The horizontal resolution of a television
system is the ability of the scanning system to resolve the horizontal details,
i.e. changes in brightness levels of elements along a horizontal scanning line.
Since such changes represent vertical edges of picture details, it follows that
the horizontal resolution can be expressed as a measure of the ability to
reproduce vertical information or lines of resolution as shown in figure 1
below.
system is the ability of the scanning system to resolve the horizontal details,
i.e. changes in brightness levels of elements along a horizontal scanning line.
Since such changes represent vertical edges of picture details, it follows that
the horizontal resolution can be expressed as a measure of the ability to
reproduce vertical information or lines of resolution as shown in figure 1
below.
The horizontal resolution in a scanning
system depends upon the rate at which the scanning spot is able to change
brightness level as it passes through a horizontal line across the vertical
line of resolution as shown in figure 1 above.
system depends upon the rate at which the scanning spot is able to change
brightness level as it passes through a horizontal line across the vertical
line of resolution as shown in figure 1 above.
In a 625-line system, there are
effectively about 410 lines of vertical resolution. The horizontal resolution
should be of the same order. Because of the aspect ratio of 4:3, the number of
vertical lines for equivalent horizontal resolution will be (410 x 4/3) = 546
black and white alternate lines, which means (546 x ½) = 273 cycles of black
and white alternates of elementary areas. For the 625-line system, the
horizontal scan or line frequency (fH) is given by;
effectively about 410 lines of vertical resolution. The horizontal resolution
should be of the same order. Because of the aspect ratio of 4:3, the number of
vertical lines for equivalent horizontal resolution will be (410 x 4/3) = 546
black and white alternate lines, which means (546 x ½) = 273 cycles of black
and white alternates of elementary areas. For the 625-line system, the
horizontal scan or line frequency (fH) is given by;
fH = number of lines per
picture x picture scan rate
picture x picture scan rate
= 625 x 25 = 15,625 Hz
As each picture line is scanned 25 times
in one second, the total line period (TH) is thus
in one second, the total line period (TH) is thus
TH = 1/ fH =
1/15,625 = 64µs
1/15,625 = 64µs
Of this period, 12µs are used for the
blanking of the flyback retrace. Thus the 546 black and white alternations,
i.e. 273 cycles of complete square waves are scanned along a horizontal raster
line during the forward scan time of (64 – 12) = 52 µs. The period of this
square wave is 52/273 = 0.2 µs, giving the high fundamental frequency of 5MHz
which is adequate as the highest video frequency in the signal.
blanking of the flyback retrace. Thus the 546 black and white alternations,
i.e. 273 cycles of complete square waves are scanned along a horizontal raster
line during the forward scan time of (64 – 12) = 52 µs. The period of this
square wave is 52/273 = 0.2 µs, giving the high fundamental frequency of 5MHz
which is adequate as the highest video frequency in the signal.
The highest fundamental video frequency
in a scanning system is given by
in a scanning system is given by
