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By the 1980s the company had moved into home theatre with four-channel Dolby Surround sound and Dolby Pro Logic. Dolby Digital (often called 'AC3') is a leap forward in digital technology bringing sound reproduction in the theatre or at home from four channels to six. |

Six-channel sound involves the front speaker channels: left, centre and right front, designed to deliver clear and precise localisation of dialogue. There are dedicated surround channels (left and right) for the reproduction of those ambient sounds, which add to the 'atmosphere' of a movie.

The sixth channel (often referred to as '.1' - as in '5.1' channel stereo) is the effects subwoofer channel for reproducing deep bass (those the low rumbling sounds that make movies impressive).

Perceptual technology
A quick explanation of 'perceptual' digital audio coding will help in understanding how Dolby Digital works. Any digital sound track, such as the one on a conventional audio CD, is recorded (coded) in a format in which the information is very 'dense'.

This huge amount of information causes two problems. One is that the amount of physical space taken up by the sound track is pretty big. The other is that when it comes to playing back the sound the amount of information or 'data' reaching the digital decoder of a sound system is often too great for the electronics to store and transmit efficiently.

The problem of too much information gets even more difficult to solve in the case of multiple sound channels such as in surround sound. If the sound system 'misses' too much information from the sound track, we end-up with something that is unpleasant to listen to - the sound track is distorted because the sound reproduction system is 'swamped' with too much information and misses big sections of it.

PCM was a big breakthrough in sound technology, but with the advent of the even greater amounts of information needed for digital video on DVD and six-channel sound, a way of handling the sound track information and the space it takes up on a disc was needed.

The first generation of PCM needs a lot of storage and transmission space, so much so that it becomes impractical for the multiple channels needed for surround sound.

If we try to add digital video a conventional PCM disc to produce a DVD we are left with almost no space and a poor picture. This is where 'new perceptual coding' comes in. The information in the sound track is still as dense; the difference is how this information is handled by the sound system.

Perceptual coding acts like a filter where only select parts of the sound track reaches the speakers. The sound is recorded on the disc and then decoded in the sound system using a type of filtering. Perceptual coding selects only the parts of the sound information that are necessary to record and reproduce a soundtrack that sounds complete.

It ignores the information your ear can't distinguished in the original blend. This results in a smaller flow of information that can be properly managed by a sound system and stored on a smaller area of the disc.

Dolby Digital adds another step to PCM and perceptual coding. The result is a sound track that needs less than one-tenth of the space of a single channel of a conventional CD stereo sound track.

How is this possible when it seems like so much information is being thrown out? The answer is partly mathematical and partly to do with the way we hear sound.

Psychoacoustic manipulation
Dolby Labs has spent over twenty years studying human hearing to define the mechanics of how we hear different frequencies of sound.

The company calls this its 'psychoacoustic' research. 'Acoustic' refers to sound and 'psycho' refers to the whole process of hearing sound. Hearing involves our ears and our nervous system and for various reasons, we hear some sound frequencies better than others even if they are at the same volume.

For instance, high pitched sounds are the hardest for us to discern and for many people an extremely low rumble is more sensed than heard, while the sound of the human voice is easy (for those without a hearing impairment) for most people to hear clearly.

The results of Dolby's research into how and which sounds we perceive have been written into a computer program (mathematical algorithm). This program is applied to the information on the disc when it is being recorded and also when it is played.

The computer program is Dolby's third generation audio coding algorithm, hence the name AC3. AC3. divides the sound into 'frequency bands'. You might say it groups the areas of the sound spectrum. Throughout the full range of sound, from low pitched to high pitched sound, the whole sound spectrum is divided. These divisions are called frequency bands. The width of each band is difference.

The difference in size of each frequency band corresponds to the way we hear sound, based on the 'physchoaucoustic' research. Each sound frequency is considered by Dolby to be the optimum for human sound perception. The resettling 'banded' sound signal is again filtered to remove extraneous sound and then sent to the six channels for surround sound.

Essentially, AC3. is a form of selective noise reduction that leaves the major elements of the original sound signal intact while 'slowing down' the rate of flow of information. A final signal that is rich in the original sound information and low in noise is sent to the speakers.

Dolby Labs maintains that even though the sound signal has been manipulated, the resulting Dolby Digital sound can't be differentiated from the 'master' recording when tested by critical listeners.

A Dolby Digital encoded sound track is compatible almost all audio-video receivers, but to hear it in Dolby Digital a decoder is needed. This isn't a problem since most manufactures incorporate the decoder into the design of their receivers. Dolby Digital has been adapted for use in a two speaker sound system. The 'pseudo-surround' effect is available in Dolby Pro Logic as Virtual Dolby Surround and Dolby Digital as Virtual Dolby Digital.

As a counterpoint to the Dolby Digital we now have the DTS system, which works on a higher information flow rate. Dolby has no plans to increase the data handling capabilities of Dolby Digital to that of DTS. This decision is based on the belief that the human ear cannot discern the difference and that by increasing the data rate for the sound track, the amount of data available for the picture on DVD is compromised. The result is said to be degraded image quality.

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