The Tasks in Standards Conversion

PAL and SECAM can be treated together for computer based standards conversion, as when digitized, the formats are identical, their only differences being in the analogue versions of the formats with the details of how the colour part of the signal is stored.

PAL differs from NTSC in two major areas - the frame rate and the number of horizontal lines that make up the picture (vertical resolution). Although PAL is said to have 623 lines, in digital video only 576 are used (the rest being in the vertical blanking area of the picture and are never displayed on TV). Similarly NTSC only uses 486 or 480. PAL is exactly 25 frames per second (fps), where each frame is made of two interlaced fields. NTSC is 29.97fps and also has two interlaced fields per frame.

The first task in standards conversion is to enlarge an NTSC field to the size of a PAL field, or shrink a PAL field to the size of an NTSC field.

The second task is to generate new frames and fields to increase the frame rate of PAL from 25fps to 29.97fps or remove frames and fields from NTSC's 29.97fps to produce 25fps

Film Conversion

Film is converted to video by different methods, depending on the frame rate (25 or 29.97) of the video format. For more details, read through the Film Effects Technical Info.

PAL

PAL (25fps) is normally converted to Film (24fps) by slowing down the video by about 4% to make it run at 24fps, and printing each frame of video to a frame of film. The reverse conversion is done by speeding up the film about 4% and matching so that it's now running at 25fp and printing each frame of film to video. these methods provide a very high standard of conversion, but the 4% speed up or down means that the audio must also be adjusted to match, and the running time of the finished program will be slightly different. This has often caused confusion to PAL movie viewers who thought that their DVD or VHS copy of the movie was cut by a few minutes compared to the cinema version. This was not the case - it's just that the movie runs 4% fast on video because of the conversion method.

Another method of converting PAL to and from film involves no speed up, but uses the addition or subtraction of 1 frame every second to make up for the 24fps to 25fps difference. This method preserved timings, but will introduce a "jump" in the video every second when the extra frame is added in as a duplication of the previous frame. On talking heads shots, this is barely noticeable, but on action it looks really bad. A variant on this adds or subtracts 1 field every half second. This involves more complicated processing to make sure the field ordering remains correct, but produces a much smoother result that is very acceptable.

NTSC

Film is converted to NTSC by the action of 3:2 pulldown. See Film Effects Technical Info for full information on this method. 3:2 pulldown can also be removed from a video telecine of film material. Apple's Cinema Tools includes a full set of features for doing this.

NTSC video can also be converted by removing 3:2 pulldown using the methods as used in Film Effects. This is useful for incorporating NTSC video in a film based project.

Cameras that shoot 24p often give the user two different modes - 24pAdvanced and 24pNormal. The 24pNormal is that which is described above using 3:2 pulldown. Advanced pulldown uses a different pattern so that the computer can extract the original film frames without having to decode the video, thus giving slight increase in quality for a native workflow. However, 24pAdvanced viewed as 29.97 without the pulldown being removed does not look as smooth as 24pNormal.