Quantcast AGC problem but good sync--copy protection? - digitalFAQ Forum
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06-04-2017, 10:29 PM
bever bever is offline
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I am digitizing some of my Disney VHS tapes so my daughter can watch them while on a long flight. I am getting an AGC type error. The video amplitude (or black level) fluctuates about every 7 seconds.
It doesnt matter Composite or Svid signal, Have tried two different VCR(s). It happens on an ATI All in Wonder 9600 and on an ATI All-In-Wonder Radeon X1800 in a separate (XP sp2) system.
I have tried at separate times each of the 3 TBC IV that come inside My DPS ES-2200T and DPS-235.
Always the same result, Rock solid (in my opinion) sync but the brightness jumps, holds for 7 seconds then goes back for 7 seconds then repeats over and over.

The example I include with this post is MPEG-2 made with MMC 8.8. and AIW 9600. The X1800 used Virtualdub with Huffyuv /lossless. Subsequently on the virtualdub copy I tried the Donald Graft deflicker filter to smooth out the ebb and flow but no luck.

Monsters,mpg.mpg

It occurs that this may be the achiles heel of my poor man surplus TBC but never had a problem on other tapes before.

My apologies if this is old hat to you veterans but I did a cursory search and did not find this exact symptom.


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  #2  
06-05-2017, 12:21 AM
msgohan msgohan is offline
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The repeating pattern probably follows the brightening and darkening pattern of the AGC-triggering MV pulses in the VBI. The top of your image is also flickering, separately from your main level jumps.

https://gfycat.com/FixedThirstyHummingbird

Small GIF version, if that link dies in future
FixedThirstyHummingbird-size_restricted.gif

MV is still present in your capture, to some degree. See that little white bar visible at the left edge of the head-switching noise? It's a signature of MV in the horizontal blanking interval that has been pushed into the visible picture area due to the head switch causing the image to skew to the right. Not all MV tapes have this, though.

VCR blue back, VSYNC shown at top
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Sweep recorded to VHS, VSYNC shown at top
Raw capture, 525i, Y8 rolling display - PV-S4670 blue back.jpg

Titanic VHS, VSYNC shown in middle. 4 VBI pulses, white VBI bar separated from pulses, plus white bar in HBI.
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Home Alone VHS, VSYNC shown in middle. 5 VBI pulses, white VBI bar right next to pulses, no white bar in HBI.
Raw capture, 525i, Y8 rolling display - PV-S4670 AVIANITA.jpg

There are probably also some residual MV pulses in the vertical blanking interval that weren't removed by the Personal TBC IV, like with the MX-1 video mixer. So the overall point here is that MV was removed enough to trick the drivers into not digitally distorting the signal, but not enough to avoid triggering AGC on the analog input side like a VCR recording would.

http://www.digitalfaq.com/forum/vide...s-showing.html
Quote:
Originally Posted by me
Apparently the MX-1 leaves some residual lines containing MV, while the MV stripper of course over-corrects by also killing the closed caption lines.


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06-05-2017, 09:13 AM
bever bever is offline
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Thanks for the breakdown msgohan

I do now see the flickering at the top of the video you noticed which I did not see before. I had already watched 12volt vids youtube videos several times back when I was trying to understand TBC (still trying). You are right they are excellent examples of copy protection. I like the one where he takes an old studio monitor and intentionally lets the picture roll to the point where the VBI is in the middle of the screen. I like the one where he shows the macrovision on the o scope also.

I guess the DPS ES-2200T and DPS-235 are not full frame TBC or the whole VBI would have been replaced including the AGC pulses. Another clue which occurs to me now is that I did see the closed caption info getting through the TBC and showing up on my preview screen in MMC.
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06-05-2017, 09:45 AM
msgohan msgohan is offline
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Quote:
Originally Posted by bever View Post
I guess the DPS ES-2200T and DPS-235 are not full frame TBC or the whole VBI would have been replaced including the AGC pulses. Another clue which occurs to me now is that I did see the closed caption info getting through the TBC and showing up on my preview screen in MMC.
Well, I don't agree that incomplete VBI replacement means that it's not a frame TBC. The MX-1 still qualifies, for example, IMO. And there's really no reason for a TBC to kill the CC lines. Some just go too far; some even black out the top of the active video.

If the DPS units passed the VBI through unchanged, the AIW cards should be showing moving video only at the top of the frame, unless you used the driver hack to remove this deliberate digital copy protection. But the driver hack doesn't help the AGC to cope with the brightness trickery.

https://www.nepadigital.com/articles...cial-vhs-tapes
http://www.biline.ca/ati_macrovision.htm "Even with the macrovision detect disabled, Macrovision does its job and renders the video signal in such a way that its very dull and loses a lot of the original color"

You should be able to trick your capture cards into showing you part of the VBI by capturing while quickly abusing the FF/PLAY/RW buttons.

Last edited by msgohan; 06-05-2017 at 10:44 AM.
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