ory signaling is carried out somewhat differently by "bit robbing." As we mentioned above, bit 8 of every sixth frame is "robbed" and used for a signaling bit. Thus each equivalent voice channel carries its own signaling. On the receive side the codec accepts the serial PCM bit stream, inputting the digit separator, where the signal is regenerated and split, delivering the PCM signal to four loca- tions to carry out the following processing functions: (1) timing recovery, (2) decoding, (3) frame alignment, and (4) signaling (supervisory). Timing recovery keeps the receive clock in synchronism with the far-end transmit clock. The receive clock provides the necessary gating pulses for the receive side of the PCM codec. The frame-alignment cir- cuit senses the presence of the frame-alignment signal at the correct time interval, thus providing the receive terminal with frame alignment. The decoder, under control of the receive clock, decodes the code character signals corresponding to each channel. The output of the decoder is the reconstituted pulses making up a PAM highway. The channel gate accepts the PAM highway, gating the n-channel PAM highway in sequence under control of the receive clock. The output of the channel gate is fed, in turn, to each channel filter, thus enabling the reconstituted analog voice signal to reach the appropriate voice path. Gating pulses extract signaling information in the signaling processor and apply this information to each of the reconstituted voice channels with the supervisory signaling interface as required by the analog telephone system in question. 6.4 LINE CODE When PCM signals are transmitted to the cable plant, they are in the bipolar mode as illustrated in Figure 6.10. The marks or 1s have only a 50% duty cycle. There are several advantages to this mode of transmission: No dc return is required; thus transformer coupling can be used on the line. The power spectrum of the transmitted signal is centered at a frequency equivalent to half the bit rate. It will be noted in bipo