| | Definition: | | In composite NTSC, PAL or S-video systems, it is necessary to scale (B-Y) and (R-Y) so that the composite ntsc or pal signal (luma plus modulated chroma) is contained within the range -1/3 to +4/3. These limits reflect the capability of composite signal recording or transmission channel. The scale factors are obtained by two simultaneous equations involving both B-Y and R-Y, because the limits of the composite excursion are reached at combinations of B-Y and R-Y that are intermediate to primary colours. The scale factors are as follows:
U = 0.493 * (B - Y)
V = 0.877 * (R - Y)
U and V components are typically modulated into a chroma component:
C = U*cos(t) + V*sin(t)
where t represents the ~3.58 MHz NTSC colour sub-carrier. PAL coding is similar, except that the V component switches Phase on Alternate Lines (+-1), and the sub-carrier is at a different frequency, about 4.43 MHz.
It is conventional for an NTSC luma signal in a composite environment (ntsc or S-video) to have 7.5% setup :
Y_setup = (3/40) + (37/40) * Y
A pal signal has zero setup.
The two signals Y (or Y_setup) and C can be conveyed separately across an S-video interface, or Y and C can be combined (encoded) into composite NTSC or PAL:
NTSC = Y_setup + C
PAL = Y + C
U and V are only appropriate for composite transmission as 1-wire NTSC or PAL, or 2-wire S-video. The UV scaling (or the IQ set, described below) is incorrect when the signal is conveyed as three separate components. Certain component video equipment has connectors labelled YUV that in fact convey ypbpr signals. |
Video Dictionary