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MillimeterKnow Your Formats, Part 2
Apr 28, 2008 12:00 AM, By Jan Ozer
Figure 1 redux: Formats and features.
Click here for a larger image.
This month, we’re tackling high-definition video formats. In the first segment, I discussed what aspect ratio is and why you care about it. In this issue, I’ll discuss the remaining items of the features table—which, through the marvels of modern digital technology, I’ve duplicated here.
Let me breeze through the next few definitions then circle back and discuss the individual formats. Of course, "codec" refers to the technology used to compress the video. All affordable HD acquisition formats use compression; otherwise the digital files would be too large for convenient on-camera storage. As you can see, there are three codecs used in this class of camera: DCT-based frame compression, MPEG-2, and AVC/H.264.
"GOP structure" refers to techniques used by the codec during compression. Technologies such as MPEG-2 and AVC/H.264 have three frame types to work with: intraframes (also called I-frames), bidirectional (B) frames, and predictive (P) frames. However, as technologies such as AVC-Intra illustrate, camera vendors don’t have to use all three frame types when choosing their formats; they can choose to go I-frame only.
When a vendor uses all three frame types in their format, it’s called a "long-GOP" format, with "GOP" standing for Group of Pictures. Each group of pictures starts with an I-frame and has multiple P and B frames within.
Data rate is the amount of data per second used to store the video stream. All compression technologies are lossy, so the more you squeeze, the more you lose. Higher data rates are generally better, but you can’t discuss data rate without considering codec and GOP structure because they interrelate so significantly. We’ll explore this in more detail below in the XDCAM HD-vs.-DVCPRO HD comparison.
Understand that there are few absolutes in the realm of compression. For example, AVC/H.264 is without question a more efficient compression algorithm than the older MPEG-2 format, so in theory, that makes it better. However, long-GOP AVC/H.264 is currently not supported by the Adobe family of editing products, so you can’t edit it in Premiere Pro or After Effects.
In addition, because long-GOP AVC/H.264 is a more advanced technology, it’s tougher to decompress during editing, so even in the few editors that can edit the format natively—such as prosumer editor Pinnacle Studio—you need a very powerful computer to achieve responsive editing. The other approach, used by Apple in Final Cut Pro, is to convert the AVCHD video into ProRes, which is easy to edit, but requires lots of disk space.
In addition, although the AVCHD spec enables data rates of up to 24Mbps, the highest data rate I’ve seen implemented is about 14Mbps, which lets vendors boost the recording time on their camcorders at the expense of quality. I also haven’t reviewed an AVCHD camcorder that had sufficiently large imagers for pixel to pixel capture, or lenses of anywhere near the quality of the average $4,000 consumer camcorder.
All told, although AVCHD is undoubtedly a better compression technology than MPEG-2, as implemented, it has delivered consistently poorer quality video with significantly fewer editing options. When the first true professional AVCHD camcorders come to market, you’ll have to look at them with a totally fresh eye.
