During the early days of color television, black-and-white sets were still expected to display what were originally color images. YIQ model separated chrominance from luminance. Luminance information is contained on the Y-channel, whereas color information is carried on I and Q channels (in-phase and in-quadrature) , in-short YIQ(Luminance, In-phase, Quadrature). In addition to providing a signal that could be displayed directly on black-and-white TVs, the system provided easy coding and decoding of RGB signals which was not directly possible.
Due to the fact that the Y-channel carries a lot of luminance information, it has a bandwidth assigned to it of 4Mhz, I-channel has a bandwidth assigned to it of 1.5Mhz, and Q-channel has a bandwidth of 0.6Mhz.
The Y component is the grayscale to drive old black-and-white TVs. The I component goes from orange to blue and the Q component goes from purple to green.
To find I-channel and Q-channel:
In-Phase = Red - Yellow Quadrature = Blue - Yellow
It is not possible to directly display a YIQ image while developing. The show function only recognizes RGB colors. If you try to display an image in another colorspace, the show function will display the wrong color. To use show, we have to use conversions.
- From YIQ to RGB conversion:
- From RGB to YIQ conversion:
Advantages:
- The advantage of this model is that more width can be assigned to the Y-component (Luminance) because the human visual system is more sensitive to changes in luminance than to changes in hue or saturation.
- For display on a screen, color television sets map these channels Y, I, and Q to R, G, and B.
Disadvantage:
- Due to the high implementation costs for true I and Q decoding, few television sets do it.
- To accommodate the bandwidth variances between I and Q, each of I and Q needs a distinct filter.
- Due to different bandwidths, the “I” filter has a time delay to match the “Q” filter’s long delay.
- In black-and-white TV only Only Y can be noticed.