Chromakey / Lumakey?

[garygraham]

Some BBC programmes in the early 1970s, such as Top of the Pops have superimposed captions that are slightly transparent. Whereas other programmes from the same period have the kind of solid captions we associate with chromakey (bluescreen). 

Does anyone know why this was? I would guess white on black captions cards were added using just a simple superimpose or possibly lumakey. Were some studios not equipped to do chromakey after being converted to colour? Or did staff prefer to do things the old way for a while? 

Must say, I quite like the effect as it has a softer edge. I was going to upload a screengrab but the forum has exceeded its storage for attachments!

[Richard Marple]

I've noticed Monty Python seemed to have both styles of captions, maybe it depended on the vision mixer's settings?

[garygraham]

Jul 27, 2021 21:06:32 GMT Richard Marple said:

I've noticed Monty Python seemed to have both styles of captions, maybe it depended on the vision mixer's settings?

That might mean preparing the captions white on black or white on blue depending on the studio! Possibly with white on black they just "crushed" the blacks on the camera output and then superimposed the caption at near 100%. 

[Peter Stirling]

Until whenever it was the the studio could only cross mix a caption so if you intensified the white of the letters you would also intensify the black of the background, this would leave a foggy black cast on your lovely main picture, so they would mix halfway.. just enough to get rid of the black cast but it could also leave the white letters with some transparency.

The luma key could (figuratively speaking) cut a hole in the picture where the white(or black on white)letters were and the nominated background colour (be it black or white) would be ignored and not seen on the picture.