4

(Apologies if this is the wrong stackexchange, it seems the most appropriate)

When rewinding a VHS tape on an analog player (i.e. a VCR hooked up to a (CRT?) TV) when rewinding, or fast-forwarding, and regardless of video format (NTSC vs PAL vs SECAM) we're always treated to the same effect: a relatively clear window into the video playing backwards (or forwards), overlayed with two or three bars of static. I found a good example on YouTube here (around the 00:06 - 00:10 time):

What causes this? How does the action of the helical-scan tape over the read head cause these artifacts?

I also wonderred: when rewinding, if the VHS tape is playing backwards, and the video signal is scanned one scanline-at-a-time, how is it the video display is not vertically and horizontally inverted? Even on early, pure-analog VHS players lacked framebuffers yet when rewinding their displayed the video in the correct orientation, how was that possible?

Improve this question
4
  • 1
    It has been a long time since I was responsible for tape decks, especially analog ones, but here is what I think is happening. The tape is moving backwards, but the head is still spinning forwards. In other words, the head does not reverse directions. That means that the head and the tracks don't line up properly, which gives you the static. That's also how the image isn't inverted: the electrical system isn't reversed, just the mechanical, tape transport.
    – Michael Liebman
    Dec 20, 2016 at 1:11
  • 1
    @MichaelLiebman If the head isn't aligned with the tape, how is it the video field(s) still fill the output frame?
    – Dai
    Dec 20, 2016 at 1:12
  • 1
    The heads are always going to output the same number of lines, whether they encounter properly aligned tracks or not. When misaligned, it reads what ever is under the head as static, since it is analog system.
    – Michael Liebman
    Dec 20, 2016 at 1:16
  • 1
    @Dai Also, note that the mechanical part of the playback doesn't output the video "one scanline at a time". The head isn't scanning lines, it's scanning video fields (262.5 lines in NTSC), each one a modulated RF 'bundle' that contains the combined mono portion of the video and a subcarrier with the color-difference portion.
    – Jim Mack
    Dec 21, 2016 at 0:42

1 Answer 1

Reset to default
6

In helical-scan video players, the tape is wrapped around a rotating drum that has one or more magnetic pickups (heads) that are the width of a track and angled slightly from true horizontal. Each track is several inches long and laid down diagonally at (roughly) the same angle as the head is tilted. (The angles differ because the relative motion of the tape and heads causes a precession, an "active angle").

When the tape is moving forward at normal speed, the rotating head aligns with the angled tracks and all is fine. When the tape moves in reverse the angles are misaligned, and when the motion is faster than normal the heads will "sample" more than one track during a video frame, and pick up some of the inter-track guard space as well (the static).

This is what you see when you rewind. Nothing about the orientation of the data on tape changes, so there's no reason for the images to be inverted. If the drum rotated in a different direction then you'd see something else.

an image from the Wikipedia page for VHS

Improve this answer
3
  • 1
    Thanks for confirming my memory and explaining it better than I did.
    – Michael Liebman
    Dec 20, 2016 at 23:37
  • 1
    @MichaelLiebman No problem. It's always good to find a use for otherwise obsolete knowledge. (-:
    – Jim Mack
    Dec 21, 2016 at 0:55
  • I understand most of your answer - but I don't understand this part "When the tape moves in reverse the angles are misaligned" - could you visually explain that with a diagram?
    – Dai
    Feb 11, 2017 at 4:24

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.