12-08-2018, 03:17 PM
Like Jeffrey, I also built the Wireless World teletext decoder. As this was well before Texas released their TIfax decoder, it was the first (and only!) decoder available for my early Stock Exchange experiments!
Like Jeffrey's, it was also rather touchy and error prone. This was mainly because the group delay specification for Teletext is far tighter than found in the average TV IF strip of the time. We also got a Decca monitor receiver which did improve things considerably but setting the optimum data slicing level was close to impossible - switch from CEEFAX to ORACLE, for example, and you would need to set it again!
In hindsight, the solution would probably have been to incorporate a peak level detector and set the slicing level at 50% of that, but it just didn't occur to me at the time!
Like Jeffrey's, it was also rather touchy and error prone. This was mainly because the group delay specification for Teletext is far tighter than found in the average TV IF strip of the time. We also got a Decca monitor receiver which did improve things considerably but setting the optimum data slicing level was close to impossible - switch from CEEFAX to ORACLE, for example, and you would need to set it again!
In hindsight, the solution would probably have been to incorporate a peak level detector and set the slicing level at 50% of that, but it just didn't occur to me at the time!
