There is no doubt that the signal bandwidth is one of the most
important factors in analog television. The signal bandwidth is measured in
megahertz. Generally speaking, the greater the bandwidth is, the more expensive
and complex the entire production and broadcasting chain are. Many kinds of
digital device are included: cameras, storage systems, broadcast systems and
reception systems.
First of all, interlace provides a fixed bandwidth with a video
signal whose frame rate is twice the display refresh rate for a given line
count. With the higher frame rate, the appearance of objects motion is greatly
improved for the higher rate is able to update their positions on the monitor
more often. If the object is still, then human vision will combine information
from multiple similar half-frames in order to produce the same perceived
resolution as progressive full frames.
Secondly, interlaced video also enables a higher spatial resolution
than progressive scan. For example, 1920×1080 pixel resolution interlaced HDTV
with a 60 Hz field rate has a similar bandwidth to 1280×720 pixel progressive
scan HDTV with a 60 Hz frame rate, but its spatial resolution is approximately
twice the resolution for low-motion scenes.
Even though interlaced video have much advantages, the bandwidth
benefits can only be enjoyed by analog or uncompressed digital video signal. If
the digital video has already been compressed, then interlacing may lead to
some additional inefficiencies no matter what current digital TV standards it
is used in. On the other hand, it has been proved that even though the frame
rate can reach twice the number of the progressive scan, the bandwidth savings
of interlaced video over progressive video is minimal. For instance, when
encoding a "sports-type" scene, 1080p50 signal almost produces the
same bit rate as 1080i50 while it actually requires less bandwidth to be
perceived as subjectively better than its 1080i/25 equivalent.
