Dolby Digital: how to make sound come to life



There are two main technologies of Dolby Digital: the Dolby noise reduction and Dolby Surround. Before the Dolby Digital surround, it was very expensive to set up a high quality stereo system in the theatre because the cost is too much, such as many speakers needed. Moreover, the direct recording and reproducing of multichannel sound were expensive and inconvenient. In 1976 the first generation of Dolby stereo system was invented and applied into the movie Star Wars (1979), which brought great visual and vocal shock to the world. In 1987, the second generation technology, also knew as Pro-Logic ImageSurround, was applied widely into no matter theatre and family entertainment equipment, such as VCD and LD. Finally, in pace with the development of digital technologies, the Dolby Digital got rid of the restriction of analogical techniques and published a new digital multichannel stereo system: Audio Coding 3 (AC3). Similar to the second generation, it could be applied in both public and home theatre. The AC3 was also known as Dolby 5.1 channel technology, which impacted the market and art a lot.

Features of Dolby Digital

Unlike Dolby Pro-Logic systems (second generation), the six audio tracks of AC3 are independently recorded and then compressed into two tracks in the discs. They will be perfectly decoded and reproduced during the playing. Although the AC3 is one kind of lossy compressions, but it has high acoustic efficiency totally acceptable by human’s ear, which has a relative narrow sensing range.

The Dolby Digital format allows for surround sound, as it uses six independent audio channels:

  •     A central speaker generally placed atop or above the screen, for reproducing dialogue.
  •     Two audio tracks for the front speakers, for accentuating the context of the sound coming from the central speaker.
  •     Two channels for the rear speakers, used for reproducing noise and the sound environment, in order to create ambiance.
  •     A channel for low frequencies (a subwoofer) and for amplifying special effects, such like explosions and earthquakes in the movies. Because of that, it is often called the boom channel. [4]


I still remember those scenes of one of my favourite movies: Batman: the Dark Knight (2008), can perfectly show that how the Dolby digital techniques can improve the experience of the audience. For instance, there was a famous chasing section in the movie: the Dark Knight is chasing his “old friend” – Joker by riding the high-tech Bat Motorbike. The sound of engines, Joker’s iconic laughing, batman’s voice over, sound of gunfire and booming are mixed together, but it is not flat and pale; under the magic of this technique, it just like a cup of cocktail, was full of acoustic dimensions and real time feelings. When I was watching this movie, the sound made me feel like to co-ride with the Dark Knight and fighting against the force of evil.

How Dolby Digital works

As mentioned above, the hearing range of human is relative narrow, from 20 to 20000 Hertz. However, a lot of animals can perceive more, such as dog, can hear sounds with frequency from 15 to 50000 Hertz. So, during the compression, we can abandon those sounds out of the range. Moreover, Dolby Digital also implements some psychology techniques during the compression. For example, if there were no signal, it would clean up all the sounds and keep the speaker completely quiet; if the audio signal comes, amplify it to cover the noise in the range.


When Dr. Dolby was a college student, he was very interested in sound recoding. In that time, he began to think that somehow to eliminate the noise without lower the quality. Through his research, he found that hearing of human beings is “lazy”, which means, as mentioned above, people are tend to select the sound. If one sound were louder, another is quieter; people would ignore the quieter one automatically. For instance, if we turn up the sound of the television, then hardly can we hear the dog barking of the neighbour. Therefore, through one kind of device, the mute signal, which contains a lot of inevitable trivial noises, such as the buzz of magnetic tape, were eliminated through Dr. Dolby’s technology. Technically speaking, The design of AC3 took the advantages of people’s selective hearing a lot: it divided the spectrum of every sound track into several bands with different bandwidths, which made the noises were close to the effective sound signals with very little frequency distance. After this process, people are easily to ignore the noises when the sounds reach them.


At the first, Dolby Digital was just a little laboratory trying to deal with the noise problems of the sound recording. But Dr. Dolby decided to enter the film market. According to him, the fist major market of noise reduction is recording industry, and then it will be the movies. So, Dolby’s technologies were applied into movies and DVDs widely since the end of 1970s. However, there was not only noise reduction, but also the stereo system in the movie. Before the Dolby, the set-up and technology of stereo was very inconvenient and expensive. So, the AC3 lower the cost and improve the quality of cinema stereo system dramatically, and also pushed up the digital technology in the film realm a lot.

However, Dolby Digital is more than just an audio coding technology. It is also a sophisticated audio delivery and reproduction system that allows both the program producer and the listener to affect how the audio program will ultimately be heard. One of Dolby’s achievements is to push up the development of home theatre.

The cooperation of Dolby Digital and those famous film production companies was almost free, so all the DVD of those companies were produced with Dolby’s digital technologies. Meanwhile, those digital sound-decoding technologies should be implemented into all the DVD players, which brought a lot of interests to the Dolby Digital. No only the cinema stereo system and home theatre, but also the car stereo and headphone, Dolby Digital contributed into them a lot and gave birth to many new digital sound technologies. We can confirm this though a series of numbers:

There are 1,499,475,600 sold products with Dolby’s identifications.

There are 606,690,060 sold products with Dolby’s AC3 technique.

There are 234,907,200 sold DVD players with Dolby’s decoding technology.

There are 273,540,900 PC DVD-ROM with Dolby’s decoding technology.

Now, Dolby Laboratory are cooperating with 1,821 identified medias, 204 identified chip manufactures and 387 identified hardware manufactures all around 50 countries. [4]

All these numbers can directly show that how successful was Dolby Digital in the market the Dolby Digital is, and how many technological shocks it brought to us. However, what has Dolby Digital done is not more than this. Dolby had already taken the traditional areas could be influenced by acoustic technologies, such as cinema and home theatre, and it still kept the mind on everything can embed its technologies.


They are many different ways that the consumers can enjoy the benefits created by Dolby Digital, such as DVD player and headphone. Moreover, there is not only 5.1-multichannel, but also many listening situation included. Here are some choices of Dolby’s format:

  • Full dynamic range 5.1-channel state
  • Reduced dynamic range 5.1-channel state (for apartment dwellers or late night 
listeners, etc.)
  • Two-channel Dolby Surround compatible downmix (which can then be Dolby Surround Pro Logic decoded)
  • Normal two-channel stereo downmix
  • Mono downmix [10]

The coding and mixing technologies are just a little part of Dolby Digital. The company and laboratory are still developing many new technologies, which is always the most important point of Dolby. Although the Dolby’s work cannot make the movie, it is just a tiny detail of the enormous film industry. But Dolby’s work is very important: the best sound track would not be better than other parts, but it not just provides a high quality sound in the cinema, but also reverts the original idea of the director and the composer to the audience greatly.

Although Dolby Digital’s reputation was growing with the amazing sound effects of those famous movies, such as Star Wars, but it was not just an entertaining effect. Dolby’s goal is to revert the overall sound effects in a very high quality, from the dialogue to the background music. Dolby’s technology is one kind of ways, or tools for file makers. It just likes the palette to the painter, which provided a plenty of colours to the director who could only choose a few colours before.

Besides Dolby Digital, Dolby Labs has developed many new versions to satisfy different needs from consumer and professionals, such as Dolby Digital EX, Dolby Digital Surround EX, Dolby Digital Live and Dolby Digital Plus. On April 2012, Dolby introduced its latest version, Dolby Atmos, a new cinematic technology, which is first utilised in Pixar’s Brave. [1]


Brave, first film to use the Dolby Atmos sound format


The Dolby Digital AC3 (Audio Coding 3) is the third-generation acoustic coding published by the Dolby laboratory. Through the implementation of the psychology theories and acoustic technologies into the digital signal processing, AC3 was the most influential audio coding and mixing technologies in the world.


Clint DeBoer 2004,What is Dolby Digital Surround? used by Audioholics, viewed 30 August 2004,<;

Gene DellaSala 2004, Dolby Digital vs. DTS: A Guide to the Strengths of the Formats, viewed 30 August 2004, <;

Jeff Tyson, How Movie Sound Works used by Howstuffworks, viewed October 2011,<;

Dolby Laboratories, Dolby Digital 5.1 Surround Sound (AC3), <;

Dolby Laboratories,Dolby Digital Cinema Details A Breathtaking Cinema Experience,<;

Advanced Television Systems Committee 2012, ATSC Standard: Digital Audio Compression (AC-3, E-AC-3) , viewed  17 December 2012, <>

Dolby Laboratories 2011, Dolby Surround 7.1Technical Information for Theaters, <;

Dolby Laboratories 2008, Dolby Debuts New Video Technologies at International CES 2008, viewed January 2008, <;

Dolby Laboratories 2008, Dolby Surround Pro Logic II Decoder Principles of Operation,


Dolby Laboratories 2000,’Dolby Digital Professional Encoding Guidelines ‘,issue 1, chapter 1 and chapter 2


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