Reverse Standards Conversion
Encyclopedia
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.
in the 60s and 70s were converted to NTSC
for distribution to non-625 line
markets. For many reasons, including the cost of video tape at the time, the original PAL master was often overwritten with new material or was simply discarded. This often left the NTSC version as the only remaining copy.
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 1/2-1/2 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.
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->NTSC->PAL) causes the artefacts previously mentioned to be exacerbated.
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.
stories The Claws of Axos
(1971) and Inferno
(1970). The resulting DVD release of The Claws of Axos also contained a documentary about the Reverse Standards Conversion process. 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. The process has since made it to a Doctor Who DVD once more for Episodes 1 - 3 of "The Sea Devils
" (1972). All stories needing RSC have been processed and are ready for release.
Historical justifications for its use
Many programmes produced by the BBC in PALPAL
PAL, short for Phase Alternating Line, is an analogue television colour encoding system used in broadcast television systems in many countries. Other common analogue television systems are NTSC and SECAM. This page primarily discusses the PAL colour encoding system...
in the 60s and 70s were converted to NTSC
NTSC
NTSC, named for the National Television System Committee, is the analog television system that is used in most of North America, most of South America , Burma, South Korea, Taiwan, Japan, the Philippines, and some Pacific island nations and territories .Most countries using the NTSC standard, as...
for distribution to non-625 line
576i
576i is a standard-definition video mode used in PAL and SECAM countries. In digital applications it is usually referred to as "576i", in analogue contexts it is often quoted as "625 lines"...
markets. For many reasons, including the cost of video tape at the time, the original PAL master was often overwritten with new material or was simply discarded. This often left the NTSC version as the only remaining copy.
PAL -> 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 1/2-1/2 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 artifactsDigital artifact
A digital artifact is any undesired alteration in data introduced in a digital process by an involved technique and/or technology.-Possible causes:...
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->NTSC->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->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 -> 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 WhoDoctor Who
Doctor Who is a British science fiction television programme produced by the BBC. The programme depicts the adventures of a time-travelling humanoid alien known as the Doctor who explores the universe in a sentient time machine called the TARDIS that flies through time and space, whose exterior...
stories The Claws of Axos
The Claws of Axos
-Writing:In late 1969, script editor Terrance Dicks contacted new writing duo Bob Baker and Dave Martin after reading a draft script they had sent around the BBC for another production, A Man's Life. After offering the duo a seven-part story in November 1969 for Doctor Whos eighth season, Baker and...
(1971) and Inferno
Inferno (Doctor Who)
Don Houghton came to Terrence Dicks with an idea for the story based on the real life Project Mohole. A smaller budget for the serial drove the idea of a parallel world, where the studio could use the same actors in multiple roles...
(1970). The resulting DVD release of The Claws of Axos also contained a documentary about the Reverse Standards Conversion process. 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. The process has since made it to a Doctor Who DVD once more for Episodes 1 - 3 of "The Sea Devils
The Sea Devils
The Sea Devils is a serial in the British science fiction television series Doctor Who, which was first broadcast in six weekly parts from February 26 to April 1, 1972.-Synopsis:...
" (1972). All stories needing RSC have been processed and are ready for release.
Further reading
Related research- Williams, Adrian and James Insell. 2005. "Reverse Standards Conversion", BBC Research and Development.