Reverse Standards Conversion

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

Many programs produced by the BBC in PAL in the 60s and 70s were converted to NTSC for distribution to markets outside the UK. 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.

Attempts to convert the NTSC version back to PAL format using the same conversion process yielded unsatisfactory results. Such double-conversions produce artifacts that usually manifest themselves as jerkiness in the picture where movement is present.

This is due to the PAL-to-NTSC conversion process. PAL and NTSC have a differing number of lines of resolution and also use a different field rate. Standards conversion techniques require that the differences between line resolution and field frequency be interpolated. This results in some of the original image data represented in the source material being lost as the signal is averaged across signal formats.

Using the same process to convert source material twice (in this example, PAL->NTSC->PAL) causes the artifacts previously mentioned during second interpolation pass (NTSC->PAL). Instead of extrapolating from the NTSC conversion to regain lost image data, the double interpolation results from the lowest common denominator in both resolution and field frequency. Even worse, converting field frequency twice results in distortion due to the different stepping rates. (This is somewhat similar to digitally re-sampling a down-sampled signal at a higher sample rate, in that the original signal and double-sampled signal often deviate significantly along where the stepping edges fall during both sampling passes.)

RSC was developed to remove the artifacts that early standards conversion methods introduced to the NTSC copies to produce a new superior master, usually in its original format (eg PAL). Instead of re-interpolating the already processed signal from the NTSC conversion, RSC extrapolates some of the original source signal by examining the temporal progression of the converted signal against the interpolation process originally used to generate the given field conversion. Additionally, RSC employed automated error-detection by re-converting the fields that fall on odd stepping intervals and comparing them to their corresponding fields in the original NTSC conversion. Field re-conversion errors exceeding the acceptable threshold can then be reprocessed against different field parameterizations until corrected. This is largely due to the fact that the NTSC field frequency is actually less than 60 frames per second by a fractional amount. Occasionally, an additional duplicate field was inserted into the converted signal to compensate. The error-detection employed provides a means to maintain frame/time synchronization despite the inability to detect and predict where these compensating fields occur within the original NTSC conversions.

An early example of material processed for commercial re-release using RSC is the 1971 Doctor Who story The Claws of Axos. The resulting DVD 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.

[edit] Further reading

• Williams, Insell. "Reverse Standards Conversion", BBC, 2005.