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Home Theater Handbook

Multichannel systems

Part 1

See also: all questions, part 2

Why are decoders and multiple amplifiers found in modern home entertainment systems?

Modern home entertainment systems have advanced decoders and multiple amplifiers that drive all of the required speakers defined by the various multichannel sound formats. They recreate in our living room three-dimensional movie soundtracks, hi-fi music, game and multimedia sounds.

What caused buyer hesitation in purchasing a multichannel system?

The rapid advances of technology brought forth new and varied multichannel formats at an alarming pace, which confused and dissuaded interested buyers. This confusion was so serious in the beginning that many prospective buyers hesitated to invest in a multichannel system out of fear that it wi 11 be soon obsolete. Lately off course things have settled down and even though the list of formats keeps increasing, most systems are compatible with the main formats, Dolby Digital and DTS.

What are some of the multichannel audio formats?

A current list of multichannel sound formats includes Dolby Digital 5.1, Dolby-EX, Dolby Pro Logic II, DTS, DTS-ES, DTS Neo, DTS 24/96, THX, THX-EX, THX 6.1, THX Select, THX Ultra, THX Ultra 2 and it continuously grows. It is understandable that such a long list of formats can create confusion about the technical characteristics of each system.

In the paragraphs that follow we shall look back into the history of multichannel formats and we shall study all current formats.

What effect did the arrival of stereo VCRs have on home entertainment?

The arrival of stereo VCRs during the 1980s for the first time allowed viewing of movies at home that had a multichannel sound format.

Which was the first home audio surround system?

Dolby Laboratories introduced in 1982 the Dolby Surround system, which was the first home surround system. This was a three-channel, four speaker version of the professional Dolby Stereo cinema system, then used at movie theaters.

How was Dolby Pro Logic superior to Dolby Surround?

The next major improvement in home theater sound came in 1987, with the introduction of the analog Dolby Pro Logic system. This was a four channel, five speaker system. Compared to the simple Dolby Surround system, the Dolby Pro Logic system, had the additional capability to reproduce audio from a central channel. It therefore reproduced sound from discrete front left, front right, central and monophonic surround speakers.

How many channel of Audio are reproduced by Dolby Digital 5.1? What is their frequency range?

The movie industry was always the driving force behind the evolution of multichannel sound. In 1987 the Dolby SR system was introduced at movie theaters, a slightly improved version of Dolby Stereo and in 1992 the optical digital Dolby Digital made its debut. The home version was named Dolby Digital 5.1. The 5.1 after Dolby Digital means that the system reproduces five ("5") discrete and full frequency range channels (left front, center front, right front, left surround, right surround) and one low frequency effects channel ("1").

What is DTS?

Soon after the introduction of Dolby Digital, Digital Theater Sound Inc. introduced the competing digital multichannel audio format DTS. The home version of the DTS system also reproduces five discrete full range channels and one low frequency effects channel.

How did the completion between Dolby Digital and DTS affect the market?

The competition between Dolby Laboratories and DTS home theater audio formats, started with the introduction of the Dolby Digital 5.1 and DTS systems. More recently it continues with Dolby Digital Surround EX and DTS-ES systems that reproduce 6.1 discrete channels, the additional channel being the rear center channel. It is very fortunate that when this competition flared, the AV component manufacturers decided to support both decoding formats, averting a quarrel that could have hurt consumers. Nowadays the vast majority of home theater DVD players and AV receivers/ decoders support both formats.

What were the capabilities of Dolby Surround?

The first home multichannel audio system was introduced in 1982, it was simple and inexpensive. Its specifications where three channel four speaker audio reproduction. The Dolby Surround system had the capability to reproduce sound from two main front channels reproduced through two main speakers and one monophonic surround channel, reproduced through two surround speakers.

What is the Matrix coding technique?

Dolby Surround system used the analog Matrix coding technique to record the audio information of four audio channels in two, which makes the system compatible with simple two channels stereo. The coding process begins with the input signal for the four channels - Left, Center, Right and Surround. The information of the central channel is split between the left and right channel. The surround channel information is also split between the right and left channel. It is however split with a phase shift of -90 degrees for the left channel and +90 degrees for the right.

What does the Matrix decoder do during the decoding of the Dolby Surround signal?

During the decoding of the signal, the decoder detects the common information of the left and right channel, whose phase is shifted by -90 degrees in one channel and +90 degrees in the other channel and drives it to the surround speakers.

How many speakers does the Dolby Surround System use? What frequencies must they reproduce?

The Dolby Surround systems used four speakers. Two full phase speakers for the front left and front right channels and two surround speakers with frequency response limited between 100 Hz and 7 kHz.

What is the main difference between Dolby Pro Logic and Dolby Surround? What decoding technique does Dolby Pro Logic use?

The Dolby Pro Logic system is analog, uses the Pro Logic Adaptive Matrix decoding technique and decodes four channels of audio information from two stereo channels. Dolby Pro Logic uses active decoding which senses the dominant sound in the soundtrack continuously and the direction of the sound origination. The decoder then adjusts the relative volume levels of the channels to steer the sound to the appropriate speaker, which enhanced the perceived channel separation. The steering logic permitted up to 3 dB adjacent channel separation.

How many and what type of audio channels are reproduced by Dolby Pro Logic?

Dolby Pro Logic reproduces four audio channels. Three channels (front left, right and center) cover the full frequency range, from 20 Hz to 20 kHz, while the fourth channel is for the surround effects and has a frequency response of 100Hz to 7kHz.

For compatibility reasons all multichannel A/V Decoders and Receivers support the Dolby Pro Logic format.

What are the usual program sources for Dolby Pro Logic?

The usual program sources for Dolby Pro Logic are stereo videotapes, stereo broadcast TV programs, analog stereo satellite programs, game consoles and specially recorded music CDs.

Dolby Pro Logic sound can also be obtained from the stereo analog outputs of the DVD players.

When was Dolby Digital 5.1 introduced? Why is it so important?

Dolby Pro Logic was good, but good is never enough. The next step in multichannel audio formats was Dolby Digital and its home version Dolby Digital 5.1. Dolby Digital 5.1 was introduced in 1995 on Laser Disc, while in the same year it was decided that it would be the mandatory sound format of DVD discs, making it the de' facto format for every home theater system.

Why is the sound quality of Dolby Digital 5.1 so high?

Dolby Digital 5.1 reproduces six channels of audio information that are digital and discrete. As a result the sound quality is superior; the signals are stable and the matching of the audio engineer's mix with the reproduced sound is very high.

Why is data compression necessary for digital multichannel audio formats?

In order for the digital encoding and sound processing of the 5.1 channels to be possible, data compression is a necessity.

Dolby Digital 5.1 uses a special compression algorithm that is based on perceptual audio coding and the masking phenomenon. When neighboring frequencies are reproduced at high levels, the human perception of sound masks certain frequencies, which would otherwise be audible. This masking phenomenon is used for the reduction of data volume. Therefore all audio information that is not perceived is removed.

Which compression algorithm is used by Dolby Digital 5.1 and what are its specifications?

Dolby Digital 5.1 uses the compression algorithm AC-3, which offers an average compression of 10:1. This algorithm allows for five full range (20 Hz to 20 kHz) audio channels and one Low Frequency Effects channel with response of 20 -120 Hz.

The system is very flexible and the decoder has the capability to downmix the AC3 data stream of 5.1 channels to four channels Dolby Pro Logic, two channel stereo and mono-phonic channel.

What are the functional advantages of Dolby Digital 5.1?

Dolby Digital 5.1, as a digital format, allows the dynamic processing of the audio and gives us great control of the audio reproduction under various conditions. For example sound can be compressed for late night audition, bass can be rerouted from the main channels to the subwoofer and we can adjust the time delay of each channel, so that the sound reaches our ears simultaneously, independently of the distance between us and the speakers.

Who supported DTS and when was it introduced?

DTS is the "other" competitive digital multichannel sound system. DTS, with the cooperation of Steven Spielberg and the support of MCA Universal, was introduced in the movie Jurassic Park.

What are the similarities of DTS and Dolby Digital 5.1?

The only similarity of DTS with the competing Dolby Digital 5.1 format is that it also reproduces 5.1 audio channels.

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