> I saw once a live show with a normal broadcast TV and a satelite TV. > While in the ads, the normal TV show then. > But the TV through satelite shows a blank screen , maybe because the ads are > localized and not shown to all country. Correct. > So ,this is a evidence that they broadcast some info in vertical retrace to > keep the regional repeaters in sync with the ads. In the UK the analogue channels use a small rectangle of black/white diagonal stripe to key the advertising drop in, and now days use teletext. The digital channels appear to use something buried in the digital data streams PDC can also indicate adverts but the standard is deliberately misimplemented by TV companies so it cannot be used to filter adverts
