5.3 Contrast

The contrast sensitivity of the eye is defined as the smallest brightness difference which is visible. In fact the contrast sensitivity is not constant, but increases proportionally to brightness. Thus whatever the brightness of an object, if that brightness changes by about 1 per cent it will be equally detectable.

The true brightness of a television picture can be affected by electrical noise on the video signal. As contrast sensitivity is proportional to brightness, noise is more visible in dark picture areas than in bright areas. In practice the gamma characteristic of the CRT is put to good use in making video noise less visible. Instead of having linear video signals which are subjected to an inverse gamma function immediately prior to driving the CRT, the inverse gamma correction is performed at the camera. In this way the video signal is non-linear for most of its journey.

Figure 5.9 shows a reverse gamma function. As a true power function requires infinite gain near black, a linear segment is substituted. It will be seen that contrast variations near black result in larger signal amplitude than variations near white. The result is that noise picked up by the video signal has less effect on dark areas than on bright areas. After the gamma of the CRT has acted, noise near black is compressed with respect to noise near white. Thus a video transmission system using gamma correction at source has a better perceived noise level than if the gamma correction...