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16-235 vs 0-255?
Hi all,
OK..I’m revisiting my continued battle with broadcast legal colours and translation to Adobe, or more broadly the digital domain.
I asked on various forums a little while ago if anyone could offer up a definitive answer to what exactly are broadcast legal colours for PAL transmission in the UK. Clearly it is WAAAYY more complex a question/answer than just that and I’ve learnt an awful lot..
However, I’m still not 100% clear on the Broadcast Colours effects in Premiere and AE. Basically we have used mainly Quantel kit or hardware based linear suites in the past which are totally capable of creating levels beyond 255. Now Photoshop et al cannot, to my knowledge, create or process anything outside the 0-255 range. This is an RGB range right? Well it seems from doing alot of reading that in the UK at least, PAL broadcast standards specify your ranges must be within 0-255, *NOT* 16-235 as has been suggested in various places..which puts my mind at ease somewhat and stops a bit of head scratching (I’d done various tests on TVCs that we had sent for broadcast and we’re deemded ‘fine’, but when tested within AE they were showing up 0-255, depsite the b/cols effect putting PAL between 16-235)
SO….I’m wondering whether infact this 16-235 range is specifying YUV values and not RGB? I tested this on the Paintbox within our Editbox which can operate in either colourspace…In the palette my white was reading 255 in RGB values, but 235 in YUV. If this is the case then surely you don’t have to worry about creating illegal colours from within Adobe products…right?
Or have I got it all screwy?!?
An garbled post to be sure, but if anyone fancies wading in to explain all this I’d be really greatful!
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1 Reply
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You are looking at difference color spaces…
Y’CbCr – this is what’s sent over SDI and recorded onto digital formats like dBeta. Legal black level is always at 16(Y’), and white level at 235(Y’). Note that the unit there is Y’.
Sometimes mistakenly called YUV. If it’s digital, they are probably referring to Y’CbCr.R’G’B’ – lots of computer-oriented programs use this. Legal range is usually 0-255(RGB), but is sometimes 16-235(RGB). This behaviour depends on what codec you are using to convert from RGB<-->Y’CbCr. Some codecs want to see 16-235 levels, most want to see 0-255.
Y’UV – when the signal gets converted into an analog composite signal, the luma and chroma signals are modulated together. The resulting composite signal has limits on its levels… usually no higher than 115~120IRE and no lower than -20IRE. Some broadcasters set the limit (for max composite IRE) lower.
For luma alone, it should usually be from 0-100 IRE (for PAL).
One an analog hardware waveform monitor, toggling the ‘LPASS’ or low pass filter will show you either the luma, or the composite signal.Pay attention to the units!
2- When it comes to composite signals, there are limits to extremely saturated + bright colors. A 255 0 0 RGB red can be illegal (even assuming your Y’CbCr/video codec wants to see 0-255 RGB levels).
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