Until tape is phased out completely, it seems prudent to briefly cover some key factors in using videotape machines.
Video recorders have five basic functions: play, record, stop, rewind, fast-forward, and pause.
To record on some machines the record button has to be held down before pressing the play button. On others you simply press the record buttons.
In either case you will probably see a red light come on a kind of universal indicator that the machine is in the record mode.
First, although the stop mode disengages the tape from the head, the pause button generally allows the tape to stay in contact with the spinning video heads, ready for an instant start in either the record or playback modes. This can create a problem.
If left in pause too long, the video heads will wear away the recording surface of the tape. This can damage the tape resulting in video noise and the dropouts we illustrated at the end of the last module.
This can also result in head clog where the microscopic gap in the video heads of an analog machine is clogged with foreign matter. To help avoid this problem, most of today's VCRs will automatically shut down after being left in pause for a few minutes.
When there is head clog, you will see a snowy picture (photo on right).
If things get even worse, you will see a full-blown "snowstorm" take over and the picture will roll, break up, and disappear.
"Playing" a head cleaning tape for about five seconds may solve the problem. If it doesn't, you may have to get a technician to clean the heads with a special solution.
Some machines have self-cleaning heads, which routinely (and somewhat superficially) clean the heads during normal VCR operations. This will generally take care of minor head clog problems.
As we've noted, a few tape machines have confidence heads with the pre-read function. These machines are able to play back the recorded signal a fraction of a second after it has been recorded.
Without confidence heads the operator can only monitor the video from the camera. This gives no indication of possible recording problems, which brings us to --
Spot-Checking a Tape
Because dropouts or head clog may not be discovered until the tape is played back, tapes should be spot checked (checked at various spots) after important recordings.
Spot checks are done by stopping the tape at the end of the recording, rewinding a meter or two (five or so feet) and checking the last seconds of the tape; then rewinding the tape to about the midpoint and checking again; and, finally, rewinding the tape to the beginning and checking the first five or ten seconds of the recording.
Although some people will only check the end of the recording, sometimes head clog problems that develop earlier in a recording will clear up toward the end -- a problem you wouldn't know about if you only checked the last few seconds.
During spot checks you should look for:
do find minor dropout problems and you can't redo the segment, an electronic
dropout compensator may be able to unobtrusively
fill in short bits of missing data as the tape is edited or copied.
Although the following differ between machines -- especially between digital and analog tape machines -- the skew control found on some VCR's controls videotape tension.
This affects the length of the video tracks as they are "read" (played back) from the videotape. Improper skew adjustment is indicated by flagging, or a bending and wavering of vertical lines at the top of the video frame.
Most skew controls have a center "indent" position that indicates a normal setting. Tapes that have been played many times, stretched, or subjected to high temperatures may require new skew settings.
A more common control is the tracking control that affects the VCR's ability to precisely (and generally automatically) align the heads with the narrow video tracks recorded on the tape. As with skew, the tracking control is only used to correct problems during playback.
On most videotape formats tracking errors show up in the form of a horizontal band of video noise (shown here). In severe cases there will be a total breakup of the picture.
Some VCRs have tracking level meters that represent a readout of the strength of the video signal. If automatic tracking fails or is not present and the video level falls below the optimum level indicated on the meter, the tracking control should be adjusted for maximum signal strength.
You may find that a tape has a tracking level that's too low to provide a stable playback. Since VCRs differ, playing the tape on a different machine may help. At least it's worth a try.
Care and Handling
The photo on the left illustrates major video timing problems that can be cleared up by running the signal through a good TBC (photo on the right).
Although TBCs can be stand-alone (separate) units, today professional video recording equipment commonly has TBC circuitry built in.
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