Quantcast JVC dropout compensation DOC? - digitalFAQ Forum
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06-07-2020, 02:34 AM
latreche34 latreche34 is offline
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I stumbled upon a tape that has a line running along its length somewhere in the middle for about 5 minutes, looks like a bad fold that has been straightened and I was amazed how the DOC in the JVC works, Basically as you can see in the screenshot it takes the last good known scan line from each field and repeats it until it receives a new clean line. I've seen this before on other VCR's where the DOC is mediocre or it doesn't exist it shows a snowy stripe instead, Big visual difference. How does your VCR handle dropouts?



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  #2  
06-07-2020, 09:21 AM
hodgey hodgey is offline
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Yeah most VCRs that are not ancient work in a somewhat similar manner. The luminance output is split and fed into a delay line (CCD on most modern VCRs) that delays it by 1 line, if the tape video signal drops below some set level the VCR switches to outputting the delayed line instead of the current video output. The same delay line is usually used for luma noise reduction as well. Chroma is usually just muted unless the VCR has fancy digital dropout compensation (e.g in combination with the digital TBC/DNR circuit) as it requires a bit more sofisticated circuitry to do that correctly.

I have also noticed what you noted that most non-JVC VCRs seem to output noise after some number of lines, while the JVCs keep outputting a delayed signal forever. One issue I have noted though is that the DOC lines especially with dropouts several lines long tend to be more offset than in many other VCRs instead, especially on my HR-S8600, HR-J681 and particularly the Philips VR1100. The older S8500, J658 and newer HR-XV20, and JVC Camcorder seems less affected. Especially noticeable with the TBC on the 8600 and VR1100 TBC enabled as the TBC can struggle a bit with recovering from the offset lines. My JVC GR-SX22E camcorder seems to have a completely different video decoding circuit than the VCRs, it doesn't suffer from the issue with offset lines, but goes into noise after some number of lines instead.

This document for the JVC HR-S9500 and other models from the same lineup mentions drop-out compensation as part of the TBC/Digital board. To what extend it works in practice I'm not sure, and later JVCs don't have a DOP connection to the 3D digital board listed in the service manual.

The Panasonics I got here seem a bit hit and miss, they lines they compensate work well, but they seem to be a bit limited in how many lines before they go into noise. I don't have a well-working newer one than the 1992 NV-HS1000 to test though. There are some panasonic SVHS VCRs that have a digital decoding process like the US AG1980P and PAL NV-HS850 and 950 and probably some broadcast models, I have no experience with any of those.

The more modern Sanyo-IC based non-SVHS VCRs I've got (Samsung, LG, Sony) seems to do slightly more lines than the panasonics before noise, the DOC lines on these end up usually being very straight.

This document has some interesting comparisons, the huge JVC Broadcast decks seem to have some very advanced dropout compensation with color.

For 8mm, the TBC/DNR equipped sony camcorders have very good DOC, including chroma, seemingly able to repeat perfect lines endlessly. I think they use a digital decoding process. For some reason the CCD-TRV218 ouputs noise after a bit instead unlike the other ones I got, it was a lower end model at the time so maybe they skimped a bit on memory or something.
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06-08-2020, 07:49 PM
NJRoadfan NJRoadfan is offline
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Anything built in the 90s is going to do dropout compensation digitally.

The Sony Hi-8 camcorders will repeat the last "good" line until the bottom of the frame if its a big drop out. You see the effect briefly at the end of recordings. The best DOC I've seen is on the Panasonic AG-DS555/545 SVHS VTRs. The machines have a full frame TBC/framesync so the outputs are pure digital framebuffer (they are meant to be genlocked to the rest of a studio). You get a pretty neat effect when you hit a crinkled part of a tape and various lines are "frozen" from different past frames.
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