equent frames frequency (e.g. 70MHz) and then re-converted to the required follow sufficiently rapidly the illusion of natural movement, as in a channel (frequency translation). cinematograph film, is provided. Signals coded in digital form are more resistant to the effects of A series of images presented to the eye at a rate above about 10 noise and distortion in transmission links, although vulnerable to per second (10Hz) gives an illusion of continuous motion but multipath distortion, and can be regenerated without loss of fidelity. accompanied by a pronounced flicker. If the rate is increased to The quality of the pictures and sound is determined primarily by the 25Hz to 30Hz the flicker is much reduced but still just noticeable, coding. Telecommunications practice is tending towards digital particularly when the images are bright. A repetition rate of 50Hz 57/4 Video transmission Movement due to line time base Movement due to line time base Figure 57.2 Simplified representation of double intrlaced Figure 57.1 Simplified representation of sequential scanning scanning provides motion virtually without flicker although for some very field are half lines, the line starting at the centre of the top line with bright pictures 60Hz or even higher may be desirable. The rate at a similar half line at the bottom of the picture, as in Figure 57.2. which video images (frames) follow one another is called the frame Since the scanning process has to be carried out synchronously in rate. the studio and in the receiver, it is necessary to provide synchroni- To transmit a series of sequentially scanned pictures at a frame sing signals that accurately define the start of the field scans and rate of 50Hz or 60Hz would require an excessively large bandwidth each line scan. Conventionally, this is achieved by transmitting two if the line resolution is more than about 300 lines. An alternative sets of timing pulses which are used by the receiver to lock its technique, universally adopted for broadcast transmission, is termed horiz