Data size discrepancy

Question

I've been doing some compression tests for a clip destined for the web. I noticed when looking at the finished versions that the file sizes shown in the Movie Info box in QT Pro are different from those shown in the finder. For one file QT Pro shows 2.9MB as opposed to the finder's 2.2MB, for another 2.9 as opposed to 2.1, and for a third QT shows 1.76 as opposed to the finder's 1.8 (not a lot but it's curious). Anybody know why I'm seeing these discrepancies?
thanks-

Answer 1

I've been doing some compression tests for a clip destined for the web. I noticed when looking at the finished versions that the file sizes shown in the Movie Info box in QT Pro are different from those shown in the finder.
Never gave it serious thought before, but since no one else wants to take a shot here, I'll give it a try. There are a number of aspects at work here. Not sure if all file types are the same, but it seemed that some of your numbers may have been reversed in your examples.
Finder File Size:
The finder file size represents the files total potential for storage and includes the actual audio/video data stored, allocated but unused storage, and file overhead which includes header, file type, creator, metadata/tags, etc. In addition, this information is given in both binary uint and binary power (2^10) equivalents where: total bytes = file size in KBs x 1024^1, total bytes = file size in MB x 1024^2, total bytes = file size in GBs x 1024^3, etc. So here we have our first discrepancy.
QT Movie Info File Size:
This entry represents the actual amount of audio/video data contained/stored in the media file. (I.e., the amount of "sequential" data contained as if it were stored in a single dimension or "linear" array.) It does not include file overhead or allocated space not used by the particular packing algorithm empoyed by a spacific codec. File overhead is probably the most "consistent" discrepancy and can probably be considered as a constant with respect to most, if not all, common QT multimedia file types. Storage allocation or packing algorithms, on the other hand, vary widely with the least efficient being the most linear in nature and the most efficient probably best described as multidimensional arrays whose block/superblock allocation may vary as a constant or vary linearily (i.e., in direct proportion to) or as a power of the current file size. Thus, the discrepancy between Finder and QT file size can vary greatly here dependent on the data density of the most recent storage block allocation.
It is likely that there are other considerations which have been overlooked here. However, these are probably the most significant. Hope this helps.