Television networks are dividing between the two operating systems of 525 and 625 lines with different field standards and at least three colour systems.
The BBC Engineering Research Department, at the mansion of Kingswood Warren, has designed the Advanced Field Store Standards Converter.
This is the input colour transcoder.
It's in here that the incoming signal is recoded.
There's nothing special about this - it's a normal comb filter transcoder.
From here, the incoming signal, still at 525 lines, goes into the interpolator and it's here that the real business of conversion begins.
That was me in 1968 describing the Advanced Field Store Standards Converter.
Eight bays' worth of equipment to exchange television pictures between different countries.
Now, 36 years later, I'm back at Kingswood Warren to tell you about Reverse Standards Conversion, or RSC, the process which has made possible the restoration of this Doctor Who.
To understand RSC, let's go back to the beginning of videotape recording at the BBC.
In 1958, the Corporation's research department demonstrated a system set to revolutionise television.
VERA - Vision Electronic Recording Apparatus - recorded television pictures electronically onto magnetic tape, making instant replay possible for the first time.
Half-inch tape ran through the giant spools and passed fixed recording heads at 200 inches per second.
But the machine could only record 15 minutes at a time.
VERA had taken six years to develop and was poised to change the face of television, but there was a rival machine made by the Ampex Corporation in the US.
Running on two-inch tape at 15 inches per second, it soon became clear that the American model would become the industry's standard.
Within 12 months, the BBC purchased an Ampex videotape machine and poor old VERA was abandoned.
The BBC's project had been led by Dr Peter Axon, whose innovations with VERA were to influence videotape development.
In a curious change of alliance, Dr Axon departed the BBC and went to work for Ampex.
Videotape recording meant that programmes could be distributed internationally without the need to transfer them to film.
But until the invention of satellite, recordings had to be flown around.
Even when it was possible to beam signals across the Atlantic, there was a problem.
British television pictures, using the PAL system, had 625 lines of resolution and refreshed the screen at 50 fields a second.
The American television system, NTSC, had fewer lines, but scanned at 60 fields per second.
This meant that an American-generated TV signal could not be directly played onto British television screens, or vice versa.
To begin with, the answer was optical conversion - a camera of one system was pointed at a monitor of the other system.
The resulting pictures suffered from flicker and were far from ideal.
The 1968 Mexico Olympics provided the impetus to find a solution.
Colour television had arrived on both sides of the Atlantic and the BBC needed to relay the NTSC pictures from Mexico live into their coverage.
Once again, the BBC turned to its research department.
The major hurdle was the need to store a field, that is one 50th of a second of television picture.
The answer was found in quartz blocks.
Rather than storing the information, the quartz delayed the signal long enough for the fields of the alternate TV standard to be constructed.
Processing by computer was not an option then and the design relied on analogue electronics.
For example, the quartz blocks were fitted with heating elements from electric irons.
This is one of the original blocks.
The signal came in to a transducer, was converted to ultrasound, went in and bounced around all over the place and finally came out milliseconds later.
Early standards converters were large, complex and in need of constant attention.
But it represented an enormous technological breakthrough and the BBC received one of several Queen's Awards to Industry for the converter.
As well as processing incoming NTSC signals, the BBC were now able to convert from PAL to NTSC for overseas programme sales.
In the 1970s, American and Canadian stations started to receive programmes such as ''Vanity Fair'', ''Up Pompeii'', ''Doomwatch'' and Doctor Who.
These did the rounds of American and Canadian stations and were sometimes wiped at the end of their tour of duty.
The assumption was that the masters were in England, but it was not always the case as many original tapes had been wiped and reused.
Videotape was very expensive in those days.
By the late 1970s, it was realised that the masters for many highly regarded programmes no longer existed.
An appeal to broadcasters in North America resulted in the return of several missing seriesin NTSC format.
For these to be screened in the UK, the recordings had to be standards-converted again from NTSC back to PAL.
This demanded an extra 100 lines of resolution, but with the scanning rate reduced from 60 to 50 fields per second.
These double conversions had unfortunate consequences.
The pictures were soft and movement even more juddery than normal NTSC conversions.
The theoretical solution was to unscramble the original with its interpolated fields.
But how could the information, now 60 fields per second, be returned to 50 without losing quality?
Back to the research engineers.
The sequence on some original PAL to NTSC conversions was analysed, unpicked and put back together.
The project, led by Jim Easterbrook, was largely theoretical, but a demonstration clip proved that the technique worked.
The motion was smooth and the pictures relatively sharp.
However, in the 1990s, the processing required to reverse-standards-convert a 25-minute programme would still have tied up a mainframe system for days.
The project was put on ice.
But as the years went by, computers became faster and cheaper.
By early 2002, the possibility of processing large amounts of video material had become a reality.
The original NTSC recordings on two-inch tape had been transferred to D1 cassette.
But a new round of research was still needed to perfect the RSC process.
A new engineer then came to the project.
James Insell had to undo the vertical interpolation applied by the original converter.
After painstaking analysis, using simulation software, the solution, involving a complex set of simultaneous equations, emerged.
In 2004, the first tapes were sent for reverse standards conversion - episodes two and three of the Doctor Who story ''The Claws Of Axos''.
Comparing a normal standards conversion with RSC shows how it overcomes the problems associated with conversions from NTSC.
The motion is smooth and the jagged edges of objects removed.
Resolution is also sharper than a normal conversion of such 525-line material.
The processing is still not in real time, but it does enable us to get the best possible quality out of the American tapes.
There are other programmes which could benefit from RSC, so this could be the start of a project to reconvert all the affected material in the BBC archive.
Finally, let us not forget the legacy of the BBC's videotape pioneer.
His work has influenced videotape development for half a century, but perhaps that is not all.
Is it just a coincidence that the first programme to be processed by RSC features aliens called Axons?
Like Dr Peter Axon, these extraterrestrials also had great technological abilities.
Was VERA, which could store and replay pictures of events in time, really the first step in a secret project to discover the secret of time travel?
Look again at the Axons' ambitions to travel in time and their ability to take on a human form.
Scientists are now asking, ''Could they be the same life form?
'' Or had the BBC's research establishment been infiltrated by aliens intent on exploiting the Corporation's foremost scientific brains?
Perhaps we will never know who was Dr Axon.
(DOCTOR WHO THEME)