Video Compression

1__ "DV = digital video." (false!)

There's some confusion about these terms. "Digital video" means any format for storing video data digitally. A DVD of "King Kong", a file on YouTube, a video podcast, and your last video shoot are all "digital video".

However, "DV" is a single specific digital video format: it's the format cameras use to record video onto tape. "DV" usually means the miniDV format that most consumer digital video cameras record with. It is a specific codec, just like the many other codecs available in QuickTime. (A codec is a tool for compressing and decompressing video.)

2__ "DV is a lossless codec." (false!)

DV is a lossy codec -- it compresses at 5:1 ratio. This means it throws out four-fifths of the input information. You will often hear people talk about "uncompressed DV", but there is no such thing. (They be trying to say that the DV file has not been compressed a second time, but technically, there is no such thing as "uncompressed DV". DV is by definition a compressed format.)

Your video camera applies the DV compression before it records the image to tape. There is no way to recover the four-fifths of the information that is thrown away. Many people think that "digital" implies "it is perfect!" Unfortunately, DV is not perfect. (Not perfect? I'm shocked! Shocked, I say.)

3__ "QuickTime and AVI are codecs." (false!)

QuickTime and AVI are not codecs, they are "media architectures". They are codec packs, or collections of codecs. They can work with files that are in a variety of different codecs (DV, MPEG2, Sorenson, etc). The media architecture contains the codec you need to play a particular file, but it is not the codec itself.

Here's an example of a nonsense statement: "I prefer AVI because it has smaller file sizes than DV." This is like comparing apples and oranges -- AVI is a media architecture, which can handle various types of codec (including codecs with a file size larger than DV).

4__ "Uncompressed video is always best." (false!)

Uncompressed video is useful for archiving, but the files are so large that they are really not useful for anything else. In fact, many computers cannot even play back uncompressed video without dropping frames.

No common means of distribution (DVD, internet, broadcast, etc) can handle files this large. You could say, uncompressed is best -- if you never actually need to play your video file!

5__ "Compressed video is always best." (false!)

Not quite -- it depends on the type of compression. For archiving, uncompressed or a lossless codec (such as Animation or PNG) is preferable, while for distribution a lossy codec (such as MPEG2 or H264) is preferable.

6__ "If you find a codec that you like, stick with it." (false!)

You may find a codec that works well for a particular situation, but that does not mean it will work well in every situation. Your choice of codec will depend on your particular task at hand -- archiving files, preparing files for DVD, preparing files for internet, etc. Getting good results may also depend on choosing a codec that suits your specific source material and its particular qualities. Is your source very static, very dynamic, noisy, overly detailed, etc? Factors like these can and should affect decisions you make about compression.

7__ "If a file is already compressed with a lossy codec, then compressing it again to a larger size will recover some of the lost information (and result in better image quality)." (false!)

No, no, no -- this is a very bad idea. You should always try to compress ONCE ONLY. Recompressing a second time will only add artifacts, and will not recover what was lost from the first stage of compression, no matter what the final data size of your file. Whatever is lost with lossy compression is lost forever. This strategy of recompressing larger gives you the worst of both worlds: more ugly artifacts, and a larger file size.

8__ "If a file has been compressed already, it's sort of a lost cause, so doesn't matter if I compress it again." (false!)

Ah, the defeatist attitude...don't be lazy! You should always want your video to look as good as it possibly can. Here's how to get the best results: export your final master version as an uncompressed file, or in a lossless codec. Then making several compressed versions from the master, one for each method of distribution you might use: one for DVD, one for broadband internet connection, one for dial-up internet connection, one for presentation directly from a laptop, etc.

Common Misconceptions About Video Compression