The surround decoder is the heart of every home theatre sound system. It processes the sound in the same way as it is done in cinemas, i.e. it supplies the front channels and the surround channels with the correct signals in each case. Different Dolby multichannel sound formats are shown below.
Dolby Pro Logic
The analogue surround technology uses four audio tracks as source material, namely left, centre, right and surround. Left and right are fed to the stereo tracks unchanged, while the centre signal is distributed equally to both tracks - adjusted in level so that the volume of the three front speakers harmonizes. Finally, the surround signal is mixed to one side normally, to the other offset by exactly one vibration phase (180 degrees). This means that in stereo playback, when the speaker on the left swings in, the one on the right swings out, and vice versa. The same effect can be achieved if the polarity of the speakers is reversed, i.e. plus and minus are reversed on one side.
Sounds that should come from both speakers equally then no longer sound localizable, but diffuse in the room. If surround-encoded audio material is played back without a decoder, exactly this effect results for the surround components: they sound diffuse, which is very desirable. With mono playback, however, the exact opposite impulses cancel each other out again, and the surrounding components then disappear.
In order to be allowed to carry the Dolby logo, the decoders still have to fulfil a number of special regulations that are supposed to ensure that the system works perfectly.
Dolby Pro Logic II
Dolby Pro-Logic II is a technology that generates 5.1 channels from the conventional Dolby Pro-Logic signal encoded in the stereo channels.
From all the Dolby Surround software offered to date, a Pro-Logic II decoder can generate the 5.1 channels. Especially because of the stereo surround channels and the full hi-fi frequency response on all channels, Dolby Pro-Logic II creates a more realistic all-around atmosphere.
But even when playing a standard stereo recording, you can prevent too much concentration on the centre channel by using the control logic of the new Music mode. This solves the problem with unnatural effects that used to occur when playing stereo sources through multi-channel systems. In Music mode, even normal stereo sources can be played back with a 5.1-channel surround sound.
Dolby Digital is the surround system of the digital world, i.e. what Dolby ProLogic or Dolby Surround is on the analogue side - a method for storing and transmitting multi-channel sound. At the same time, Dolby Digital is also a method for reducing the data volume of digital sounds. Similar to Dolby Digital works another method DTS, which practically makes the same demands on the Dolby Surround system.
Advantages of Dolby Digital:
- perfect channel separation
- the stereo is also in the rear
- full bandwidth on all channels
- additional bass channel
In summary, these advantages lead to improved spatiality, more impressive effects and even more realistic sound reproduction.
Digital multi-channel systems always work with independent ("discrete") channels, so that the audio tracks cannot influence each other. The only exception: If sounds occur equally on several channels, data reduction methods may omit this sound where it is quieter. This is because it is assumed that the human ear perceives a sound only from the direction in which it is dominant.
Digital sound systems can usually handle one to six channels. In contrast to analogue surround, two separate channels are then provided at the rear; in addition, there is another channel for low-frequency effects (low-frequency effects channel, LFE), which supplies subwoofers or woofers.
Therefore, the channel configuration is designated with the number of channels and a .1 behind it; common for movies is usually 5.1, which is also the maximum for Dolby Digital, MPEG-2 can up to 7.1, DTS in various versions also at home maximum eight. The rear centre channel used in 6.1 and 7.1 systems (Surround EX, DTS ES) is generated from the stereo tracks of the effect channels and does not have its own discrete audio track.
Dolby True HD
Dolby TrueHD was developed by Dolby Laboratories specifically for HD DVDs and Blu-ray Discs as a lossless multi-channel sound format. The competing format is DTS-HD Master Audio. Meanwhile, game soundtracks also use this format.
The format supports up to eight channels on HD-DVD and Blu-ray Disc at 96 kHz sampling frequency, and up to 6 channels at 192 kHz, each with a 24-bit word width. The maximum data transfer rate is 18,432 kbit/s. The format offers the possibility to reproduce film soundtracks in a particularly high dynamic range with a wide frequency range (20 - 20000 Hz) and the finest resolution.
The Dolby Atmos format allows an unlimited number of soundtracks. The sound objects are stored digitally with coordinates for the position in space, volume and other parameters. At the respective point in time in the film, the corresponding sound object is then played. The sound objects in-game soundtracks are stored in a similar way. For example, a sound object can be triggered several times.
Home theatre systems with Dolby Atmos or cinemas with Dolby Atmos have so-called ceiling speakers, where sound effects can also come from above.
Dolby Atmos is backwards compatible with older systems such as 5.1 or 7.1. The first generation, the Dolby Atmos Cinema Processor, supports up to 128 individual sound objects and up to 64 separate output signals. Dolby Atmos can assign a single sound object to each speaker.