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Gamma (HD/SD Monitoring variance)
(note: I have also posted this in the Kona forum not knowing if the issue is AE or Kona related).
On a Kona 2 system we have found a strange behaviour when monitoring between HD/SD in After Effects.
We often colour grade in After Effects previewing out to a HD/SD broadcast monitor. We have noticed that if you monitor out 625i/RGB and 1080P25/RGB there is quite a difference between in gamma.
The scenario we have been tripped up in is grading a HD project (monitoring in HD out of AE) getting everything right and then outputting an SD version and the image is all washed out.
If for a moment we assume this output variance is not an error and is correct we have found 3 work-arounds;
1. Take the HD render (using a lossless format) and reimport into AE and then use the “expand to 601 luma levels” option in the interpret footage, drop that in a SD comp and re-render.
2. Alternatively you can use an adjustment layer and levels to simulate this effect to bypass rendering HD, reimporting and then re-rendering the 601 luma adjust.
3. Use the Kona control panel to force the gamma to 601 (rather than set to auto) so the difference in monitoring out when grading in HD and SD is purely resolution and gamma space is identical?
What is doing my head in and need some help with is;
1. Is what we are seeing a Kona/AE error or is there a massive difference between SD (601) and HD (709).
2. If it is correct why doesn’t this variation seem to occur if you are doing this in Final Cut (suggests to me this is an error).
3. Which of the workarounds seem most valid and technically safe to use.
4. If 709 is vastly different when is 709 actually relevant to us (only HD broadcasting?)
Interestingly, if we start grading in SD and then want to reconform the project to HD it will be all crushed, there is no obvious workaround to this and I suspect the render method would be no good as you have potentially clipped info you will never get back. Furthermore there is no compress 601 luma option even if you had not clipped. The only option would be to try and recreate a levels setting while working in floating point that adjusts it prior to rendering and brings sub-black and overbright info back to the 709 range.
One other thing I have noted (still in AE) is that if we montior SD in either 625iRGB or 625iDVRGB it makes no perceptual difference (I don’t have an external WFM to check). But in 108025RGB and 108025DVRGB the later (DVRGB) perceptually looks a lot closer to both the 625 outputs potentially solving this problem. (I am assuming the DV indicates a YUV conversion).
(Note the 108025DVRGB output seems to be flakey, sometimes outputting a corrupt like image that can be corrected by toggling back and forth between the RGB and DVGRB setting).
Well if you have made this far and understood my long-winded explanation and actually think you can offer insight into this I would love to hear from you.
To say this has been doing my head in is an understatement!
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1 Reply
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According to Mario Orazio in this week’s “TV Technology,” “NTSC [has]a display gamma of 2.2; 625-line SD…assumes 2.8; and BT.709 has been called anything from 2.22 down to 1.92 or so (at least one source says 1.8).”
But, according to Orazio, it’s the luma differences that will really get you.
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