Comparison of Video Codecs - Video Quality

Video Quality

The quality the codec can achieve is heavily based on the compression format the codec uses. A codec is not a format, and there can be multiple codecs that implement the same compression specification – for example, MPEG-1 codecs typically do not achieve quality/size ratio comparable to codecs that implement the more modern H.264 specification. But quality/size ratio of output produced by different implementations of the same specification can vary, too.

Prior to comparing codec video quality, it is important to understand that every codec can give a varying degree of quality for a given set of frames within a video sequence. Numerous factors play a role in this variability. First, all codecs have a bitrate control mechanism which is responsible for determining the bitrate and quality on a per-frame basis. A difference between variable bit rate (VBR) and constant bit rate (CBR) creates a trade-off between a consistent quality over all frames, and a more constant bitrate, which is required for some applications. Second, some codecs differentiate between different types of frames such as key frames and non-key frames, differing in their importance to overall visual quality and the extent to which they can be compressed. Third, quality depends on prefiltrations, that is included on all present-day codecs. Other factors can also come into play.

For a sufficiently long clip, it is possible to select sequences which have suffered little from the compression and sequences which have suffered heavily, especially if CBR was used, in which the quality between frames can vary highly due to different amounts of compression needed to achieve a constant bitrate. So, in any one long clip such as a full length movie, any two codecs may perform quite differently on a particular sequence from the clip, while the codecs may be approximately equal (or the situation reversed) in quality over a wider sequence of frames. Press-releases and amateur forums sometimes select sequences known to favor a particular codec or style of rate control in reviews.