ered time slot is structured exactly as described above for a 64-kbps channel, whereas the other time slots have no such structure. All channels have a frame structure in the case of multiple B and H channels; the initial channel controls 11 Note the use of ISDN terminology for channel types (e.g., B-channels). Turn back to Section 12.4.2.1 for a description of ISDN user channels. 12 This is the standard Nyquist sampling rate discussed in Chapter 6. TELEVISION TRANSMISSION Bit number 8(SC) Octet number Frame alignment signal Bit-rate allocation signal Encryption control signal Figure 16.12 Frame structure for a 64-kbps channel (i.e., a B-channel). [From Figure 1/H.221, ITU-T Rec. H.221 (Ref. 22).] most functions across the overall transmission, while the frame structure in the additional channels is used for synchronization, channel numbering, and related controls. The term I-channel is applied to the initial or only B-channel, to time slot one of the initial or only H channels, and to TS1 on and H12 channels. REVIEW EXERCISES 1. What four factors must be dealt with by a color video transmission system trans- mitting images of moving objects? 2. Describe scanning, horizontally and vertically. Define a pel or pixel (besides the translation of the acronym). If the aspect ratio of a television system is 4:3 and the width of a television screen is 12 inches, what is its height? NTSC divides a television image into how many horizontal lines? European systems? How do we achieve a sensation of motion in TV? Relate this to frame rate and flicker. In North American practice, the time to scan a line is 63.5 usec. This time interval consists of two segments: what are they? What is the standard maximum voltage excursion of a video signal? Just what are we measuring here? 9. Give two definitions of a composite signal. 10. At a TV receiver, about what S/N is required for an excellent picture? REFERENCES 11. If we were to measure S/N, we would measure S and we would measure N. In common TV practice, what measurement units are u