Audio formats for the General Public
Dolby Digital is a process of digital encoding and reproduction allowing for the encoding of the six channels necessary for Home Cinema in one sole digital stream. The first major difference with Dolby Surround is that Dolby Digital only exists in the form of a digital stream, whereas Dolby Surround can in fact be encoded on a normal stereo audio recording. This procedure consequently requires a digital support. This is why it is not present on video cassettes, cable TV or analogue satellite transmissions. We do however find it on Laserdiscs, DVD’s, and shortly on digital cable and satellite. The DVD or Laserdisc player should, however, be equipped with left and right analogue audio outputs as well as the digital output. This is the case of all DVD players, but not the case of all Laserdisc players.
Second difference: the six channels are the same as those found after decoding and filtering in a Dolby Surround ProLogic amplifier (right, left, centre, Surround right, Surround left and bass), but they are not obtained in the same manner. In Dolby Surround, the only channel that is really encoded is the Surround channel, which feeds the rear speakers. The two others (centre and bass), are not encoded on the recording, and are only the result of filtering and extraction done more or less well effectuated by the amplifier.
Nowadays, the mixing of a film soundtrack is entirely done in digital format. To make Dolby Digital, we simply recover the six digital channels mixed in the studio, and thanks to a mathematical operation called multiplexing, it is possible to bring them together in one sole digital stream. In addition, as the data is digital, it is possible to realise another mathematical operation consisting of compressing this data in order to take up the least space possible on the support. This operation of compression calls upon very powerful mathematical algorithms allowing the Dolby signal to take up 10 to 12 times less space than if it had not been compressed. In all, two mathematical operations allow for the reproduction of the six original channels without filtering or extraction. The extraordinary advantage of Dolby Digital remains particularly in the fact that all the channels, except those reserved for low frequencies, operate while retaining the integrity of the sound spectrum. The two Surround speakers now each possess a channel resituating high fidelity sound, with the bass channel operating between 20Hz and 200Hz. This encoding and restitution procedure is called 5.1, because we indeed have 5 full channels, and 1 bass channel with a reduced bandwidth. This sixth channel, of which the bandwidth is limited, is set apart and called .1.