In the first case, the widescreen pin allows to indicate the current signal format, which allows widescreen TVs to adjust the image width, and widescreen-capable standard TVs to compress the scan lines of the 576i image vertically to a letterbox shape portion of the picture tube. In the second case, the widescreen SCART signal is never active and the signal source performs the adaptations itself so that the image has always a standard format as a result. Some sources assume that the TV is always capable of widescreen functionality and hence never perform the adaptations. Some sources will not even issue the widescreen signal or maintain it at the same level all the time. Other sources might offer the option of truncating the sides, but not of letterboxing, which requires significantly more processing. Notably, the circuitry of the early widescreen MAC standard decoders (e.g. the "Visiopass") could not letterbox. The limitations apply mostly to satellite TVs, while DVD players can always at least letterbox and often "zoom".