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QuickTime & Noise Problems

Q Our application converts movies made from digital film to CVID (CinePak) using Movie_tool_box, but we can't overcome the noise problem when the temporal quality is set to 512 or 1024. We need to compress for time direction. Is there a way to remove the noise?

A The temporal quality should be set to normal (512 or less) for the initial compression value. Sequences with minimal to moderate motion should obtain adequate compression at the 'normal' setting. Sequences with large amounts of motion probably need to have the compression settings adjusted to a setting below normal. If the compression ratio is not adequate at the normal level, it should be set somewhere between low and normal and adjusted up or down until the desired bandwidth result is obtained.

As the temporal quality is decreased, the bandwidth of the sequence will decrease also. However, decreasing the temporal quality increases the number of artifacts (blocks and/or streaks) in the image.

In your case, adding temporal compression is apparently increasing the noise level. The noise you're experiencing is probably the result of pixel level values that change rapidly over time, causing a 'living' background of various pixels that are constantly changing values. This could be due to tape quality, but if you are converting from a pure digital resource to QuickTime, tape quality issues are eliminated.

Make sure that the contrast levels are calibrated, so that black is black, and white is white. The more pixels you have with the same contrast level, the more CinePak will adjust the values to similar or near-similar pixel values, which could cause additional noise.

Noise can also be generated by the background of the video image. For example, a constantly changing background, such as trees or grass moving in the wind, can cause noise due to the constantly changing pixels in the background. The CinePak codec tries to adjust these constantly changing values to something near average, causing pixel flashes.

Adding key frames to reduce the temporal values between the key frames should result in better quality images. As you increase the size of the movie, you also increase the bandwidth needed to load the movie.

You can use a combination of these techniques to deal with the noise, but as you can probably tell, this is a tuning process, so the results you get will vary with the original images, making it difficult to predict your results in all cases.

[May 01 1995]

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