Insert Editing With A Flying Erase Head Explained
Mar 30 '01 (Updated Apr 01 '01)
The Bottom Line For editing work, flying erase heads are irreplaceable.
All VCRs and camcorders use so-called helical scanning system. The tape moves along the rotating metal drum that has video heads (and in case of Hi-Fi models, audio heads). The drum may include 2 video heads located at the opposite ends of the drum, 4 heads located at 90 degrees of each other, or more heads in professional models.
You can see this drum if you open the lid of your VCR's tape compartment and look inside. During operation, it rotates at 1,8000 RPM (30 revolutions per second).
The rotation of the drum is directed with an angle to the tape so it fills the entire tape's surface with magnetized lines that contain information about image - brightness of different areas and such. It is impossible to achieve good still picture with two heads - it will be distorted, noisy and usually has a vertical "noise line". 4-headed models allow the perfect still picture/slow motion and several recording speeds.
In other words, the drum is tilted relatively to the direction of tape travel. By the way, if you happen to adjust the “Tracking” (usually the automatic tracking does it for you), what you really did is change the angle at which the drum is tilted in respect to the direction of the tape travel, so it would match the angle, at which the drum in the VCR that recorded the program was tilted.
So the tape is filled with diagonal lines with information (video, and in case with stereo VCRs, stereo audio) and the lines that go along the tape, which a recorded by a stationary heads – mono sound, control signals and such.
Before you can record anything properly (without video noise), you need to erase the previous recording. Otherwise the picture will be noisy. Since the “regular” erasing head is stationary located before the recording heads on the drum (in relation to the tape travel direction), the recording starts before the erased area reaches the drum and there is an area at the end of the recording, which is not erased, but not recorded.
So, if you want to insert some video in the middle of already recorded program, you get noisy signal in the beginning of the “inserted” recording, and no signal (video, audio and control) at the end of it for several seconds.
But this is just a part of the problem. Since video signal is recorded diagonally, in the beginning the erasing head erases parts of the frames of video, same thing at the end. This creates other unpleasant effects.
To improve the situation, the “Flying Erase Head” was introduced. It is located right on the drum before the recording head and erases only the areas that will be recorded.
Flying erase heads cost extra, and that’s why you won’t find them in the cheap VCRs. Additionally, there is no benefit from them during playback. But for editing work, they are irreplaceable.
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