This is more of a curiosity than anything that might affect actual captures.
Several days ago I asked if my capture system had AGC in the thread
"Is this an example of AGC changing levels?". I had experienced automatic level shifting while paused on an image that was over the brightness 235 upper limit. The shifting brought the over exposed limit to well within the 235 limit. Some times it occurred other times it did not. Today I found that whether it happened or not was based on which capture device I used.
Today, I ran into the automatic level shifting again so I went back to my engineering roots and figured out where the change is coming from one step at a time. The starting workflow included: Mitsubishi HD-2000U vcr --> Panasonic ES15 --> Diamond VC500 capture device --> Windows 7 Pro with
VirtualDub 1.9.11. I used S-video for all connections.
Step 1: Eliminated the ES15. Automatic level shifting still occurred.
Step 2: I suspected the vcr because Sanlyn said the VC500 did not have AGC. I went through all four combinations of turning on and off the 3D-Y/C and DNR+TBC features. Level shifting still occurred.
Step 3: I now suspected the VC500. I moved the video connection to my Hauppauge 2250 capture device. Surprise! The auto level shifting did not occur while paused on an image. I ran the same combinations of the vcr 3D-Y/C and DNR+TBC features and the level shift did not reappear. I also tried different paused images that were out of bounds.
With the VC500, the right end of the histogram moved left in small increments once a second until the level was well within the 235 boundary. The VC500 only affected the white level. I tried images with blacks below the lower limit and no automatic change occurred.
I ran further tests to determine if this action affects video that is running versus paused. While paused, I stepped the video forward and back a frame at a time to see if the adjusted level reset or stayed the same. The adjusted level stayed in place. However, hitting the play button always put the levels back to where they were with the overly bright scenes going past 235.
A few days ago I thought consistently over bright scenes would cause the level to change automatically so I ran some overly bright scenes. I didn't see any change. Either the level shift does not occur during play or the shift is so slow I couldn't see it.
CONCLUSION: I think the level shift during pause is a VC500 protection feature to prevent CRT and plasma based displays from burning in.
The following images are from my prior post and show the amount of shift over about 20 seconds while paused on one frame.
8-16-18 vc500 capture before gain change.jpg
about 20 seconds later:
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Update: A family vhs video that has considerable over limit scenes did show the VC500 adjusting the upper end just like the paused video.