from Hacker News

PAL Colour Recovery from black-and-white ‘telerecordings’ (2008)

by madflame991 on 8/28/22, 9:53 AM with 44 comments

• by hilbert42 on 8/28/22, 11:55 AM

  In Australia before the official introduction/launch date of PAL colour television in 1975 it was a requirement of the then, now defunct, broadcasting regulator, the ABCB - Australian Broadcasting Control Board for television stations to remove any colour content from their TV broadcasts. (During the conversion period leading up to the launch stations would run a mixture of B&W and colour material within their stations).

  To comply, stations would strip the colour burst from the TV video sync block before it was broadcast. This infuriated many propeller-head techies and nerds, myself included.

  To overcome the problem, the 4.43 MHz colour subcarrier in the broadcast video which wasn't deliberately stripped out was used to reconstitute the colour burst. This was achieved by modifying standard PAL colour TV sets (which weren't that difficult to obtain) with the addition of some subcarrier-extracting filters and appropriate phase-locking/modifying circuitry. This was a bit tricky, as the reference phase was no longer there and the fact that it was a PAL signal (PAL - Phase Alternating Line encoding).

  In fact, I recall at the station I was working for at the time we had a modified TV set in the engineering department working in colour from off-air signals (one of my colleagues was a past master at tweaking up sets this way).

  Perhaps a bit of broadcasting history trivia but it sure shows the colour recovery technique in this story wasn't the first effort.

  Edit: Incidentally, the same trick was used on source material such as quadruplex videotape that already had the burst stripped at other locations.

• by timonoko on 8/28/22, 10:10 AM

  Black&White-tv was almost HD, 625x625. Then they added 3Mhz color-carrier in 1966 and it was 300x300 with this color-furze on top. This sucked so much. There was nothing I wanted see in living color. Especially winter-sports were mostly BW.

  I remember that color movies sucked also in 1950s. Technicolor has annoying fuzziness around objects. See Wizard of Oz.

• by londons_explore on 8/29/22, 8:49 AM

  This is a perfect use for image to image ML models...

  Throughout one recording, the phase shift caused by the distortion of the glass screen is probably approximately the same - and therefore could be learned.

  Then for the actual decoding, certain elements of the frame should be of approximately known colours - for example someone's face should be skin colour. That then informs the colours for neighbouring objects, since over a small area phase is consistent.

  Applying such techniques repeatedly over the whole video, trying to minimize inconsistencies, I'd bet you can get really good results.

• by Jaruzel on 8/28/22, 10:48 AM

  This was used to good effect to 'recolour' the black and white versions of old Dr Who episodes due to the colour originals having been lost/destroyed.

• by powlow on 8/28/22, 11:56 AM

  This is from 2008 - what are the newer developments in this space?

• by dannyw on 8/28/22, 11:56 AM

  A great video series on analog TV and color:

  https://youtu.be/dX649lnKAU0

  https://youtu.be/InrDRGTPqnE