When I switch on my TV, audio is heard immediately but video comes only after some time. Why?

 When any TV is switched on, electric power is supplied to both audio and video sections. The video section consists of a cathode ray tube which is commonly called as picture tube. An electron beam is used to sweep the screen to display the pictures. This electrons beam is generated within the CRT using a filament which can emit electrons only when heated. This heating process requires sometime. Audio section does not need any such ‘warm up’ to start functioning and so we hear the sound as soon as we switch on.