A picture element (pixel) is the smallest area in an image that can be reproduced by the video signal. The size of a pixel depends upon the size of the scanning spot, the number of scanning lines and the highest frequency utilised in the video signal. Its vertical dimension is equal to the distance between two scanning units, or centre to centre distance between two adjacent spots. Its horizontal dimension will also be the same.
With an aspect ratio of 4:3 the number of pixels on a horizontal scanning line will be 4/3 times the number of pixels on a vertical line. The total number of pixels activated on the screen reproduce the picture.