In order to explain what a video file format is we first need to first answer two questions:
First Question - What is a Video File?
A file like myvideo.mov, myvideo.wmv or myvideo.mpg is a file that contains video and audio data. It also contains data about the file itself. This is illustrated in the following diagram:
So a video file is a file that contains, a video stream, an audio stream and data about the file itself. The next picture is an actual example of a video file as seen in the Sony Vegas video editing software. Your can see that the file contains one “video data stream” and one “audio data stream”:
A video stream is simply a sequence of video frames and similar for an audio stream. Many file formats support multiple video and audio streams but to keep it simple our file has only one video and one audio stream. The second image below shows some of the other information that is also contained within our media file:
This tells us that our example file is an MPEG-2 file and that it contains 2 streams. The audio stream has been recorded in stereo at 48KHz and lasts 37.28 seconds. The video stream has a frame size of 720×576 and the number of frames per second (fps) is 30. The video stream has the same time duration audio stream.
Second Question - What is a Video Codec?
The video and audio data in your files is encoded (compressed) using a codec. WMV and MPEG-4 are codecs. So in the file above the video and audio data is compressed using the WMV codec which is a Microsoft Windows proprietary codec. When the file is played back using Windows Media Player it is decompressed by the same codec.
Codec stands for Compressor \ Decompressor. Codecs are needed in order to reduce the size of your video files so that they take up less space on your PC’s hard drives. A codec is simply a piece of software that uses a set of mathematical rules to compress your file for storage and decompress it for playback.
Typical codecs used to compress\decompress video files for streaming are:
- WMV
- MPEG-4
- H.264
Finally, What is a Video File Format?
A video file format is simply the way in which video, audio and meta-data is organised in a file. Typical file formats used for streaming video are:
- Quicktime ( .mov files)
- Windows Media ( .wmv files ), and
- Flash Video ( .flv files )
The specific details of each file format differ but the key point to remember is that files created in any of the above formats contain the three items discussed above.