RGB Modes
In these modes complete lines are transmitted one after another. These
are generally very robust modes which are not susceptible to tuning
inaccuracies. Slight frequency errors will cause the image to appear
lighter or darker, but the colours are not affected.
Each line has three component parts, one for each of the three primary colours - red, green and blue, which
are transmitted one after another. The
order in which the three RGB colour components are sent depends on the
mode.
Each of the three component parts of the
line represents the gray scale values for the respective colour moving
progressively from left to right of the image. The same process is then
repeated twice more for the other two colours, again scanning the same
line from left to right.
In most of these modes the lines are separated by sync pulses. The
three RGB colour components of each line may also be separated by short black
porches. The function of these porches is somewhat obscure but they may be
connected with the colour processing which took place in dedicated scan
converters. They also provide a small border which is not normally
displayed but which prevents picture detail from one side of the image appearing
on the opposite side if there is a slight synchronization error. In modern computer systems
the black porches have no real function, but
they must be taken into account in the encoding and decoding software to avoid
colour mis-registration.
Once the three colour components have
been received they can be recombined to reconstruct the line,
pixel-by-pixel, reading the red, green and blue values from the
appropriate positions in the three line components. If the values are
not read accurately from corresponding positions in the three line
portions this will result in colour mis-registration, visible as colour
fringes around any sharp edges in the image. This is another reason to
maintain accurate timing at both the transmitting and receiving ends.
22 January 2008
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