Reverse Standards Conversion

Reverse Standards Conversion or RSC is a process developed by a team led by James Insell at the BBC for the restoration of video recordings which have already been converted between different video standards using early conversion techniques.

Many programmes produced by the BBC in PAL in the 1960s and 1970s were converted to NTSC for distribution to 525 line markets. Because of the cost of video tape at the time, the original PAL master was often overwritten with new material or simply discarded. This often left the NTSC version as the only remaining copy.

PAL and NTSC have a differing number of lines of resolution and also use a different field rate. Traditional standards conversion techniques adopted interpolation as a way to cater for the differences between line resolution and field frequency.

What the original BBC converter tried to do (using the limited technology of the day) was to minimise judder by choosing either one 50 Hz field or a half-and-half mix of two 50 Hz fields, whichever was "nearer" to the temporal position of the target 60 Hz field. This gives a sequence like this (Nx = 60 Hz field x, Py = 50 Hz field y):

Simply taking the nearest raw field would produce a peak to peak "error" of 0.83, instead of 0.33.

This approach of interpolation results in some of the image data present in the PAL source material being merged between lines / fields of the resultant NTSC version.

Attempts to convert the NTSC version back to PAL format using traditional conversion processes yielded unsatisfactory results. Such double conversions produce artifacts that manifest themselves as jerkiness in the picture where movement is present, and in soft-looking pictures.

Using interpolation processes to convert source material twice-over (in this example, PAL to NTSC to PAL) causes the artefacts previously mentioned to be exacerbated.