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Weird Compression Ratio

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Riddle me this: 2 files, almost the same size. One file contains 16Bit 44.1KHz PCM stereo audio, the other is the same song, but in 44.1KHz 5.1 AC3 Dolby Digital wave format. Using WinRAR's best multimedia (lossless) compression methods, the AC3 wave data compresses to 31% of original size and the PCM wave data compresses to 67% original size. This doesn't seem correct. For instance, the same video uncompressed and compressed with MPEG will yield WinRAR results on the order of 50% original on the uncompressed video and 98% on the MPEG video. AC3 is highly compressed as it is, so why is it the WinRAR can squeeze the file so much more? I did a little sanity checking and verified that the source and compressed files' CRC checksums are the same, and visual inspection of the waveforms show that they are the same. The kicker is that extracting the files from the WinRAR archive, burning them to an audio CD, and playing them through an AC3 decoder indicates no problems with the data at all.
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[b]Please note that the following is speculation only.[/b] The AC3 data might have more redundant information in it than the plain old PCM data. Thus, WinRAR could squeeze the AC3 data down farther relative to its original size. Like I said, it's just speculation. -Danny

Grace, Peace, V, and Hz,



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