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Technical DV avi question
I have been suffering from the dreaded “dark video” when I open the NTSC DV avi files I have “captured” from my Sony TRV-38 into Premiere Pro 1.5.1. I have two goals: to color correct my footage, and to try to figure out where my problem is originating. I have scoured the Internet, forums, and some books, but I have 3 technical questions to which I cannot seem to find the answer. (Actually 4, but the last is different.)
1. My understanding of miniDV is that my camera transcodes and compresses the RGB data it acquires into YUV (or YCrCb) space and stores the result. I copy exactly the data file to my computer through the Premiere capture process using firewire. Am I correct in assuming that it is Premiere (or After Effects) that is transcoding the YUV data back to RGB for me to edit?
2. Is there any way to, say adjust levels on the original data? Is there a program or setting that converts to RGB at a higher bit depth to reduce rounding errors? I am wondering especially whether luminance artifacts are occuring because of a variation in the DV codec used internally in Premiere that might be different with another conversion method.
3. My actual problem is that I have a very compressed luminance. When I look at the luminance waveform in the Color Finesse plugin for After Effects 6.5 it appears that my files tend to range between 10 IRE and 80 IRE at absolute best. Even recording black I get luminance above 5 (RGB each 10+). Does Premiere add some sort of setup? According to Adam Wilt’s web pages the digital signal should range in RGB 0-255 (maybe 0-235, but some cameras have superwhites). Do I just have an exposure/encoder problem in my camera, or is there some subtlely in interpreting DV files? I just want to pin down the problem.
(4.) This is different, but I can not ever capture the first ~2 seconds over firewire from my TRV-38. What is strange is that I can step frame by frame through those 2 seconds, but when I try to grab a clip including them it either fails with an error “bad timecode” or just skips the beginning. Is this a general problem with miniDV, or maybe just my camera? Adobe says it supports the TRV-38, but it is not in the list when I control the camera – could that be it? I’ve tried setting preroll from 4 seconds to 0 with no effect.
Any clarification on any of these questions would be greatly appreciated.
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2 Replies
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Premiere should not automatically modify the DV video you capture. If the video appears too dark, it probably only has to do with the way your computer displays it (overlay video). If you were to use an external monitor (or output it back to tape), you would see the video is identical to the original source. You might want to check if your video card lets you adjust the brightness of overlay video.
As for capturing the first 2 seconds of tape footage, have you tried capturing manually instead of batch capture? Batch capture isn’t recommended for the first 5 seconds, so I usually capture those manually. But the best advice is to always let the tape roll for a few seconds at the begining of the tape when you shoot.
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JF Robichaud
Low Budget Productions -
Thanks for the suggestions. What I “see” on my computer monitor or television are approximately the same. I don’t have $700 to drop on a broadcast monitor, so I will have to live with that. (I eliminated the possibility that it was an overlay artifact by looking at the numerical values for pixels and scopes in Color Finesse.)
After playing with this for the last month, I have come to the conclusion that most (maybe all) of the issues are camera related. But I will finish with another question…
(4.) Basically, I will just have to waste the first few seconds of any new tape. No big deal now that I know. I have tried every possible capture combination without success. It appears that the camera takes a second or so to come up to speed when starting from a stopped state. Normally it will preroll so that things are running fine by the in point. At the beginning of the tape, the camera tries to preroll, hits the end, and takes longer than normal to start up again.
This would seem to defy the claim that I am just “copying” the DV file over firewire, but I suspect that the camera itself is not sending any data unless it is up to speed, and so Premiere just starts capturing when it sees the first frame. I can scan through the frames because the camera is in a stepping mode in preview, not a playing mode. To recover those lost frames, I would need to capture them frame by frame from the signal sent in stepping mode – not worth my effort to figure out.
3. I have managed to record superwhites that peg out RGB at 255 by extreme measures, so it appears that the upper end of my dynamic range problem is also camera related. There are several variables, but turning off image stabilization gives me a boost in exposure. Also, the auto exposure seems to not always go high enough. I’ll just have to be careful to manually set the exposure to max in typical room lighting, and bring out the rest in post. These are not scripted shots.
Somewhat more speculative:
The black-level problem may just be a “feature” of the camera’s DV encoding algorithm. Recording with the lens cap on just does not allow going below RGB values around 12. After re-reading several sources for the umpteenth time, it seems that 601 conformant data would be between RGB 16-235. I had thought that Premiere expanded the RGB range to 0-255… I still find this confusing.Finally, I have noticed that if I apply a gamma correction of about 1.1-1.2 to all of my footage, the overall luminance seems MUCH better on my computer monitor. I have not tested this correction by writing back out to tape yet. The idea for this came from the claims I have read that many DV cameras typically record with a slightly lower gamma (1.1) than the standard (1.2) in order to roughly counteract the 7.5IRE setup added to North American NTSC output. Is this just hearsay?
The essential questions:
If I do color correction, what RGB values should my edited footage respect? 16-235? 0-255? Should I add a default gamma correction? There has to be a better way than eyeing it on a broadcast monitor. I presume there is a well-defined standard, I just can’t quite grasp it yet.Does Premiere compress the range on encoding to MPEG2 or DV, or will it just pass the values through?
I’m certain it will become clear some day…
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