ed within the studios, and as of the 625 line standard. Digital sound/data signals are inserted into far as the transmitter, it is impractical to transmit a frequency the video channel during the line sync periods and then frequency Ultraviolet Infrared light light 700 nm Wavelength Figure 57.5 The electromagnetic spectrum of visible light. Short wavelengths such as these may be quoted in Angstrom units (10-10 m) 57/8 Video transmission Video carrier Clusters of monochrome modulation sidebands Sidebands Sidebands 15 625 15625 harmonic cycles cycles 15 625 Unused spaces Figure 57.6 In a monochrome television transmission the sidebands are in groups around harmonics of 15625Hz leaving spaces in which colour information may be inserted modulated with the video signals on a single carrier. Although blanking periods using binary phase shift keying. Data bit rate digitally processed, the video signals are thus transmitted in anal- 20.25MHz. ogue form. The receiver decoder demultiplexes the signal, restoring D-MAC/packet has the same data rate as C-MAC but with the the YUV signals to their original parallel relationship, making them data signal in duobinary form inserted on to the baseband signal suitable for display as conventional 625 line YUV signals. as part of the multiplex rather than directly modulating the A number of MAC systems have been developed not only for DBS carrier. applications but also for use as a multiplexed signal within studios 4. D2-MAC/packet has a format similar to D-MAC but the data or for OB contribution links, electronic/satellite news gathering, etc. rate is half that of D-MAC (i.e. 10.125Mbit/s). It can thus carry There are also several different standards relating to the way in four high quality audio channels or eight medium quality chan- which the digital audio/data signals are transmitted: nels. 1. B-MAC. This system developed by Scientific-Atlanta for sat- The 625 line D-MAC/packet system was adopted as the UK DBS ellite cable distribution links or DBS can carry six digital audio stan