08-30-2024, 07:42 AM
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Post vhs capture (capturing 720x480 NTSC). I've been trying to read up on the best way to remove the vhs head switching noise from the bottom.
My PC is old. For my AVISynth script, I tried capturing in Virtualdub and can capture video, but 90% of the time, the audio is out of sync. I've been capturing in AmarecTV with very good results. I really cannot see a difference in my caputured video from Virtualdub compared to AmarecTV, so AmarecTV it is.
Anyway, I've just added some lines to my AVISynth script.
I believe the correct procedure is to crop 6 lines horizontally from the bottom of the capture? My script also crops 8 lines from each side.
Then, I'm adding a black border the same size (6 pixels) back to the bottom.
The script is below. Just verifying this is correct. I want to keep the correct aspect ratio.
Code:
SetFilterMTMode ("QTGMC", 2)
#DirectShowSource("F:\amarectv310\captures\stvhs1.avi")
DirectShowSource("F:\amarectv310\captures\stvhs1.avi", fps=29.97, ConvertFPS=True)
EnsureVBRMP3sync()
#ConverttoYUY2(interlaced=true, matrix="rec601")
AssumeTFF()
QTGMC(Preset="slower", FPSDivisor=2, EdiThreads=3)
Crop(8,0,-8,-6)
# Full sale black border using color_yuv hex color notation
ADDBorders(0, 0, 0, 6, color_yuv = $008080)
Spline36Resize(720,540)
Prefetch(1)
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10-29-2024, 04:41 PM
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The best way is to just mask it with black. Otherwise you'll risk AR damage, as well as stretch/alias the remaining image.
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10-29-2024, 05:05 PM
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This doesn't get discussed much and probably isn't applicable here because you've already done your captures, but it's actually possible not to get the head switching noise on fist generation tapes with VHS to begin with if you slightly adjust the head switching point.
This video shows how simple it is on a modern JVC. The warning they give is that doing it a bunch of times could loosen things up, but I don't think that's as big of a risk as say messing with the tape guides which are often threaded into plastic whereas this is threaded into aluminum.
https://www.youtube.com/watch?v=AqGmOiU41i8
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10-29-2024, 05:12 PM
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Quote:
Originally Posted by aramkolt
This doesn't get discussed much and probably isn't applicable here because you've already done your captures, but it's actually possible not to get the head switching noise on fist generation tapes with VHS to begin with if you slightly adjust the head switching point.
This video shows how simple it is on a modern JVC. The warning they give is that doing it a bunch of times could loosen things up, but I don't think that's as big of a risk as say messing with the tape guides which are often threaded into plastic whereas this is threaded into aluminum.
https://www.youtube.com/watch?v=AqGmOiU41i8
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You're vastly understating the risks here. Even I don't screw around with this. Playing with fire.
The creator of that video, latreche here on this forum, says as much. You must read that full description.
Pay close attention to his comment: "it will wear out the screws if done frequently."
I need to do it on a few important tapes, but using an S-VHS VCR with TBC that I can afford to "oops" (ie, not my Panasonics).
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10-30-2024, 12:31 AM
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I only bring up the head switching because it's never really talked about as a possibility, not saying that it should routinely be done. I think you could probably do it several hundred times before anything really wore out, those screws are just holding a small PCB in place, so they don't have to be tightened very much and they have quite a bit of thread to spread the load.
The math as I've come up with if you're not wanting to mask the noise would go as follows:
First remove 16 pixels of the "side black bars" these have to do with the sync pulse/colorburst that causes a CRT to know it is time to start a new line and end a line, but do not contain any video frame data.
Your frame's actual video contents is now 704x480.
704 would normally be squeezed horizontally to 640 pixels. That means each of the 640 pixels represents exactly 1.1 pixels to expand back to 704 horizontal resolution.
If you remove 6 of the lower lines to get rid of the head switching noise, you would need to remove 4/3 that many pixels from the sides, which is 8 pixels from the correctly scaled 640. If we expanded that back to the 704 pixels we were originally working with means 8x1.1=8.8 pixels which is close enough to 9 pixels.
So in total you'd remove the 16 + 9 = 25 total side pixels from the starting 720x480 file and remove 6 lines from the bottom resulting in a starting window that is 695x474. You can then scale that to any 4:3 ratio that you want. You do lose those extra 9 side pixels in doing it this way.
We were only off by 0.2 pixels with the calculation, so in a system that started off as 704 pixels, 0.2 pixels worth of error is over 99.97% accurate to the starting 4:3 ratio, but you do lose 1.5% of your horizontal picture.
If you instead removed 4 lines from the bottom, you'd need to remove 4/3 that may pixels from the sides which in this case is 16/3 = 5.33333. Since that is looking at the correctly scaled 640 horizontal resolution, we need to expand that by the 1.1 pixels to give us how many pixels that would be in the 704 equivalent scale from our starting file = 5.33333x1.1 = 5.87 pixels to remove from the sides which is very close to 6 pixels.
So in short:
If you remove 6 rows from the bottom, you need to trim a total of 25 horizontal pixels (of which 16 is side black bars for a pre-scaling resolution of 695x474
If you remove 4 rows from the bottom, you need to trim a total of 22 horizontal pixels (of which 16 is side black bars for a pre-scaling resolution of 698x476
I've never really seen the math done or explained this way, but seems pretty clear to me that those are the cropping tradeoffs = 1.5% horizontal loss if removing 6 lower lines, or less than 1% horizontal loss if removing 4 lines all while the aspect ration remains 99.97%+ accurate to the correct aspect ratio.
...But there's a much bigger can of worms if you want to get technical - analog NTSC actually has 486 lines vertically (well, two fields of 242.5 lines) and capture cards usually only capture 480 of them. That does actually change the original aspect ratio some more, but I've never really heard of anyone in modern times trying to compensate for that. For home videos though, I do believe that all 486 lines are present on the tapes as video data. Wouldn't be too hard to check using a Blackmagic or AJA capture card which does capture all 486 of those lines. The actual math on it would be pretty simple, add 6 lines of padding or trim even more horizontal resolution to make up for those 6 lines not being there in the capture.
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10-30-2024, 05:34 AM
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Quote:
Originally Posted by aramkolt
I think you could probably do it several hundred times before anything really wore out,
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I highly doubt it. Even a dozen adjustments to screws can micro-strip (both head and shaft), and allow tiny movements. Since VCRs have tiny tolerances, often a mere 1mm or less, the deck will become unreliable.
Again, playing with fire.
A reason that I hate these sort of discussions is because idiots will read it, do it, maybe use it (if the deck was not total f'd up in the process) -- then resell the VCRs as "tested" and "working" on Facebook/eBay/etc. When you start to screw around with more complex stuff like this, you get weird errors that are almost unsolvable. Those errors are at the bottom of a troubleshoot list. It ruins the VCR for future users, and makes it almost impossible to refurb.
I've been having a heck of a time trying to refurb decks since about 2021. In the 2000s, I could self-repair/maintenance my decks without issues. In the 2010s, I could refurb back to like-new specs, sometimes even refurb better than JVC specs. But now? Everything is shot and deteriorated. Over-cleaning heads, and wrongly cleaning heads, is a primary reason. But it that, during the pandemic, everybody thought of themselves as capable of DIY repair for anything. Well, no, that was stupid. Our video community is now dealing with the outcomes of monkeys with sticks opening VCRs, and pretending to be a tech.
I can still, to this day, self-repair/maintain my own decks without issue. And when I get facility decks, those tend to be fine, even if far out of spec. But anytime I acquire decks from individuals, it's a mess. The amusing part is that individuals think their crap is worth more than a facility decommission. Some of these folks are out of their damned mind, wanting refurb'd prices for it.
Anyway, discussions like this will only hasten the demise of extant gear.
I've always said that gear will disappear far faster than the media (VCRs, TBCs, CD/DVD/BD readers, etc). But apparently some folks have said (imagine that Jeff Foxworth redneck voice) "I can make it go quicker!"
As parents and teachers used to say, think before speaking. Words can have large implications and consequences.
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