NTSC to PAL Conversion After-Effects
NTSC and PAL are the two dominant formats for modern video production. NTSC is the format for North America and Japan, whereas PAL is the standard in Europe and the Pacific. The biggest difference between the two is in speed: NTSC runs at 29.97 FPS (Frames Per Second). PAL, however, runs at 25 FPS, closer to the cinema film standard of 24 FPS. Incautious conversion between these formats can cause several problems.
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Audio Sync Problems
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When 29.97 FPS NTSC video is converted to 25 FPS PAL, pictures will display at a slightly slower rate. This difference is too slight to be noticed by the untrained eye, but over time, the disparity will grow until speech and sound effects are played out of synchronization with the picture. This can be avoided during video conversion if a time-distortion effect is added to the audio track. The effect will force the audio track to conform to the length of the video track.
Frame Judder
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This effect occurs when the software converting 29.97 FPS NTSC video to 25 FPS PAL video is not instructed on how to handle the extra frames. Instead of flowing smoothly from frame to frame, some frames will double-up or flash out of sync onscreen. This can be solved by running a de-interlacing filter. Video images are made up of thin lines, interpolated so fast that the eye sees them as a whole image. Applying the process of de-interlacing to footage converts these lines into stable images free from judder.
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"Mouse-Toothing"
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A side-effect of improper de-interlacing, "mouse-toothing" refers to patterns that sometimes form in fast-moving footage. The jagged effect can be seen on the edges of moving onscreen objects. The term comes from the effect's resemblance to a very small set of tooth-marks. Mouse-toothing can be avoided by running a "blend" de-interlace procedure rather than the more simple "pull-down" procedure. This procedure is available in professional video software such as Final Cut Pro and Adobe Premiere, but may not be available in programs geared toward the amateur editor.
Color Loss
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PAL and NTSC devices operate at different electronic cycle speeds. PAL machines operate at 25 Hz (Hertz), whereas NTSC devices operate at 29.97 Hz. Many hardware-based forms of conversion do not account for this difference. The result is that basic image information is retained, but the color channels are not transmitted correctly, resulting in colored NTSC footage becoming black-and-white PAL video. This problem can be avoided if you don't use hardware-based conversion tools.
Image Distortion
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PAL and NTSC footage differ in their aspect ratio (visual dimensions) with the standard size for PAL digital video 720 x 576 pixels while NTSC is 720 x 480 pixels. If this difference is not accounted for, NTSC video will appear stretched when converted to PAL. Compensate for this distortion by using conversion equipment that allows you to modify the aspect ratio during the conversion process.
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References
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