Overview

The original purpose of the Vertical Blanking Interval (VBI) was to allow early television receivers to have very simple sync recovery circuits. Modern receivers are not affected if extra waveforms are placed in the VBI, so broadcast engineers can now use this precious resource for test, control and data signals (Figure 2.7.1).

Figure 2.7.1: The PAL Vertical Blanking Interval (VBI) contains 'unused' lines. Modern sync recovery circuits will still work even if the VBI is filled with test waveforms and data.

In Europe (and other 25 Hz regions) a popular text and data distribution system called teletext has used spare capacity in the VBI (Figure 2.7.2). Variants of teletext have been used for broadcasting Internet, private data and control signals. Several advanced forms of teletext have been designed but have not been widely used for a number of reasons, including incompatibility with legacy decoders, bandwidth scarcity and the high cost of developing the newer decoders. The imminent arrival of newer, interactive, digital technologies means that the more advanced forms of teletext may never materialise. Teletext is in use in over 30 countries, including Australia, Austria, Belgium, China, Denmark, Finland, France, Germany, Hungary, India, Ireland, Italy, Malaysia, the Netherlands, Norway, New Zealand, Poland, Portugal, Singapore, Spain, Sweden, the UK and the United Arab Emirates.

Figure 2.7.2: In some European countries the VBI is now virtually full of teletext and other data. The diagram shows VBI of ITV in the UK. Line 335 (arrowed)...