Information Technology Reference
In-Depth Information
Figure 5.16:
In an interlaced system, a horizontal edge is present in
one
field only and so is presented to the
viewer at frame rate not field rate. This is below the critical filcker frequency and is visible as
twitter
.
In order to return to a continuous signal, a quincuncial spectrum requires a triangular spatio-temporal low-pass
filter. In practice no such filter can be realized. Consequently the sampling sidebands are not filtered out and are
visible, particularly the frame rate component. This artifact is visible on an interlaced display even at distances so
great that resolution cannot be assessed.
Figure 5.17(a) shows a dynamic resolution analysis of interlaced scanning. When there is no motion, the optic flow
axis and the time axis are parallel and the apparent vertical sampling rate is the number of lines in a frame.
However, when there is vertical motion, (b), the optic flow axis turns. In the case shown, the sampling structure due
to interlace results in the vertical sampling rate falling to one half of its stationary value.
Figure 5.17:
When an interlaced picture is stationary, viewing takes place along the time axis as shown in (a).
When a vertical component of motion exists, viewing takes place along the optic flow axis. (b) The vertical sampling
rate falls to one half its stationary value. (c) The halving in sampling rate causes high spatial frequencies to alias.
Consequently interlace does exactly what would be expected from a half-bandwidth filter. It halves the vertical
resolution when any motion with a vertical component occurs. In a practical television system, there is no anti-
aliasing filter in the vertical axis and so when the vertical sampling rate of an interlaced system is halved by motion,
high spatial frequencies will alias or heterodyne causing annoying artifacts in the picture. This is easily
demonstrated.
Figure 5.17(c) shows how a vertical spatial frequency well within the static resolution of the system aliases when
motion occurs. In a progressive scan system this effect is absent and the dynamic resolution due to scanning can
be the same as the static case.
