What exactly does bitrate mean in an video/audio file?

Question

I use ffmpeg to convert videos from one format to another. Is bitrate the only parameter which decides the output size of a video/audio file?

Solution

Yes, bitrate is essentially what will control the file size (for a given playback duration). It is the number of bits used to represent each second of material.

However, there are some subtleties, e.g. :

• a video file encoded at a certain video bitrate probably contains a separate audio stream, with a separately-specified bitrate
• most file formats will contain some metadata that won't be counted towards the basic video stream bitrate
• sometimes the algorithm will not actually aim to achieve the specified bitrate - for example, using the CRF factor. http://trac.ffmpeg.org/wiki/x264EncodingGuide explains how two-pass would be preferred if targeting a specific file size.

So you may want to do a little experimenting with a particular set of options for a particular file format.

Comments

[Нет войне]

It does depend on the algorithm used and the material, though. Some encoded audio may not sound any clearer at 192 kbps, and one video algorithm may produce a 1000 Kbps file of better perceived quality than another algorithm rendering at 3000 Kbps.

[joey]

Agreed, the material, i.e. content of the video matters a lot. For example, a video with a lot of motion will be perceived differently depending on the algorithm.