Quantcast Purpose of colorbars when capturing VHS tapes? - digitalFAQ Forum
  #1  
11-10-2012, 05:13 PM
JasonCA JasonCA is offline
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Hi All,

I'm not getting the purposes of Colorbars in conjunction with capturing of VHS tapes? I can tweak the proc amp and settings so that the colors are recorded correctly in respect to the colorbar charts, but what significance does this have towards correctly capturing of VHS tapes?

My whole setup can be calibrated to color charts and ready to capture color chats quite well, but that doesn't seem to mean much when you finally put a VHS tape in and start capturing it? This is considering that the colors recorded on the tape may be off or black levels could be off when played back from the VHS device to a computer for capture. Is the colorbars used merely to see if you can get a clean signal through the chain of devices?

In my setup and for background, I'm using the following chain:

[JVS VHS (2MB TBC)] -> [Data Video 1000] -> [BVP-4] -> [ATI USB 600].

I can feed colorbar info into the Data Video 1000 and then record it with the ATI USB 600. But I guess I think, "So what?". Really what is that proving except perhaps that my video CAN go through the chain of devices OK?

However, when I put in a VHS tape. I still would need to adjust the proc amp per VHS tape? Or am I wrong?

Jason
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  #2  
11-10-2012, 07:12 PM
NJRoadfan NJRoadfan is online now
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Color bars are meant to be a reference to calibrate your equipment to. Unless your recordings were semi-professional and included them, don't bother. It is standard practice in professional video to record bars and tone for about 30-45 seconds at the beginning of a recording using the equipment capturing the event. That way when another studio plays the tape back, they can easily calibrate the output from that tape to be correct in color/hue and brightness. To properly do the adjustments, you need a video waveform monitor and vectorscope. More information can be found in this thread if you are interested.

How to calibrate your NTSC CRT video monitor/equipment

For capturing home movies, its best to eyeball the color when using a procamp, since you have no reliable reference to compare to.
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11-11-2012, 07:20 AM
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Color bars are for calibrating the hardware. But colorbars on a VHS tape can be ignored.
In essence, these don't do anything for you.

There's been several calibration discussions here in the forum over the years. In addition to what NJ posted, there's:

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11-11-2012, 06:06 PM
jmac698 jmac698 is offline
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Recording colorbars through your equipment let's you calibrate your ati capture card to a known standard. When capturing an unknown VHS, this is a good place to start. The tape may not be exactly the standard and color can be a preference anyhow, so feel free to adjust it how you like. One thing you should be certain of though, is to ensure you capture the full available range of brightness and not cut off any shadows or highlights, likewise miss any saturated colors.

If you can put colorbars on a new recording, do so. If you have an old recording with colorbars, adjust your capture settings to get the correct display.

I wrote an Avisynth script for this, but the easiest way is to capture with Dscaler which has a calibration routine built in. It shows you how close the colors are and you have to adjust the capture card settings to make them closer.

You can get a correct capture of colorsbar on tape without a calibrated monitor, nor do you need a vectorscope. Dscaler provides the readings in software or there's other ways to do it. A calibrated mnonitor would be important for color grading.
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11-13-2012, 03:14 AM
JasonCA JasonCA is offline
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Quote:
Originally Posted by jmac698 View Post
One thing you should be certain of though, is to ensure you capture the full available range of brightness and not cut off any shadows or highlights, likewise miss any saturated colors.
Well said! Yet, how can I use colorbars to facilitate this? Colorbars seem to push the limits of the full range of colors that can be captured. So it seems to be useful possibly beyond hardware calibration?

My frustration like perhaps others is this: how do you capture all that data OFF the VHS and ensure you have transferred it correctly to your computer? Ultimately you can't keep the VHS tape forever without degradation over the years. And, VHS tapes takes do take up space where I may not be always able to keep all of them. Or, somehow the tapes could be accidentally damaged over the years. I'd like to eventually junk the VHS tapes knowing I've gotten everything off of the VHS tape that's possible. We can read the VHS signals off the VHS tape...but some reason...it seems so complicated to get the VHS signals preserved properly in a digital format as if it were being read from a VHS tape.

But when you say, "ensure you capture the full available range of brightness and not cut off any shadows or highlights, likewise miss any saturated colors"...are you saying that a waveform monitor can be used to ensure that the video signal stays between 0 to 255? Or how does one ensure that all that data get's from the tape into the computer or that the full range of brightness, highlights are NOT cut, and colors are not overly saturated? If I can capture the signal like it was played from a VHS, then wouldn't I be able to later (perhaps even years) do post work on the saved video files and obtain descent results?
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11-14-2012, 10:59 AM
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I understand your concern about capturing in the best quality. To ensure that you have the full range of signal, you can use the histogram display in Virtualdub.

The histogram looks like some white "hills". Make sure the left-most hill is lined up to the left edge of the point labelled "16" and the right-most hill is lined up to the "235". To move the hills left or right use the brightness slider of the capture settings, to horizontally shrink or stretch the hills use the contrast control. You may find you have to use both; for example the contrast could move your left hill and also your right hill, so you have to use brightness to slide the left hill back into place.

When you do this, it should be on a scene where there is black and bright light. If there's sun or a lightbulb in the scene, it's for sure a good time to adjust the rightmost hill. Where there seems to be a deep shadow, is a good time to check the left hill.

I should also mention, to include even noise so it doesn't clip. Most people don't realize it, but when denoising an image, clipped noise will change the noise statistics and make the denoised result at the wrong levels. This then results in losing details in highlights and shadows.
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11-15-2012, 12:56 AM
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I've always found the best way to color correct is by using the calibration graphs, color bars, etc, to calibrate the hardware. And then use your eyes for the values. You've probably seen video on TV that was too bright, too dark, under saturated, over saturated, too loud, too soft, etc -- and that's almost always the end result of robotic corrections without much human override. Even the best AI still cannot replicate the human condition of decision making for the best overall values.

There's two schools of thought on this:
(1) Always do what the tools tell you.
(2) To hell with the tools, I'll rely on my eyes/ears to determine what's best.

The problem is that:
(1) Some tools are simply not very good.
(2) Most people are simply not very good.

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