TELETEXT FILTERS

A RANGE OF DIP PULSE SHAPING FILTERS
FOR TELETEXT SYSTEMS

To avoid data errors the fast pulses produced by data generators need to be band limited prior to insertion into a video channel.

When conventional low pass filters are used unacceptable time domain ringing of the pulse results.  This range of teletext filters is designed to band limit while avoiding this problem.  Due to technical and historic reasons three types of filter are in current use. They are based on the Gaussian, Raised Cosine and Cosine Roll-off realisations. Each has a slightly different characteristic. The cosine roll-off type is the only one that band limits the frequency spectrum to 5.5 MHz so ensuring that a 625 line video system is transparent to the pulse.