next up previous
Next: Compression Up: Issues in HDTV Previous: Distribution - terrestrial? satellite?

Interlaced versus non-interlaced

The maximum vertical resolution promised by a particular TV system is greater than the actual observed resolution. The reduction in resolution is due to the possibility of a picture element (pixel) falling "in-between" the scanning lines. Measurement gives a effective resolution of about 70% of the maximum resolution (the Kell factor) for progressively scanned (i.e. not interlaced) systems. If the image is interlaced, then the 70% factor only applies if the image is completely stationary. For non-stationary interlaced images this resolution falls to about 50%.

Interlacing also produces serrated edges to moving objects, as well as flicker along horizontal edges (glitter) and misaligned frames. As a consequence of the many problems associated with interlacing, a number of the HDTV proposals are for progressively scanned (not interlaced) service. Notice that these apply both to new ideas for HDTV, and to upgrades of the existing NTSC, PAL and SECAM systems as well. (Although initiating progressive scanning on conventional service does create compatibility problems, some of these techniques offer improved performance to NTSC/PAL/SECAM without the associated problems of moving to "true" HDTV.)

Dave Marshall