Answers.Body: <p>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").</p>
  
  <p>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). </p>
  
  <p>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 <em>drum</em> rotated in a different direction then you'd see something else.</p>
  
  <p><a href="https://i.sstatic.net/z9MGB.jpg"><img src="https://i.sstatic.net/z9MGB.jpg" alt="an image from the Wikipedia page for VHS"></a></p>
  
Answers.ContentLicense: CC BY-SA 3.0
Answers.CreationDate: 2016-12-20T23:37:29.457Z
Answers.Text: Thanks for confirming my memory and explaining it better than I did.
Answers.UserDisplayName: Michael Liebman
Answers.ContentLicense: CC BY-SA 3.0
Answers.CreationDate: 2016-12-21T00:55:26.477Z
Answers.Text: @MichaelLiebman  No problem. It's always good to find a use for otherwise obsolete knowledge. (-:
Answers.UserDisplayName: Jim Mack
Answers.ContentLicense: CC BY-SA 3.0
Answers.CreationDate: 2017-02-11T04:24:20.170Z
Answers.Text: 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?
Answers.UserDisplayName: Dai
Answers.ContentLicense: CC BY-SA 3.0
Answers.CreationDate: 2016-12-20T14:18:17.753Z
Answers.LastActivityDate: 2016-12-20T14:18:17.753Z
Answers.UserDisplayName: Jim Mack
Body: <p>(Apologies if this is the wrong stackexchange, it seems the most appropriate)</p>
  
  <p>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): <div class="youtube-embed"><div>
                  <iframe width="640px" height="395px" src="https://www.youtube.com/embed/QnkWzu4OGjI?start=0"></iframe>
              </div></div> </p>
  
  <p>What causes this? How does the action of the helical-scan tape over the read head cause these artifacts?</p>
  
  <p>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?</p>
  
Comments.ContentLicense: CC BY-SA 3.0
Comments.CreationDate: 2016-12-20T01:11:14.707Z
Comments.Text: 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.
Comments.UserDisplayName: Michael Liebman
Comments.ContentLicense: CC BY-SA 3.0
Comments.CreationDate: 2016-12-20T01:12:19.090Z
Comments.Text: @MichaelLiebman If the head isn't aligned with the tape, how is it the video field(s) still fill the output frame?
Comments.UserDisplayName: Dai
Comments.ContentLicense: CC BY-SA 3.0
Comments.CreationDate: 2016-12-20T01:16:57.513Z
Comments.Text: 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.
Comments.UserDisplayName: Michael Liebman
Comments.ContentLicense: CC BY-SA 3.0
Comments.CreationDate: 2016-12-21T00:42:33.753Z
Comments.Text: @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.
Comments.UserDisplayName: Jim Mack
Tags: <video><vhs><tape>
Title: What causes the VHS rewind effect? And how did rewind preview work?
UserDisplayName: Dai
ThreadText: # What causes the VHS rewind effect? And how did rewind preview work?
  
  (Asked by: username_0 on 2016-12-20)
  
  (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?
  
  **Tags:** video, vhs, tape
  
  > **Comments:**
  > **username_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.  
  > **username_0**: @username_1 If the head isn't aligned with the tape, how is it the video field(s) still fill the output frame?  
  > **username_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.  
  > **username_2**: @username_0  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.  
  
  ---
  
  ## 1 answers
  
  ### Answer by username_2 on 2016-12-20. Score: 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](https://i.sstatic.net/z9MGB.jpg)](https://i.sstatic.net/z9MGB.jpg)
  
  > **Comments:**
  > **username_1**: Thanks for confirming my memory and explaining it better than I did.  
  > **username_2**: @username_1  No problem. It's always good to find a use for otherwise obsolete knowledge. (-:  
  > **username_0**: 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?  
  
  ---
  
  