Explain the Purpose of Color Bars

Tech Talk

Why Color Bars?
I hear so much about color bars. I have a Videonics MX-1 and love it; I can use it to make color bars but I'm not sure why I should. Could you please give me a reason why and tell me what is the purpose.

Charles Foley

Good question. Color bars are one of those things we've all seen from time to time in TV broadcasts and on videotapes, but their usefulness is not commonly understood. Color bars represent a standard color test, used to calibrate equipment for copying, recording, broadcasting or just playing back video. It's standard practice to record between ten seconds and one minute of color bars at the beginning of any tape so that anyone can adjust his or her equipment to accurately represent the correct hues recorded on the tape. Professional video technicians would use color bars in conjunction with a vectorscope and a waveform monitor to check the color accuracy against established norms. If you have a printed color bar chart for comparison, you can make an "eyeball" judgement about the similarity between the colors on the chart and the colors on your screen. If you don't like what you see, a processing amplifier (or proc amp for short) will correct problems--or, if you're just playing a tape on television, a couple of twists of the hue control might do the trick.

Audio Onramp
Is it possible to connect a Y-adapter to the audio input jack of a record VCR, then use the extra input to add the output from a microphone, CD player, cassette deck or other audio device to the audio signal?

Mark Slobodnik
Metairie, Louisiana

The Y-adapter, in the situation you have described, is actually acting as a passive audio mixeressentially a way to mix two audio signals together without adjusting their relative signal strengths. If you used such a device with, say, a CD player to record background music onto your tape, you'd get both signals mixed together at whatever relative strengths they had when they entered into the Y adapter. Adjusting the output volume of the CD player to match levels with the program audio would be a cumbersome way to edit, but it would work.

However, if you decided to use a standard microphone as your secondary input, you'd run into problems right away. Microphone signals are very weak compared with standard audio signals that VCRs, camcorders and CD players output (all of which are commonly referred to as "line-level" signals). The easiest way to match the microphone's signal strength to that of the line-level signal of your VCR or camcorder is to use a true audio mixer that accepts both microphone and line-level inputs.

Another way to add an extra audio track to your videos: use a record VCR that has the audio dub feature. After recording your source material with the program audio, rewind the tape and put the deck into audio dub mode. Then record music, narration, sound effects, etc. onto this track, which you can then mix with your original program audio.

Tracking Issues
I recently traveled to the Galapagos Islands and filled four two-hour tapes with my Sony 8mm camcorder. After coming back, I was able to view these tapes clearly, but when I tried to view some previously shot tapes, I had no sound and a badly damaged picture. I took the camcorder to a repair shot and the diagnosis was "tracking heads off." The shop fixed the problem, but now I have a new problem: when I play the Galapagos tapes, I get no sound and a badly damaged picture. Is there anything to be done to fix these four tapes?

Henri Hoffmann
Jacksonville, Florida

Here's what you can do to recover your Galapagos tapes: take your camcorder back to the person who fixed the tracking. Pay him or her to a) restore the tracking to the way it was before, b) make copies of the tapes, then c) fix the tracking problem once again. Alternatively, if you live near a video post-production facility, you might be able to rent some time on a professional Hi8 deck with tracking controls that allow you get a clean picture from the mis-aligned tapes. Either way, this will result in a second-generation copy of your original tape, and it might cost you a little bit of money; even so, it's better than nothing.


Sun, 02/01/1998 - 12:00am