Thursday, January 22, 2015

Benefits of Interlacing

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.