Spatial Preprocessing - A Practical Guide to Video and Audio Compression - page 556

525 on 625 line raster

Widths scaled to preserve aspect ratio

Figure 34-5 525 lines on 625-line format.

The second quite common example is when 405-line UK transmissions are being dis-
played on 625-line TV (or being shown in the United States). The output quality will be
better if the picture size is reduced so that it occupies a 405-line window in the middle of
the 525-line raster rather than scaling it to fit. Over-scanning of TV sets will account for
some of the margin around the edges. The relative sizes of these margins are shown in
Figure 34-6.
This leaves an unacceptably large blanked region around the picture when it is
broadcast in Europe, so 405-line TV is often scaled up to fit 625 lines. Otherwise the mar-
gin around the image is probably too wide. The 1:1 scaling is too small and 1:2 is too large.
So a non-integer scaling factor is used. This yields some very nasty line-based artifacts.
Oddly enough, 405 would probably look better on HDTV because a 1:2 scaling in the ver-
tical axis would be feasible.
The third case is when you are just enlarging an area of picture. Obviously, non-inte-
ger scaling factors will be problematic.

405 on 625 line raster

405 on 525 line raster

Figure 34-6 405 lines on 525 and 625 rasters.