MPEG video compression - The MPEG - page 139

Figure 5.8: An object moves past a camera, and is tracked on a monitor by the eye. The high temporal frequencies
cause aliasing in the TV signal, but these are not perceived by the tracking eye as this reduces the temporal
frequency to zero.
Whilst this result is highly desirable, it does not circumvent sampling theory because the effect only works if several
assumptions are made, including the requirement for the motion to be smooth.
Figure 5.7(b) shows that when the eye is tracking, successive pictures appear in different places with respect to the
retina. In other words if an object is moving down the screen and followed by the eye, the raster is actually moving
up with respect to the retina. Although the tracked object is stationary with respect to the retina and temporal
frequencies are zero, the object is moving with respect to the sensor and the display and in those units high
temporal frequencies will exist. If the motion of the object on the sensor is not correctly portrayed, dynamic
resolution will suffer. Dynamic resolution analysis confirms that both interlaced television and conventionally
projected cinema film are both seriously sub- optimal. In contrast, progressively scanned television systems have
no such defects.
In real-life eye tracking, the motion of the background will be smooth, but in an image-portrayal system based on
periodic presentation of frames, the background will be presented to the retina in a different position in each frame.
The retina separately perceives each impression of the background leading to an effect called background
strobing .
The criterion for the selection of a display frame rate in an imaging system is sufficient reduction of background
strobing. It is a complete myth that the display rate simply needs to exceed the critical flicker frequency.
Manufacturers of graphics displays which use frame rates well in excess of those used in film and television are
doing so for a valid reason: it gives better results!
The traditional reason against high picture rates is that they require excessive bandwidth. In PCM form this may be
true, but there are major exceptions. First, in the MPEG domain, it is the information which has to be sent, not the
individual picture. Raising the picture rate does not raise the MPEG bit rate in proportion. This is because pictures
which are closer together in time have more redundancy and smaller motion distances. Second, the display rate
and the transmission rate need not be the same in an advanced system. Motion-compensated up-conversion at the
display can correctly render the background at intermediate positions to those transmitted.
[ 1 ] Kelly, D.H., Visual processing of moving stimuli. J. Opt. Soc. America , 2 216-225 (1985)
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