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This     article     was     more     relevant     when     analogue     video cameras    were    all    the    rage.    Nowadays    we    have    digital sensors   in   our   digital   cameras   and   resolution   refers   more   to the   horizontal   x   vertical   storage   and   display   resolutions   of the    digital    video.    However,    analogue    cameras    are    still common in security video so this article is still useful. When   selecting   a   video   camera   we   are   most   interested   to know   how   sharp   the   picture   is   going   to   be   and   to   make   an informed     decision     we     need     some     way     of     making     a comparison.   The    sharpness    of    a    video    picture    is    directly related    to    its    bandwidth,    which    strictly    speaking,    is    the difference   between   the   lowest   frequency   (usually   25Hz   for PAL   or   30Hz   for   NTSC)   and   the   highest   frequency   that   the device   can   pass   without   any   significant   loss   of   detail.   In other words we want to know the frequency response. In   the   case   of   a   video   device   with   inputs   and   outputs,   we   can simply   inject   test   signals   and   read   the   results   off   a   calibrated display.   But   what   do   we   do   with   a   camera   which   only   has   an optical    input    or    a    TV    monitor    which    only    has    an    optical output? A   method   was   devised   to   quantify   bandwidth   (or   sharpness   / resolution   /   definition)   as   a   simple   figure   of   merit   referred   to as   "Lines".   The   more   lines,   the   wider   the   bandwidth   /   the sharper   the   picture   /   the   finer   the   resolution   /   the   better   the definition... you get the idea, a bigger number is better. OK,   so   what   are   these   mystical   lines?   Well,   to   start   with "Lines"    was    a    very    unfortunate    choice    of    words.   As    you probably   know,   a   colour   video   picture   is   made   up   from   a series     of     regularly     spaced     horizontal     lines     which     are modulated   to   produce   light   and   dark   shades   of   red,   green and   blue   to   fool   our   eyes   into   seeing   a   rectangular   picture with a whole rainbow of colours. In   the   PAL   system   there   are   two   interlaced   fields   of   312.5 lines   each   which   effectively   gives   us   625   lines   per   frame. The   50Hz   field   rate   reduces   the   severe   flicker   we   would experience   from   a   25Hz   frame   rate   (there   are   two   fields   per frame). The   NTSC   system   consists   of   two   interlaced   fields   of 262.5   lines   each   yielding   525   lines   per   frame   at   a   60Hz   field rate (30Hz frame rate). These   lines   are   NOT   the   lines   we   are   talking   about   when   we are    talking    about    sharpness    /    resolution    /    definition!    The numbers   of   these   horizontal   lines   CANNOT   and   DO   NOT change!   PAL   has   625   horizontal   lines   and   NTSC   has   525, end   of   story!   Now   that   we   have   that   out   of   the   way,   let's continue.   In   the   PAL   system,   these   horizontal   lines   are   64uS
(64   microseconds)   long,   but   12uS   are   not   useable   for   video because    they    contain    the    synchronisation    and    blanking pulses,   so   there   are   52uS   left   for   video   (and   we   don't   get   to see   all   of   that   due   to   overscan).   Let's   say   we   have   a   video picture made up from dark and light vertical lines. If   we   look   at   such   a   video   signal   on   an   oscilloscope   we   will see    a    square    wave    which shows      us      that      a      single vertical   "Line"   is   either   a   dark or   light   one.   Basically,   if   there were   104   lines,   there   would be   52   of   these   dark/light   pairs in       the       52uS       available, therefore   each   pair   would   be 1uS     wide,     equalling     1MHz (52/.000052   =   1,000,000),   so 104   lines   would   be   equal   to 1MHz.   VHS   vcrs   were   often quoted   as   having   a   resolution of   240   lines   (120/.000052   = 2.3MHz),     and     S-VHS     vcrs were    often    about    400    Lines (200/.000052 = 3.84MHz) and so on. How   is   the   number   of   lines   measured?   1.   In   the   case   of   a camera   an   accurate   printed   test   chart   is   photographed   to   fill the   visible   area.   The   video   output   from   the   camera   is   viewed on   a   waveform   monitor   and   the   resolution   determined   by comparison   with   calibrated   markers   on   the   test   pattern.   2.   A monitor   is   fed   with   a   test   pattern   with   calibration   markers embedded   and   where   the   pattern   becomes   a   mushy   grey the   frequency   is   beyond   the   equipment’s   ability   to   resolve   it into   discernible   Lines.   3.   A   VCR   was   tested   with   a   playback tape   to   determine   the   frequency   response,   then   a   record   / play   test   was   performed.   The   result   in   MHz   can   easily   be converted to Lines mathematically. So   to   recap,   Lines   (of   resolution   /   etc)   has   nothing   to   do   with how    many    horizontal    lines    the    picture    is    made    up    from, because   for   PAL   or   NTSC   that   can't   change.   It   is   determined by   observing   how   many   discernible   vertical   lines   are   able   to be   reproduced   by   the   device   under   test.   Lines   (of   resolution   / etc.)    is    just    another    way    of    expressing    bandwidth    or frequency   response   that   is   applicable   to   opto - electronic   and electronic   video   devices   so   their   frequency   response   can   be compared. (c) 2000 Quest Electronics abn 99 064 323 255 Test chart fragment from bealecorner.com  
Analogue Video Resolution - Lines vs MHz