Whether you’re a professional video editor or an amateur that occasionally uses iMovie or Windows Movie Maker for home movies, you’ve probably noticed the bit rate setting during the process of exporting your video. What does bit rate mean in video? How do we know what the right setting is for a particular video?
What is Bit Rate?
A digital video (in its original state) is really a string of still images that is tied together in a single video file. In a lossless video format, these files can be extremely big, taking up gigabytes of space for just a few minutes of video. Because of this, lossy codecs have been developed that turn these giant files into much smaller, manageable ones. They do this by taking out data that remains constant from each frame, and only storing data that actually changes. This allows videos with still backgrounds to be much smaller than they would be if the background had to be present in every single frame.
The side effect of video compression is in a drop in quality. What is kept still in a picture might not look exactly the same way once compressed, especially if each frame shares a portion of the overall bit rate. Setting the bit rate too low means that each frame has a restricted amount of space to work with. The result is a more pixelated and distorted image. A bit rate that has been set too high will take up a lot of space on your hard drive, and make uploading the video to YouTube a much longer process.
In short, the bit rate is the amount of bits that can be used in one second of video. If your video has 30 frames per second, it will require a higher bit rate than one shot at 24.
What Should the Bit Rate Be?
Determining the right bit rate to set your edited video at can be tricky. You need to balance space requirements with quality, and I have yet to meet a team of video editors that agrees on what this magical number should be. There are some variables to take into account when deciding on a proper setting:
- Source Bit Rate: What bit rate is the source file recorded at? Exceeding this number doesn’t help the quality, and will only increase the size of the file. While some codecs may differ, the general rule of thumb is to keep the bit rate at or below the source.
- Size of the Video: Are you exporting a video at 720p or 320p? The larger a video is, the greater the bit rate needs to be in order to maintain picture quality involving more pixels.
- Codec Used: Some video codecs offer better quality at lower compressions. Depending on the codec, you might be able to get away with reducing the bit rate without experiencing a great amount of perceived quality loss over another codec. Because there are so many variations on even the most popular of encoding systems and codecs, determining the best bit rate for your video can be a matter of trial and error.
- Where You’re Uploading: YouTube takes video that you upload and converts it into several different size and bit rate formats that it supports. Because of this, you want to be mindful of the bit rate YouTube uses when deciding what to encode your video to. For example, YouTube uses a variable bit rate averaging at 2 Mbps for its 720p video stream. Because its compression systems are so fine tuned, its 2 Mbps 720p video may look better than the one you come out with. It’s a good idea here to encode your video above YouTube’s for this very reason. For videos on LockerGnome, many of them are presently encoded at 5 Mbps or above.
- Amount of Movement: The more movement present in a video, the more bits are required to maintain image quality. A single speaker talking to a camera mounted on tripod while standing against a still background makes compressing a file down much easier than an action sequence that takes place in a park filled with trees on a breezy day. Everything that moves in your video requires extra bits to handle. Setting your bit rate too low may result in terrible pixelation and even the appearance of objects that appear to split themselves as they move across the screen.
Over all, finding the right bit rate is a matter of trial and error for most video editors. There is no golden rule or magical number to set, and anyone that tells you otherwise is doing so on the assumption that all compression software and codecs work the same. Sadly, they really don’t.