figure 19.12 Signal-space diagrams for m-state quadrature amplitude modulation: (a) four state; (b) eight state ; (c) sixteen state; (d) thirty-two state called a word or a byte. An 8-bit byte is sometimes called an octet. discrete time. If a continuous signal has a bandwidth W, then the In a time division multiplex (TDM) system, the coder and decoder minimum sampling frequency, fs, that can be used is fs = 2W. This are required to perform their operations within the time slot of one is called the Nyquist rate. Subsequent removal of all frequency channel. They can therefore be common to all the channels of a components except the original baseband by a low pass filter at the TDM system, as shown in Figure 19.14. receiver recovers the original signal. For telephony, speech samples are usually encoded in an 8-bit If the sampling rate is less than 2W, the lower sideband about code. Since sampling is at 8kHz, a telephone channel requires binary digits to be sent at the rate of 8 X 8 = 64kbit/s. It was shown overlaps the base