Introducing high-resolution line graphics in UK teletext using differential chain coding

A method is presented to integrate high-resolution line graphics in the UK Teletext system. Differential Chain Coding (DCC) enables an attractive combination of efficient graphics representation and a low-cost consumer decoder. Separate graphics overlay pages are linked to the text page by means of a ‘ghost row’, a concealed row of the text page. This separation of text and graphics guarantees compatibility with the existing teletext format. One major problem of DCC is its sensitivity to errors. Two fundamentally different approaches have been considered to solve this problem: error-correcting codes and protective measures to decrease the impact of errors. Experimental results indicate the feasibility of including DCC graphics in public Teletext systems, for bit error rates up to 5 × 10-3. For the higher BERs, repeated reception of the cyclic transmissions of teletext pages appears to be essential. This leads to better results than most error-correcting codes, and the decoder can remain simple and inexpensive.