System and method for converting component video signals to a composite video signal compatible with the PAL standard

DETAILED DESCRIPTION OF THE INVENTION OVERVIEW FIG.
1 shows an exemplary prior art system which generated a digital PAL composite video signal from digital component signals Y, U and V which have a sampling rate that is incompatible with the desired PAL sampling rate of 17.
734475 MHz (hereinafter 17.
7 MHz).
In this exemplary system, the sampled data Y, U and V signals are separately resampled by the respective interpolators 110, 112 and 114 to provide samples at a rate which is compatible with the PAL standard.
Next, the U and V samples are band-limited by respective 11 tap low-pass filters 118 and 120 so that they occupy a band of frequencies consistent with the PAL standard.
Each of the signals, Y, U and V is then scaled by a respective gain factor Y.
sub.
G, U.
sub.
G and V.
sub.
G by multipliers 116, 122 and 124, respectively.
The scaled U and V signals are then used to modulate respective sampled data color subcarrier signals applied to the multipliers 126 and 128.
These modulated signals are then summed by an adder 130 and added to the scaled luminance signal by an adder 132 to produce the digital PAL composite video signal.
The circuit shown in FIG.
1 uses 36 multiply operations and 33 add operations in the three 12-tap interpolators, 22 multiply operations and 20 add operations in the two low-pass filters, three multiply operations for gain adjustment and two multiply operations and two add operations for modulation.
This results in a total of 63 multiply operations and 55 add operations.
By contrast, the circuit described below accomplishes the same function using only 24 multiply operations and 25 add operations.
This is possible because a single 12-tap interpolator is used to interpolate, band-limit and modulate the two color difference signals U and V.
To better explain the operation of the invention, it is helpful to first describe the PAL composite video signal mathematically.
It is generally known that Y+Ucos(f.
sub.
p).
+-.
Vsin(f.
sub.
p) represents an analog PAL signal, i.
e.
, the luminance signal Y added to two suppressed subcarrier signals at a frequency of f