Join Jeff I. Greenberg for an in-depth discussion in this video Learning compression terms like CBR and VBR, part of Compressor 3.5 Essential Training.
When anybody ever talks about compression, there are a couple of key terms that you just have to know beyond things like transcoding and encoding. The first term is codec. The word codec is actually a made-up word. It's made up from the word compression, the first two letters, and decompression, d-e-c. So that's how we get the word codec, and it really talks about what kind of file you're handling. It's not enough to say you're handling a QuickTime file. You have to tell me the codec and I really think of it as like a flavor.
When we talk about compression in general, be it video or any other sort of compression, there are really two flavors: lossless compression and lossy. You probably use lossless compression quite a bit in your lives. Anytime that you take a file when you compress it to send to somebody else, say a Microsoft Word file, you're zipping up that file. That file was a lossless compression. It would be awful if you sent that file to somebody and it didn't have all the letters, or it didn't have all the vowels. On the other hand, a lot of our video is being used in what's called a lossy compression.
We're willing to throw out and lose information. When you deal with JPEGs, you're dealing with a codec from the Joint Photographic Experts Group, JPEG, that specifically is meant to throw out stuff your eye can't see. An example of some compressions that we use for video might be things like DVCPRO-25 or ProRes or MPEG-2. They are all in their ways lossy compression. When we talk about compression, the other big issue we need to talk about is whether it's got a constant bit rate or it's got a variable bit rate.
A lot of the codecs have a constant bit rate and I'm just going to bring up some settings here so you can see this. For example, I'll go to the Animation codec. The Animation codec has a data rate we can't even touch. It's a preset data. It's using the same amount of compression or no compression on every single frame. That's what makes it a constant bit rate. This constant bit rate, the actual frame, is called an intraframe.
It begins with the letter I. On the other hand, codecs like MPEG-2, these very lossy codecs, they're using what's known as a variable bit rate. A variable bit rate allows me to vary my work. It sits back and it does an analysis. Instead of putting all the information on every frame, it puts all the information on one frame. It actually has an intraframe and then a series of the frames are just the changes.
If you would imagine me standing against a green wall or some solid color, right now it would send you the first frame of me standing there and then just the changes as my jaw moves. This variable bit rate allows it to get a higher and less-- I'd like to think of it as robbing Peter to pay Paul, to improve your compression. The terms for this change, for MPEGs we call them B and P-frames, these changing frames. They're what's known as delta frames. You can see this idea here of we have an I-frame, a full frame, followed by some bidirectional B-frames, and actually look back at the I-frame, being followed by predictive frames that look all the way back at the prior I-frame.
You don't have to be a super genius with these things. You just need to be aware that we use VBR for a lot of our compressed files that are going for distribution, and we tend to use CBR, a constant bit rate, for our files that are meant for editing.
- Learning the basic rules of compression Understanding transcoding and completing an HD cross-convert Building DVDs directly from the Compressor interface Adding chapters and art to a podcast with Compressor Building batch templates and creating job actions
Skill Level Beginner
Q: Can Compressor reduce a QuickTime movie file to the smaller MP4 or FLV format?
A: Technically, Compressor (or any other compression software) can reduce the files as much as you’d like. However, depending on the codec, data rate, and content of the files, the image quality will suffer. If the files are a slideshow with mostly static images, you can likely make great reductions in size. If the file contains more dynamic video, quality will suffer if you try to keep the size small..