Friday, December 12, 2008

Ghosts of Analogue Past - 1

Fascinating to read of the colour restoration of a Dad's Army episode that only survived as a monochrome film recording - and also amusing to read some of the half-arsed technical 'explanations' given of how the restoration has been done. Hats off to James Insell and the team - and boos to anyone who confuses this with 'colourisation'.

I'm kicking myself for not thinking this was possible, actually: in a previous existence I worked alongside the old FR (film recording) machines, which were antiques even back then (early 70's). They were indeed nothing more than film cameras, slaved to 25fps, pointed at bright TV monitors. But pretty good, sharp TV monitors, and all nicely contained in a light-proof box. By all accounts, pretty good stock was used too.

In the PAL system, need I remind anyone, the colour information is transmitted (or 'coded' as was the phrase) in the analogue signal around 4.43** MHz - it's analogue, and the frequencies correspond to horizontal detail. The full luminance bandwidth was 5.5 MHz, so the colour signals sit amongst the fine detail - or to be blunt, above the detail that the TV broadcast chain, including the TV, of the time could actually display. But the film recording equipment was fed by closed circuit PAL baseband signals within TVC, hence no transmission errors, and it would appear, the colour signal wasn't filtered out of the feed to the monitor. This latter detail surprises me - it was usual to feed through a notch filter to avoid the 'colour dots'. Maybe there weren't enough notch filters to go round.

So, the colour signals are there on the film, to some extent - and with the wonders of HD telecine and a great deal of digital processing, we get to see the original again.

More technical detail at James Insell's (or his group's) wiki pages

Less technical detail, but you get to see the footage, at last night's Newsnight

Programme itself airs on Saturday next.

**Precisely the colour sub carrier is 4.43361875 +/-1 MHz. Some numbers just lodge in the brain.