Interlaced Video - Benefits of Interlacing

Benefits of Interlacing

One of the most important factors in analog television is signal bandwidth, measured in megahertz. The greater the bandwidth, the more expensive and complex is the entire production and broadcasting chain (cameras, storage systems such as tape recorders or hard disks, broadcast and reception systems such as terrestrial, cable, and satellite transmitters and receivers, or the Internet, and end-user displays such as television sets or computer monitors).

For a given line count and refresh rate, analog interlaced video reduces the signal bandwidth by a factor of two.

Given a fixed bandwidth instead, interlace can provide a video signal with twice the display refresh rate for a given line count (versus progressive scan video at similar frame rate, for instance 1080i at 60 half-frames per second, vs. 1080p at 30 full frames per second). The higher refresh rate improves the portrayal of motion, because objects in motion are captured and their position is updated on the display more often, and when objects are more stationary the human vision combines information from multiple similar half-frames resulting in the same perceived resolution as progressive full frames. This technique is only useful though, if the source material is available in higher refresh rates. Cinema movies are typically recorded at 24fps, and get no real benefit from common interlacing techniques.

Given both a fixed bandwidth and high refresh rate, interlaced video can also be seen as providing a higher spatial resolution than progressive scan. For instance, 1920×1080 pixel resolution interlaced HDTV with a 60 Hz field rate (known as 1080i60 or 1080i/30) has a similar bandwidth to 1280×720 pixel progressive scan HDTV with a 60 Hz frame rate (720p60 or 720p/60), but achieves approximately twice the spatial resolution for low-motion scenes.

However, the bandwidth benefits only apply to analog or uncompressed digital video signal; with digital video compression, as used in all current digital TV standards, interlacing introduces some additional inefficiencies. Tests performed by EBU have shown that the bandwidth savings of interlaced video over progressive video are minimal even with twice the frame rate, i.e. 1080p50 signal produces roughly the same bit rate as 1080i50 (aka 1080i/25) signal, and 1080p50 actually requires less bandwidth to be perceived as subjectively better than its 1080i/25 (1080i50) equivalent when encoding a "sports-type" scene.