Manic compression
Table of Contents
- Introduction
- Types of compression
- QuickTime’s Animation codec
- Flash Video
- Adobe Media Encoder
- Compression for iPod
- Conclusion
Types of compression
There are two kinds of video compression—lossless and lossy. Lossless compression lowers the amount of data in the video without any visible quality loss (although I know quite a few television engineers who will debate this point vigorously). Lossy compression degrades the video to some extent, sometimes noticeably, sometimes not. It all depends on the codec used, and the settings of that particular codec.
Codec stands for compressor/decompressor. There are hardware-based codecs (like the codec used in a Digibeta camera) and software-based codecs (like the On2 VP6 codec used in Flash 8 Video Encoder). The video on DVDs is compressed with the MPEG-2 codec, which is also used for satellite and digital cable transmission (you know how the picture falls apart a bit on some of those esoteric channels on your digital cable? That's MPEG-2).
Both QuickTime and Windows Media include many codecs, and knowing which one to use when is a fundamental in digital video postproduction.
Let's take a look at the most common workflow for digital video. A MiniDV camcorder records a compressed video signal to tape with its internal lossy DV codec. DV compression is 5:1, meaning for every five bits of data, only one is written to tape; the other four are discarded. The resulting loss in quality is not generally noticeable to the untrained eye, but for those of us who do this for a living, the loss of color range, digital artifacts, and noise in the compressed video is easy to recognize.
When you capture your tape to hard disk using Adobe® Premiere® Pro or other software, the video becomes an AVI (Windows Media) or QuickTime file written with its DV codec.
Figure 1: The AVI Video Compression dialog box in Adobe After Effects®, showing the Microsoft DV Codec selected in a pop-up menu showing all the AVI codecs on my system.
Essentially, the compressed DV data is copied from videotape to hard drive via a Firewire cable, with no further compression taking place. If all you were to do is edit the video without rendering anything and lay it back to tape, it wouldn’t be compressed further.
But, if you render a DV codec clip with the DV codec, it will be compressed again (5:1 lossy compression). That means you take something where four bits of every five have already been discarded, and then discard four bits out of five of that remaining one bit. In other words, you wind up with something that looks like it went through a meat grinder. It's not pretty.
So how to conquer this? Render uncompressed, or even better, to a lossless codec. Select No Compression (see Figure 1) and your rendered video will suffer no quality loss. You will, however, wind up with a huge file, so you'll need to have plenty of hard disk space if you're doing lots of renders. A better solution is to render using the Animation Codec, which is part of QuickTime.
