11. Do all videos use Dolby Digital AC-3 with 5.1 channels ?
Dolby Digital soundtracks are very popular with many commercial DVD-Video releases. However, the DVD specification does not require that the soundtrack have to be Dolby Digital. There are other formats such as MPEG audio, PCM audio, and DTS. Dolby Digital, DTS, and MPEG audio record audio samples in compressed format while PCM audio samples are uncompressed.
The full name of Dolby Digital or AC-3 is Dolby Digital 5.1. The name describes the sound distribution on six channels in space. All the six channels are compressed into one stereo file, which is unpacked with a special decoder and divided into six audio columns. According to the AC-3 format the sound is heard from five wide-band columns: left, right, rear left, rear right, central and from one narrow band, which is used for low frequency effects (LFE) only. This LFE effect is better felt than heard in the movie theatres. The number 5.1 indicates five general channels and one LFE channel. Dolby Digital is a great step forward comparing to Dolby Surround, concerning perception of the spatial sound. The following illustration shows an ideal setting to enjoy Dolby Digital soundtracks.

The Dolby Digital format supports soundtracks in a variety of formats such as mono, dual mono, stereo, Dolby Surround stereo, Dolby Surround 5.1 (also commonly known as AC-3). DVDs with DTS audio soundtracks are required to include a Dolby Digital audio track. Each DVD disc can have up to 8 different soundtracks. Commercial NTSC DVD-Video usually contains the first soundtrack in Dolby Surround 5.1 and the secondary soundtrack in Dolby Digital Stereo.

Some DVD packaging has small letters or icons under the Dolby Digital logo that indicates the channel configuration. In many cases, there are often more than one Dolby Digital version of a soundtrack: a 5.1-channel track, and a track remixed for stereo Dolby Surround. Your DVD player may indicate the playback of a Dolby Digital audio track while your receiver indicates Dolby Surround. This simply means that the DVD disc contains a two-channel Dolby Surround signal encoded in Dolby Digital format.