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Best ways to confirm a noisy image…
It’s a bit weird, but i have a show that is providing us with excessively noisey rushes. They claim they’re looking at the footage on a grade 1 CRT at their end and cannot see any noise but when i look at it on my grade 1, my JVC DTR24L4D HD LCD and a consumer LCD i can clearly see a fair amount of noise, more than i would want. The rushes they are providing are HDCAM shot on a Sony HDW-790P which i would have thought would be able to give pretty clean blacks. Because the rushes are 1080/50i and they are viewing on a SD CRT this will naturally make the picture they see look a little less noisey right? The footage is interior in a mock-nightclub, so there are areas that are lit and others which are in shadow.
In Blue Only mode i can see the noise very clearly although they claim they have also looked at it in blue only too. Other than by pure visual inspection and a monitor just showing the blue signal are there any other ways to test for noise in an image? I have access to HD Tek scopes etc. My understanding is that most noise is present in the blue signal hence using the blue-only function of the monitor to reveal it. Would anyone more engineer-ey care to elaborate?
Any thoughts welcome
Cheers
Andi
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4 Replies
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My 1st reaction is that using an SD monitor (even if it is a Grade 1) to assess an HD feed is relatively pointless. Not only is the resolution a quarter of an HD screen, you’d be looking at a down converted (processed) feed, not the actual source picture.
Also, have the monitors been aligned recently? Even slight variations in the set up can make an image look more or less noisy in the blacks.
I’d look up the Tektronix manuals for how to measure noise (there’s info on their web site if you haven’t got the books to hand). I think it varies depending on which scope(s) you’ve got.
[N.B. I wouldn’t say that “most” noise is in the blue channel but it is usually a bit worse in the blue channel than the others because the blue will have more gain in the camera head. But that’s a slighly pedantic point – paying particular attention to the blue channel is valid.]
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Thanks Andrew,
I will check to see the alignment status of both their monitors and ours, and thanks for the advice. We’ve got a Tek WVR7000 that i can play with, i’ll try and hunt down a manual for it, and i’m all about pedantic points! Trying to learn what i can about it at the moment so any advice is greatfully received, could you tell me why more gain in the blue? Or any good resources for this sort of nerdy info? Many thanks,
Andi
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Basically the chips (of whatever variety) aren’t perfectly sensitive across the whole colour spectrum, so the channels need slightly different amounts of gain to get a properly colour balanced picture. The chips will also produce a certain amount of thermal noise, which is not related to the light that’s falling on them, so the channel which has the most gain in the camera will show the most noise.
There are a few places to get technical info. Digging into the Tektronix website is a good place to go (lots of useful techy stuff on there if you have a hunt), Quantel’s digital factbook has been a good glossary of terms for many years (google for it) but light on in-depth details.
Also google for “ITU-R bt.709”, which is the international spec for HD video (I believe the ITU is paywalled, but you can find pretty much everything you need to know without paying if you’re persistent). TBH, wikipedia has some good pages but (as with everything on wiki) it’s worth following up the external references as the wiki pages themselves are generally somewhat simplified.
If you’re really nerdy (in the best sense of the word) this place has many interesting diversions
https://www.bbc.co.uk/rd/pubs/papers/index.shtml and
https://www.bbc.co.uk/rd/publications/(don’t know why the first one isn’t coming up as a link – should be ok to copy &paste)
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Andrew,
Many thanks for all the useful info and insight. It’s my day off today so I shall put the kettle on and settle in for an epic nerd-fest. Long live the COW!
Cheers,
Andi
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