Image Processing Reference
Comparing NTSC with PAL (60 Hz Versus 50 Hz)
To convert between NTSC and PAL services, the change in scan lines and the horizontal res-
olution is usually dealt with by simple interpolation in the X- and Y-axes. The frame rate is
much harder to process because simply removing frames when going from 30 to 25 Hz leads
to a somewhat jerky experience. Going the other way, from 25 to 30 Hz, requires the intro-
duction of extra frames. Working with fields rather than frames does not solve the problem.
Pulldown techniques that work for film do not work very well for video because the inter-
lace scanning fields are not photographed at an identical moment in time. Therefore they can-
not be averaged or repeated without some strange combing effects being introduced.
Duplicating a single field is unsatisfactory because it has a spatial and temporal relationship
to the adjacent fields before and after. Figure 5-1 shows these field-timing differences.
Digital systems with multiple frame stores provide the means to carry out the nec-
essary temporal interpolation. Very good time-base conversion is possible with desktop
tools such as Adobe After Effects and Apple Final Cut Pro. There are of course other alter-
natives to these, depending on the platform you use and the budget you have available.
American TV transmissions run at approximately 30 frames per second—the actual frame
rate is 29.97 frames per second. Because interlacing is used, this is really 60 fields per sec-
ond. In the UK, Europe, and some other parts of the world, the values of 25 frames and 50
fields per second are applicable. Mixing content from both systems, or playing American
content back in the UK and vice versa, causes problems.
Resampling the 2D raster image is simply a case of averaging or interpolating adja-
cent pixel values according to whether you are scaling down or up. Changing the frame
rate is much more complicated because you must interpolate movement from one frame
to the next and it is not happening uniformly across the picture.
Interlaced content is even more complex because pixels only correspond to one
another in alternate fields, and adjacent fields are offset by the field time (one fiftieth or
sixtieth of a second).
Thankfully you don't have to worry about this too much as there is now some very
good software that makes changes to the frame rate. Products such as Final Cut Pro and
60 Hz fields
O E O E O E O E O E O E O E O E O E O E O E O E O E O E O E
50 Hz fields
Figure 5-1 Frame rate relationships.