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?

Answer 1

Bitrate describes the quality of an audio or video file.

For example, an MP3 audio file that is compressed at 192 Kbps will have a greater dynamic range and may sound slightly more clear than the same audio file compressed at 128 Kbps. This is because more bits are used to represent the audio data for each second of playback.

Similarly, a video file that is compressed at 3000 Kbps will look better than the same file compressed at 1000 Kbps. Just like the quality of an image is measured in resolution, the quality of an audio or video file is measured by the bitrate.

Answer 2

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.