Reverse Standards Conversion or RSC is a process developed by a team led by James Insell at the

BBC The British Broadcasting Corporation (BBC) is a British public service broadcaster headquartered at Broadcasting House in London, England. Originally established in 1922 as the British Broadcasting Company, it evolved into its current sta ...

for the restoration of video recordings which have already been converted between different video standards using early conversion techniques.

Historical justifications for its use

Many programmes produced by the BBC in

PAL Phase Alternating Line (PAL) is a color encoding system for analog television. It was one of three major analogue colour television standards, the others being NTSC and SECAM. In most countries it was broadcast at 625 lines, 50 fields (25 ...

in the 1960s and 1970s were converted to

NTSC NTSC (from National Television System Committee) is the first American standard for analog television, published and adopted in 1941. In 1961, it was assigned the designation System M. It is also known as EIA standard 170. In 1953, a second ...

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 to NTSC conversion (c. 1968)

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): * N0 = P0 (ideal = 0, error = 0) * N1 = P1 (ideal = 0.83, error = 0.17) * N2 = (P1 + P2) / 2 (ideal = 1.67, error = -0.17) * N3 = (P2 + P3) / 2 (ideal = 2.5, error = 0) * N4 = (P3 + P4) / 2 (ideal = 3.33, error = 0.17) * N5 = P4 (ideal = 4.17, error = -0.17) * N6 = P5 (ideal = 5, error = 0, start of next group) 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.

Double conversion problems

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 Jerkiness (sometimes called strobing or choppy footage) describes the perception of individual still images while watching a motion picture. Description Motion pictures are made from still images shown in rapid sequence. Provided there is suf ...

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.

Summary

RSC was developed as an alternative to double-conversion. Use of RSC bypasses the generation of the artefacts that would be introduced in a normal NTSC-to-PAL conversion, and actually reverses the early standards conversion method used to create the NTSC copies. RSC is the result of reverse engineering the method of conversion inherent in the old traditional BBC PAL to NTSC converter. RSC attempts to separate the information from the merged lines and fields of the NTSC conversion. One of the problems inherent in this is that of increased noise. RSC employs techniques to minimise the resultant noise - both in the separation process itself, and in preparation of the NTSC material prior to processing through use of HF linear filtering.

Programmes recovered by this process

Early examples of material processed for commercial re-release using RSC are the ''

Doctor Who ''Doctor Who'' is a British science fiction television series broadcast by the BBC since 1963. The series, created by Sydney Newman, C. E. Webber and Donald Wilson (writer and producer), Donald Wilson, depicts the adventures of an extraterre ...

'' stories '' Inferno'' (1970) and '' The Claws of Axos'' (1971). The resulting DVD release of ''The Claws of Axos'' also contained a short documentary about the Reverse Standards Conversion process, presented by Jack Pizzey. It includes a split screen comparison between the source NTSC version and the final RSC processed version. The "Inferno" DVD does not feature this comparison. RSC was also used to restore the " Undertaker Sketch" from ''

Monty Python's Flying Circus ''Monty Python's Flying Circus'' (also known as simply ''Monty Python'') is a British surreal humour, surreal sketch comedy series created by and starring Graham Chapman, John Cleese, Eric Idle, Terry Jones, Michael Palin, and Terry Gilliam, w ...

'', which had been cut from the BBC's PAL master tape for being in (deliberately) bad taste.

Historical justifications for its use

PAL to NTSC conversion (c. 1968)

Double conversion problems

Summary

Programmes recovered by this process

See also

Further reading