Information Technology Reference
In-Depth Information
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 is performed near the display.
Figure 5.9:
CCIR Rec.709 reverse gamma function used at camera has a straight line approximation at the lower
part of the curve to avoid boosting camera noise. Note that the output amplitude is greater for modulation near
black.
In practice the system is not rendered perfectly linear by gamma correction and a slight overall exponential effect is
usually retained in order further to reduce the effect of noise in darker parts of the picture. A gamma correction
factor of 0.45 may be used to achieve this effect. If another type of display is to be used with signals designed for
CRTs, the gamma characteristic of that display will probably be different and some gamma conversion will be
required.
Figure 5.10:
The sensitivity of the eye to noise is greatest at low frequencies and drops rapidly with increasing
frequency. This can be used to mask quantizing noise caused by the compression process.
Sensitivity to noise is also a function of spatial frequency. Figure 5.10 shows that the sensitivity of the eye to noise
falls with frequency from a maximum at zero. Thus it is vital that the average brightness should be correctly
conveyed by a compression system, whereas higher spatial frequencies can be subject to more requantizing noise.
Transforming the image into the frequency domain allows this characteristic to be explored.
5.4 Colour vision
Colour vision is due to the cones on the retina which occur in three different types, responding to different colours.
Figure 5.11 shows that human vision is restricted to range of light wavelengths from 400 nanometres to 700
nanometres. Shorter wavelengths are called ultra- violet and longer wavelengths are called infra-red. Note that the
response is not uniform, but peaks in the area of green. The response to blue is very poor and makes a nonsense
of the use of blue lights on emergency vehicles which owe much to tradition and little to psycho-optics.
