Newsgroups: comp.robotics
Path: cantaloupe.srv.cs.cmu.edu!das-news2.harvard.edu!news2.near.net!howland.reston.ans.net!pipex!uunet!timbuk.cray.com!driftwood.cray.com!kilian
From: kilian@cray.com (Alan Kilian)
Subject: Re: Indexing VCR Project...
Message-ID: <1994Dec9.103339.10389@driftwood.cray.com>
Lines: 96
Nntp-Posting-Host: poplar021
Organization: Cray Research, Inc.
Date: 9 Dec 94 10:33:38 CST

This sounds like a terriffic project. 

Some things to consider:

We use a Betacam recorder to do our single-frame animations and I have
studied it a little bit. Here's how it works as far as I can tell:

 - The tape rewinds a little bit.
 - The tape starts to play forward and gets up to speed.
 - The tape speed and head speeds are adjusted to match the incoming
   video signal so that the head is in just the right place with respect
   to the tape jsut as the incoming video signal gets to a vertical synch
   pulse.
 - The tape also has a frame number written into each video frame. THis
   is called SMPTE time code. (I think) The VCR is reading the beginning
   of each video frame and comparing the SMPTE timecode with the timecode
   that you asked to be recorded, and when it gets to the frame just before
   that, it switched the heads from playback to record, and ...
 - The VCR records one frame.

   Then the VCR slows down, stops, and rewinds a little bit and waitd for 
   another "record a frame" command

The tricky bits:

 - If you go from pause, to record to pause, the tape is not moving at the 
   proper speed, and the tracks are not lined up right.

   Like this:


  Normal speed regular recording:

                       +--------End of frame #1
                       |    
                       |   +--- End of frame #2
                       |   |
                       V   V
  ---------------------------------------------------------------------------
  /                   .   .                                                 /
  \                  .   .                                                  \
  /                 .   .                                                   /
  \                .   .                                                    \
  /               .   .            <==== The tape is moving this way        /
  \              .   .                                                      \
  /             .   .                                                       /
  \            .   .                                                        \
  ---------------------------------------------------------------------------
              ^    ^
              |    |
              |    +--- Beginning of frame #2
              |
              +-------- Beginning of frame #1


   If the tape is moving slower than normal:

                   +--------End of frame #1
                   |    
                   |   +--- End of frame #2
                   |   |
                   V   V
  ---------------------------------------------------------------------------
  /                .   .                                                    /
  \               .   .                                                     \
  /               .   .                                                     /
  \               .   .                                                     \
  /              .   .             <==== The tape is moving this way        /
  \             .   .                                                       \
  /            .   .                                                        /
  \            .   .                                                        \
  ---------------------------------------------------------------------------
              ^    ^
              |    |
              |    +--- Beginning of frame #2
              |
              +-------- Beginning of frame #1


  Those lines should both be straight of course, but the second ones should
  be more vertical than the first ones.

  Now you can see that if you record a bunch of frames like example #2
  and then play them back with the tape moving at a normal speed, the heads
  will move from one frame to another and you will get that nasty noise
  bar in the middle of each displayed frame. I don't know if tracking has
  enough adjustment to take care of this. It might.


  That's it for me. Have fun,
   
                     -Alan Kilian
-- 
 -Alan Kilian    kilian@cray.com 612.683.5499 (Work)
 "We built a humongous telescope in space for a few million dollars, but it 
  didn't work, so we spent a few million more and fixed it." -The Hubble Story.
