Re: Laser Disc Rambling

[gatech!chinet.chi.il.us!katefans@EDDIE.MIT.EDU (Chris Williams)]

Chris here,

> Color is another interesting issue.  VHS video tape has no built in
> time base correction which leads errors in color reproduction.

  Richard Caldwell is wrong about the function and purpose of a time base
corrector. Color correction is done with an analog process amplifer.
A Time Base Corrector corrects (what else) the horizontal timing errors
caused by speed variations of the heads moving relative to the tape and
the tape moving relative to the head stack. The rotating head moves fairly
quickly, scanning the tape at an angle recording one field per pass. There
are two heads, one for even fields and one for odd. The tape moves fairly
slowly though, and variations in the speed crop up due to the difficulty of
moving something slowly and accuratly. These "time base errors" show up as
the tendency of vertical lines to become fuzzy.

        ------=------             ------=-----
        ------=------   Correct   -------=----   Time
        ------=------    Time     ------=-----   Base
        ------=------    Base     -------=----  Errors
        ------=------             ------=----- 
       
   A reference color signal is used to correct the timing of each line to
the ideal standard by delaying the video signal varying amounts. The T.B.C.
in a LD player corrects for speed variations of the rotating platter. 
  Sorry to cast aspersions on Richard's omniscience.

                                   Chris Williams of
                                      Chris'n'Vickie of Chicago
                                         katefans@chinet.chi.il.us

P.S. In the on-going e-mail personal-size flame-war between Richard and me,
   he has once again pulled his favorite retorical trick, the old "me and
   this mouse in my pocket" one, where he calls upon invisible people who
   agree with him. As I haven't had any mail complaining about my stuff
   in quite a while (since Drukman), would _anyone_ who has written
   _anything_ to Richard talking about me please send me a copy. I'd like
   to see it and find out _what_ he is on about.