Proudly making Quest AV equipment in Australia since 1988
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abn 99 064 323 255  also t/a Quest Electronics ®  and Quest AV ®
Phone and SMS ... 0431 920 667 Mail ... POB 348  Woy Woy  NSW   2256
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To   answer   the   second   question   first,   you’re   only   likely   to need   a   TBC   these   days   to   stabilise   and   adjust   the   output from    a    VHS,    Betamax    or    8mm    VCR    (if    you    can    find    a working   one)   for   the   purpose   of   capturing   the   video   and audio   to   a   computer.   The   video   output   from   digital   cameras using   memory   cards   can   be   edited   directly   without   going through analogue capture, thus preserving the quality. When   a   tape   is   recorded   or   played   in   a   VCR   it   is   subjected to    a    range    of    complex    mechanical    forces    which    cause instability   in   the   video   signal   timing.   The   most   dynamic   and troublesome   of   these   is   caused   by   the   video   heads   which protrude   ever   so   slightly   (this   is   called   tip   penetration)   from the   spinning   head   drum.   As   the   head   strikes   the   tape   and begins   its   helical   path   down   the   tape,   it   stretches   the   tape   a tiny amount and the tape springs back. Because   of   this   stretching   and   springing   back   of   the   tape some   lines   will   be   shorter   and   some   lines   longer   than   they should   be.   This   gross   time   distortion   (severe   timebase   or sync   jitter)   of   the   first   lines   of   video   usually   reduces   as   the head   proceeds   further   down   the   tape.   All   lines   are   affected, but   the   first   10%   to   20%   at   the   top   of   the   picture   are   the worst   and   cause   the   effect   known   as   flagging   or   flag-waving which    can    be    seen    on    some    older    TV's    and    any    video monitors    with    slow    horizontal    synchronisation    (long    time constant). It   is   important   to   realise   that   ALL   analogue   video   cassette recorders   (VCR's)   do   it   and   they   all   do   it   slightly   differently, so   the   errors   which   are   recorded   will   always   be   a   bit   different to   the   errors   caused   during   playback,   especially   in   a   different machine.   A    first    generation    tape,    an    original    master,    will exhibit   two   layers   of   timebase   error   upon   playback,   even   in the   machine   that   recorded   it   because   the   mechanical   system is    affected    by    so    many    variables    (temperature,    friction, humidity,   gyroscopic   effects,   etc.)   that   that   is   the   best   you can   ever   hope   for.   Other   sources   of   time   distortion   are   wow and   flutter   which   may   be   caused   by   varying   tape   tension, dragging     brakes,     sticky     guides,     the     video     cassette mechanism, power supply variations, etc, etc. Every   time   an   analogue   tape   copy   is   copied   to   another   tape, the    timebase    errors    are    compounded    further,    eventually making   it   impossible   for   a   VCR   to   synchronise   with.   The signal   becomes   unrecordable,   but   a   TV   or   video   monitor may   still   show   a   recognisable   picture   because:-   1.   Basically, the   inertia   of   an   electron   beam   and   it's   controlling   circuitry   is virtually   zero   compared   to   a   mechanical   servo   system   thus allowing   the   electron   beam   to   follow   the   unstable   video AND 2.    Your    eyes    are    easily    deceived.    Professional    video monitors   have   operational   modes   such   as   underscan,   pulse cross   and   slow   sync   which   can   expose   timebase   errors,   but domestic TV's are designed to hide them. To     correct     timebase     errors     when     making     copies     or transferring   to   a   DVD   recorder   or   PC   you   use   a   timebase corrector.   There   are   two   main   types   of   TBC   -   external   and internal.   The   external   TBC   usually   has   at   least   two   inputs, one    for    composite    video    and    one    for    S - Video    (Y/C). Composite   video   fed   to   the   TBC   is   most   often   converted   into two    streams    of    digital    data    (one    for    the    Y    or    luminance portion    and    one    for    the    C    or    Chroma    portion,    similar    to S - Video)   and   stored   in   memory   (a   video   frame   store).   The original    unstable    sync    is    only    used    to    synchronise    this process and is stripped off. A   very   stable   crystal - locked   pulse   generator   is   then   used   to read   the   data   back   out   of   memory.   This   causes   all   horizontal lines   of   video   to   be   restored   to   the   same   length   (or   duration). They    are    then    converted    back    to    analogue    video    and provided   with   new,   very   stable   sync   signals. At   any   time,   the digital   memory   effectively   contains   a   whole   frame   (1   x   frame =   2   x   fields   of   312.5   lines   =   625   lines   for   PAL)   which   are proceeding    through    it,    line - by - line    in    a    first - in,    first - out fashion. Because   of   this   digital   process,   the   video   coming   from   a TBC   exhibits   none   of   the   time   jitter   that   is   seen   at   the   output
of   a   vcr   and,   at   least   as   far   as   the   sync   is   concerned,   is   a first    generation    signal    which    is    easily    recorded.    Internal TBC's   in   VCR’s   such   as   the   excellent   JVC   HR-S7600AM   are pretty   much   the   same,   but   usually   only   need   to   store   a   few lines    of    video,    often    15    or    16    lines,    because    they    have control   of   the   vcr's   servos   and   can   control   gross   mechanical errors directly. External TBC's   have   some   interesting   features.   For   instance, they   always   output   a   continuous,   stable   video   signal   -   even   if there   is   no   input   or   if   the   input   is   'garbage'   such   as   random noise   from   a   tuner   with   no   RF   signal   being   input   or   perhaps playback   of   a   tape   with   a   huge   crease   and   major   dropouts. Some    TBC's    allow    you    to    select    whether    you    will    have colourbars,   a   black   screen   or   a   freeze - frame   upon   loss   of input   -   you   will   see   this   last   effect   on TV   when   the   microwave link is lost. A   few   up - market   and   all   professional   TBC's   will   provide   an extra   input   to   allow   another   (highly   stable)   video   signal   to synchronise   or   'gen-lock'   the   output   of   the   TBC.   This   was useful   when   you   needed   to   mix   two   analogue   video   signals together.     Most     TBC's     have     controls     for     black     level (brightness),    video    gain    (contrast),    chroma    level    (colour saturation), and enhancement / filtering (sharpness). Internal   TBC's   in   domestic   vcr's   usually   DON'T   have   video signal   adjustments   or   genlock   capability   and   usually   they don't   output   any   signal   when   stopped,   except   for   whatever signal   may   be   being   fed   to   the   active   input   (known   as   e-e   or electronics   to   electronics   mode).   Note   that   the   e-e   mode   of   a vcr   with   an   internal   TBC   does   not   usually   output   a   stable signal    and    (in    most    if    not    all    cases)    cannot    be    used    to timebase correct the signal being fed through it. Not   so   long   ago   in   a   domestic   analogue   editing   situation   it was   usual   to   find   that   an   original   miniature   camcorder   tape was   copied   to   a   full-sized   VHS   or   SVHS   tape   and   that   this tape   was   then   copied   piece - by - piece   to   another   tape   during the   editing   process   and   that   this   edited   master   was   used   to make   the   final   copies   -   all   without   a   TBC   in   sight!   No   wonder those   old   tapes   look   so   bad   compared   to   today's   digital!   It   is important   to   realise   -   and   this   really   can't   be   understated   - COMPOUNDED      TIMEBASE      ERRORS      CANNOT      BE REMOVED,    only    the    current    errors    due    to    the    playback process   can   be   removed.   A   TBC   cannot   extract   a   perfect video      signal      from      several      compounded      layers      of time - distortion.   A   grotty   tape   can   be   made   recordable,   but the   copy   will   still   look   grotty   (though   stable)   because   EVERY analogue    generation    has    to    be    TBC'd    to    maintain    the stability   of   the   video.   Once   the   video   is   captured   in   digital form    it    may    be    possible    to    remove    more    errors    using freeware such as VirtualDub and AVIsynth. Analogue   video   mixers   are   getting   hard   to   find   now,   but   units such    as    the    Focus    Enhancements    (previously    Videonics) MX - Pro   and   Edirol   V - 4   have   digital   TBC's   built - in   to   allow digital   effects   and A/B   roll   editing.   If   you   have   one   of   these   it should   be   used   whenever   transferring   or   copying   analogue video   to   ensure   that   the   final   copies   are   100%   stable   and   of the   highest   visual   quality.   This   is   especially   important   when transferring   your   old   Betamax,   VHS,   SVHS,   8mm   and   Hi8 tapes   to   DVD   recorders   or   for   video   capture   into   a   computer. There   are   still   a   few   TBC’s   available   such   as   the   CTB - 100, and   video   standards   converters   such   as   the   CDM - 820   that also perform timebase correction. *TBC = Timebase Corrector / Time Base Corrector (c) 2000 Quest Electronics abn 99 064 323 255
What is a TBC* and why would I need one?