Colour bar

[jezbond]

I can see what the colour bar is and I read somewhere that it shoudl be used as a marker (?) at the beginning of each tape (shoot?), but why? what is it for? does little old me going from DVX100 into computer need to know about this/use it? Thanks for your help.

[J_Barnes]

Color bars are a holdover from analog video. You would record color bars on the tape as a reference of how that camera records color and luminance. Since the color bars are standard, you could check a waveform monitor and vectorscope when playing out of a different machine to see if the color bars were off. If the color bars were off, all the video on that tape would also be off by just as much, thus you'd use a time base corrector (for NTSC) to adjust the signal to the standard.

A TBC is similar to the Proc Amp included as a filter in most NLE's.

With DV, the signal is transferred to the computer as raw data and the head alignment has no effect on the quality of the image, thus I don't really see much use in recording color bars on a tape that's not intended to go into analog machines.

If you're going to broadcast, you still need colorbars at the head of the master tape.

I could be wrong about this though, I've just never bothered to use them on DV tape going into an NLE.

[taubkin]

I don't use colorbars for capture, but I use them for 2 reasosn.

1 - I can calibrate whatever monitor I will be playing it on

2 - Tape starts are usually a bad place to have info and sometimes your capture program will need some pre-roll timecode to batch capture.

So I adopted this standard (it's my standard, but still...): 15 seconds of bar before the beggining of each mini DV tape I record (raw material). 1 minute bar on displaying VHSes (Baad dropouts on the first minute). Bars between different pieces on a same master tape.

[J_Barnes]

Christ, I can't even begin to tell you how many tapes I've gotten from various sources where they just started running at the beginning of the tape with their subject talking. No preleader, no black, no bars, no nothing...just play and go.

If nothing else, recording bars at the beginning of your tape would completely eliminate this and reduce the number of editor-director murder cases.

[jezbond]

Thanks both of you. That's a good help. Basically for all these reasons I reckon I should do it! So is it best then to record some colour bar at the beginning of each new take, each set up, each day's shoot, or each tape? (Didn't quite understand about dropouts and needing a minute...could you please explain it to me?). Thanks so much.

[Bill_Bolton]

I'm getting ready to send a tape over to the local access station and they explicitly told me "no bars and the tone just hurts my ears, so don't include it either". Well as you may have guessed, they are real small time and wanted a DVD instead of tape. Ahhh the joys of video.

[J_Barnes]

Thanks both of you. That's a good help. Basically for all these reasons I reckon I should do it! So is it best then to record some colour bar at the beginning of each new take, each set up, each day's shoot, or each tape? (Didn't quite understand about dropouts and needing a minute...could you please explain it to me?). Thanks so much.

Oh no...not before each take...

The theory behind color bars is that each video recording device records slightly differently. (This pertains more to analog)

So, you've got these standard reference points of color and luminance (color bars), and you record them on tape before you shoot. It's really the camera's recording process that's really being measured, and any difference in how the camera records...say luminance for example...is shown in the difference between the color bars on tape and the universal standard for color bars as seen on a waveform monitor.

Say your camera records 100 IRE white as 108 IRE...you don't need to know what IRE's are, but they're measures of luminance. So, your camera records the brightest white as 108 IRE...but the standard is 100 IRE. When you record those 100IRE color bars on your camera, they'll be recorded as 108 IRE.

When you play back that tape on another machine and send it through a monitor that shows the scale of luminance, you'll see that the color bars are reading 108 IRE and you'll adjust them down to 100 accordingly.

It's just a standard at the begininig of each tape recorded on a specific device that tells you how that device records a given video signal. For example, if you took that tape out halfway and put it into another camera...you'd ideally want to record color bars again to reference that recording device.

It would serve no purpose to record between every shot, every set up, or any other inverval other then changing the recording device.

What we're saying about the beginning of the tape is this:

You need leader at the head of your tape that doesn't have crucial information on it. Meaning, your interview cant start at 00:00:00:00:00 because all editors, playback devices and capturing utilities need a few seconds to get the tape up to speed and assure proper sync. If you've got something important at the zero part of your tape, it's going to be lost because you can't capture that part.

We're saying it might be a good idea to record at least 20-30 seconds of color bars at the beginning of a tape, simply to insure that you aren't recording in the "pre roll" section of the tape that can't be captured.

It doesn't really have much to do with dropouts and you could do just as well to roll tape for 30 seconds with the lens cap on.