Information Technology Reference
In-Depth Information
Figure 5.14:
The vertical/temporal spectrum of a non-interlaced video signal.
Interlace is actually a primitive form of compression in which the system bandwidth is typically halved by sending
only half the frame lines in the first field, with the remaining lines being sent in the second field. The use of interlace
has a profound effect on the vertical/temporal spectrum. Figure 5.15 shows that the lowest sampling frequency on
the time axis is the frame rate, and the lowest sampling frequency on the vertical axis is the number of lines in a
field. The arrangement is called a quincunx pattern because of the similarity to the five of dice. The triangular
passband has exactly half the area of the rectangular passband of Figure 5.14 illustrating that half the information
rate is available.
Figure 5.15:
The vertical temporal spectrum of monochrome video due to interlace.
As a consequence of the triangular passband of an interlaced signal, if the best vertical frequency response is to be
obtained, no motion is allowed. It should be clear from Figure 5.16 that a high vertical spatial frequency resulting
from a sharp horizontal edge in the picture is only repeated at frame rate, resulting in an artifact known as
interlace
twitter
. Conversely, to obtain the best temporal response, the vertical resolution must be impaired. Thus interlaced
systems have poor resolution on moving images, in other words their
dynamic resolution
is poor.
