As I mentioned before, bit rate is how much data is sent each time period (usually measured in seconds). The higher the bit rate, the more information is sent. With more data, there are fewer artifacts.
So what’s too high a bit rate?It really depends on the footage. When you burrow deep down, it all depends on what percentage of all the frame’s pixels are changing and how many of them look similar to each other.
But that answer really doesn’t help. What you really have to do is view your program and evaluate how it looks. Even then it can be difficult to see where the problems might be.
Some situations where compression struggles are when all the pixels change every frame over a period of time, for example, a dissolve. If you use a 30-frame dissolve to dissolve between different scenes, every pixel changes at each of those 30 frames.
What about a cut?
Cuts can be a problem. You might see artifacts right after a cut if you step frame by frame through the compressed file. However, there are two reasons why a cut might not be a problem.
- First, compression bit rate is often specified in average bit rate and maximum bit rate. If a particular frame is difficult to compress and still looks good but the frame before it is simple, the compressor might use more bits over time on the later frame and less on the first. This may help with a cut transition.
- Secondly, compression systems take advantage of the fact that our eyes find it more difficult to pick up details in moving images. Although you may see artifacts when stepping frame by frame through the scene, if you watch in real time, the artifacts are usually hidden by the wholesale change from one scene to the other. Our eyes just don’t pick it up.
The fact that our vision system gets fooled is one reason why motion compression can work so well. If you were to watch everything frame by frame, you would be surprised by the number of artifacts.
Next time, I’ll show you how your edit software can indicate where compression is affecting your video.
Michael Guncheon is an accomplished editor who has cut a wide range of projects, including music videos for Prince, a documentary on Genesis, and numerous commercials and corporate pieces. A partner at HDMG, a Minneapolis video production and post-production company, Guncheon has written several books on DSLRs and is the author of the Kodak Digital Photo Guide. He has presented his talk on shooting with HDSLRs at Twin Cities Public Television, WGBH in Boston, PBS in New York, the Hollywood Post Alliance and at the annual SMPTE conference in Hollywood.