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blutach 7 Jan 2005 07:43 AM

FAQ: Compression percentage explained
Compression percentage explained

Many people are asking about the compression percentage and what results in a good quality burn.

Put simply, it is not the compression percentage which matters but the bitrate which is devoted to the video encoding.

For example, if a movie was originally encoded with a "high" bit rate (say 6 Mbps - "mega bits per second") and compressed by a lot, (say to 50%), on average, this would result in a bitrate of 3 MBps.

But if the movie was originally encoded with a "low" bitrate (say 3.5 Mbps) and was compressed only by 20% (to 80%), the resulting average bitrate would be 2.8 Mbps -- worse than the 50% compression!!!

(There are more technical aspects regarding quality that involve I, P, and B frames, and variable vs fixed encoding and transcoding, but these are beyond this FAQ.)

Now, what you can detect as "bad quality" will most likely depend on the final bitrate and the size of your screen. If your are projecting onto a huge screen, the higher numbers are probably necessary (say over 3.5 and preferably 4.5+). But on a regular TV, you can probably get away with the 3 Mbps range (and even lower).

IfoEdit has a neat bitrate calculator in its Tools menu. First, take the total amount of space devoted to your title - DVD Shrink will tell you this in its main screen (it may be 4,464MB for a reauthored DVD). Then, simply enter in the number of audio streams and subtitle streams. If you have a DTS stream add one to the number for audio. IfoEdit will calculate the average bitrate based on this data.

Videocalc has an even better freeware bitrate calculator, which allows you to calcuate, very accurately, the average bitrate of your DVD. Try it out! :)


jmet 22 Feb 2005 04:05 PM

This thread has been closed/locked, as it is for information purposes only. If you have a question regarding this thread. Please start a new thread in the DVD Shrink - General Discussions sub-forum.

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