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Dolby Surround

Dolby Surround Sound

A surround sound decoder is central to any home cinema system. This is where the sound signals are processed for use by the system’s multiple speakers.

With surround sound formats, it is important to note that there are two different types: Analogue and digital. 

Analogue systems

always use Dolby Surround coding for movie soundtracks or music. Dolby Surround was the very first true surround format to become popular with theaters and uses four channels: left, right, centre and surround (the signals for the rear effect speakers). A later development of Dolby Surround is the ProLogic norm which has recently been even further enhanced in the Dolby Pro Logic II format.

Digital surround systems are available in two basic varieties on the market, namely Dolby Digital and DTS. As opposed to analogue systems, they each use completely different coding so that each decoder is adapted for one specific software. Digital surround systems are also called discrete surround systems, because each audio channel is separately encoded.  

Dolby Pro Logic

This analogue surround technology uses four sound channels: Left, right, centre and surround channels. In the left and right channels, the stereo tracks are played back without alteration and the centre signal is comprised of the even distribution of the left and right channels. The levels are adjusted so that the volume from the three front speakers harmonizes without any one speaker overwhelming the others. The surround signal is produced using phase shift technologies. This allows the sound produced by the rear speakers to sound more diffuse, as background noise naturally would.

This principle was not pioneered by Dolby - it already existed as "quadraphonic“ sound. This simply refers to a system that uses four independent signals. Really any manufacturer can implement quadraphonic sound technology in the production of movies and music as well in audio hardware. In order to use the Dolby Logo, however, a decoder must fulfill a series of specifications that ensure an optimal acoustic result.

Originally, there were home cinema systems available with decoders that only bore the logo "Dolby Surround.“ These were passive decoders since the signal only passed through simple switches which divided it into 3 parts – left, right and surround. The centre signal was not separately generated. At the present time, however, one can only find “Dolby Pro Logic” decoders that reproduce what Dolby cinema sound already used: The processing of the signal for better channel separation. This prevents, for instance, dialogue from emanating from the surround channels - a very distracting sound effect, especially when the characters producing the dialogue can be seen in front of the viewer on the screen. This active switch appraises the so-called dominant sound direction and ascertains which channel produces the highest levels and amplifies this further. Pro Logic decoders also contain certain control elements such as the ability to adjust levels, make adjustments according to speaker size and to adjust the delay interval according to the room size. In this way, Pro Logic decoders are equipped for just about any eventuality.

Dolby Pro Logic II

Dolby Pro Logic II is a new technology that encodes 5.1 surround sound from traditional stereo signals. A Pro Logic II decoder is able to produce 5.1 channels from any of the Dolby Surround software available today. Dolby Pro Logic II can take all traditional stereo content from music to films and games and create a realistic, surround sound image thereby eliminating the unnatural effects that otherwise spoil the playback of stereo sources on multichannel systems. In the music mode, even traditional stereo recordings can be played back as 5.1 surround sound.

Software for analogue decoders

As we have seen, surround sound can even be generated from traditional stereo recordings based on the audio cues regarding space inherent in these recordings. Only one type of recording eludes surround: Mono. With mono recordings, the decoder moves the sound to the centre speaker since it cannot recognize signals for other channels. But just about any type of stereo sound can be used by a surround decoder. This is true for all types of movies. Even those that don’t advertise Dolby formatting will have some sort of surround coding. With television feature films, it is sometimes the case that corners were cut with Dolby licensing and alternative encoding systems were used. With  television series or shows, Dolby is increasingly used, but less often with music or games.

To check which productions were licensed by Dolby, check out the Dolby website www.dolby.com. Note that even when Dolby technology is used, it’s possible that the rear channels remain silent. This is then because the director and sound engineers wanted it that way, as is the case with Woody Allen films.

A new era of sound technology

As its name suggests, Dolby Digital is the digital equivalent of Dolby ProLogic and Dolby Surround – a way of storing and transmitting multiple channels. At the same time, Dolby Digital is a format that minimizes the size of digital audio files, a feature that we will, however, not be covering extensively here. 

The advantages of Dolby Digital

The advantages over Dolby Surround are:
- Perfect channel separation
- The ability to have stereo in the rear channels as well
- The ability to include the full frequency range on all channels
- An additional bass channel

All told, these advantages lead to an improved 3-dimensionality, more powerful effects, and even more realistic sound reproduction. One should note, however, that the digital surround tone cannot work wonders all on its own. The quality of the sound system and amplifier or A/V receiver will, of course, play an important role as will the fact that not every movie makes full use of the possibilities presented by Dolby Digital.

Digital multichannel systems constntly work with independent channels that are not able to influence each other with a few exceptions. If sound comes from multiple channels at the same time, a data reduction process may remove the sound where it is softest. This is based on the fact that the human ear will only recognize sound coming from the direction in which it dominates.

Digital sound systems can usually process up to six channels. As opposed to analogue surround systems, the back channels are supplied with two separate signals. In addition, there is a low frequency effect channel (LFE) for the subwoofer. This last channel is the .1 in a 5.1 system – which is the maximum number of channels that Dolby Digital supports. MPEG-2 can support up to 7.1, SDDS in cinemas up to eight full channels, and DTS up to eight. 6.1 and 7.1 system (Surround EX, DTS ES) employ a rear centre that is also supplied by the stereo channels of the surround speakers and uses its own independent sound channel. There are also the first discrete multichannel systems with over 6 channels such as DTS Discrete 6.1.

Software for digital decoders

All digital systems can be separated into two basic categories: Two channel stereo and true multichannel systems. For instance, Dolby Digital is only a digital surround system when more than two channels are used. With older movies, no discrete tone tracks were mixed. Instead there is only an analogue encoding for the two channel optical sound from 35 mm film. In order to hear surround sound from these DVDs, the Pro Logic Decoder will have to be used. The same is true for MPEG. MPEG-1 always has two channels and MPEG-2 can use up to eight channels. The appropriate material for a digital decoder should have 5.1 channels or 4.0 – a format popular with music that employs 4 equal channels.

As opposed to analogue systems, digital processing formats are in no way compatible with each other. Each software requires its own decoder. With Dolby Digital, it’s relatively easy: Every hardware solution can make use of every software. Every DVD player has a digital decoder yet it only produces two channel tone. The same applies to digital televisions in the USA. The term Dolby Digital Decoder only applies when at least 6 channels are processed.

What one needs

Integrating a Dolby Digital Decoder into a home cinema system is actually quite easy. Nearly all contemporary home cinema receivers use an integrated decoder for Dolby Digital signals. Older receivers that were exclusively designed for Dolby Pro Logic cannot be upgraded.

But many DVD players have their own decoders at their disposal. These can then be connected to a receiver with a six channel input via six cinch cables. These devices originate from the transitional period between analogue and digital surround sound. The settings for the surround loudspeakers used and the individual levels (Speaker Management) are made directly via the DVD player.

Dolby Digital Decoders are less common in televisions. If, however, a Dolby Digital Decoder is integrated, the DVD player can run over the TV.

The first essential preconditions for Dolby Digital are of course the source and playback device. With all DVD players, you can be assured that all data will be sent in a Dolby Digital format. One must however note whether the signal is transmitted via cinch (coaxial) or Toslink (optical).

Which amplifier?

Since all channels are equal, the amplifier used should produce a similarily even power – if all speakers are the same. However if, for instance, the two front speakers contain integrated subwoofers, they will require substantially more power than if they were to leave the bass frequencies to an external subwoofer. An ideal scenario is to have five equal loudspeakers, each responsible for a range above 80 Hertz. Since Dolby Digital makes it possible to experience an entirely different low range potential, the power will be needed in the bass range. For this reason, it is wise to choose a system with an active subwoofer.

Essential for Dolby Digital when it comes to amplifiers is the following: Two power amplification channels for the surrounds must be present. With analogue Dolby, one was enough. Those who opt for an all-in-one unit such as an A/V receiver shouldn’t worry about this. Those who want to continue using their older equipment may run into problems here.

How to future-proof a decoder

Dolby Digital is widely used around the world. For this reason, the standard is considered to be well future proofed for the coming years. Along with DTS, other systems such as the rival high-end audio formats DVD Audio and Super Audio CD are worth watching as they are both capable of rendering six channels, in part without data compression or with lossless compression. In this case, the Dolby Digital Decoder will be able to be used and the pre-amplifier will have to have six channel inputs.

One detail concerning the decoder can really ensure future compatibility: Multiple digital inputs. If the decoder cannot be accessed from the outside, for instance because it is integrated in a DVD player, it is already possible to connect it to another source. Decoders with only one input will already be obsolete once television offers digital multichannel tone or if the system should, for instance, be expanded with a DVD player with another regional code.

Along with multichannel support, another point that will ensure future compatibility is the resolution. Sound signals with DVDs already come with a 24 bit resolution and a 96 kHz sampling rate. DVD audio can go as high as 192 kHz which allows frequencies up to 100 kHz be realized. Even Dolby Digital can exceed CD standards of 16 bit and 44.1 kHz. Those who want to go beyond these possibilities should keep an eye on current standards.

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