3) Dolby Surround, Dolby Digital - what is that?

Dolby Surround, or Dolby Pro-Logic, is a four-channel analogue technique which became established in the 1970s, firstly in cinemas, then in the home. It requires only two audio channels to store the information. Center and rear channels are added to the two front channels in coded form and contain neither low nor very high frequencies. For reproduction, Dolby Pro-Logic requires a corresponding decoder and merely a four-channel surround amplifier because both rear speakers carry the same signals. The feature films made in the 1970s and 1980s can also be received in Dolby Surround via a television.

Dolby Digital, or AC 3, on the other hand is a real five or six-channel technique (also known as 5.1) which only became popular in the 1990s. All five surround channels are completely independent of each other and also contain the lowest and highest frequencies. Therefore, Dolby Digital places higher demands on the center and rear speakers. The sixth channel contains only the very lowest frequencies and is connected to the digital surround amplifier via a separate subwoofer output. However, this channel can also be wired into the front channels or all five surround channels.

The five-channel "dts" delivers similar results to AC 3; however, it requires a different or additional decoder. Up to now, digital surround sound has only been available from digital versatile discs (DVD) or the German television broadcaster Pro7.