codec accepts 24 or 30 voice channels, depending on the system used; digitizes and multiplexes the information; and delivers a serial bit stream to the line of 1.544 Mbps or 2.048 Mbps. It accepts a serial bit stream at one or the other modulation rate, demul- tiplexes the digital information, and performs digital-to-analog conversion. Output to the analog telecommunications network is the 24 or 30 nominal 4-kHz voice channels. Figure 6.9 illustrates the processing of a single analog voice channel through a codec. The voice channel to be transmitted is first passed through a 3.4-kHz low-pass filter. The output of the filter is fed to a sampling circuit. The sample of each channel of a set of n channels (n usually equals 24 or 30) is released in turn to the pulse amplitude modulation (PAM) highway. The release of samples is under control of a channel gating pulse derived from the transmit clock. The input to the coder is the PAM highway. The coder accepts a sample of each channel in sequence and then generates the appropriate 8-bit signal character corresponding to each sample presented. The coder output is the basic PCM signal that is fed to the digit combiner where framing-alignment signals are inserted in the Frame alignment 3.4kHz Channel 1 Voice channel Sample Channel PAM stream Coder Digit combiner input (transmit) output Channel n Transmit clock Timing recovery Channel 1 Channel n 3.4 kHz Voice channel Receiver Frame Digit output clock align separator input Channel 1 Channel PAM stream Signaling Decoder (receive) Channel 1 interface Channel n Signaling processing From transmit clock Channel n Transmit signal digits Figure 6.9 Simplified functional block diagram of a PCM codec or channel bank. 6.4 LINE CODE appropriate time slots, as well as the necessary supervisory signaling digits corresponding to each channel (European approach), and are placed on a common signaling highway that makes up one equivalent channel of the multiplex serial bit stream transmitted to the line. In North American practice supervis