Communications Technology Handbook, 2nd Edition

39 Video on demand systems

Using digitally compressed video signals, it is possible to transmit good quality images at bit rates as low as 2 Mbit/s. By also employing spectrally efficient modulation schemes, it becomes possible to transmit up to eight NTSC, PAL or SECAM television signals within a 6 or 8 MHz bandwidth. The current TV frequency spectrum is thus capable of supporting up to 200 standard programmes. But how are these new programme channels to be allocated? The quality of the programme material must be acceptable to the viewer and also economically viable to the producer! One approach is to use a number of channels (ten) to radiate the same programme, but with starting points staggered in time (typically five minutes) to allow the ...