This Audio Coding system, also known as Dolby AC-3, allows audio to be compressed, to reduce the bitrate and the storage requirement.  Dolby Digital  is supported by all NTSC DVD players (DVD players sold in regions of the world such as the US and Japan that utilize the NTSC television standard).  By utilizing AC-3 or a similar audio compression system, the bitrate of the video stream can be increased, allowing for greater video quality.  Although the audio quality is slightly reduced, the difference is not significant, and the benefits in improved video quality will usually make the trade-off worthwhile.