The Evolution of Dolby

June 7, 2018 General Studies

Dolby Surround is the consumer version of the original Dolby multi-channel analogue film sound format Dolby analogue. Dolby Surround or Dolby Pro Logic is based on basic matrix technology. When a Dolby Surround soundtrack is created, four channels of sound are matrix-encoded into an ordinary stereo two channel sound track by using phase shift techniques. A Pro Logic decoder/processor unfolds the sound into the Dolby 4. 0 surround. This consists of left and right, center, and a single limited frequency-range mono rear channel which are matrix-encoded onto two audio tracks.

These two tracks are then carried on stereo program sources such as videotapes and TV broadcasts into the home where they can be decoded by Dolby Pro Logic to recreate the original four-channel surround sound experience. A system with out this decoding method would play back the audio as standard Stereo. Dolby Surround delivers four-channel surround sound via regular stereo VHS, TV and FM radio broadcasts, DVD discs, video games and CDs. Four channels (left, centre, right, and surround) are encoded into two-channel soundtracks and then recovered on playback by means of a Dolby Surround Pro Logic decoder.

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Dolby Surround is a two-step encode/decode process involving both recording and playback. When a Dolby Surround soundtrack is produced, four channels of audio information, left, centre, right, and surround, are encoded onto two audio tracks using equipment manufactured by Dolby Laboratories. These two tracks are then carried on stereo program sources such as video tapes and TV broadcasts into your home, where they can be processed by a Dolby Surround decoder to recreate the surround sound experience. There are two kinds of decoders: basic Dolby Surround and Dolby Surround Pro Logic.

Both recover the surround information from encoded program material and feed it to a pair of surround speakers placed up on the side walls adjacent to the listening area. In a basic Dolby Surround system, left and right front speakers are fed with the entire program in normal stereo without any processing, while the surround speakers are fed with a surround signal derived by a relatively simple passive matrix decoder. As with regular two-channel stereo, the left and right front speakers create a “phantom” centre channel but only for those relatively few viewers seated on centre.

By using the same directional enhancement system found in professional Dolby Stereo cinema processors, Pro Logic decoders derive a separate centre channel to keep dialogue and other central sounds firmly localised on the video screen. Pro Logic also supplies higher separation among all four channels and more accurate sound positioning, which along with the centre channel enable a greatly expanded listening area. Dolby Surround Pro Logic is the best way to accurately reproduce the Dolby Stereo theatre experience in your home and will ensure that you hear Dolby Surround soundtracks as the producer intended.

The basic Dolby Surround Decoder features passive matrix decoding to derive the surround channel and three output channels, left, right and surround. Surround channel frequency response is limited to 7kHz plus modified Dolby B-type noise reduction. Surround channel time is variable from 15ms to 30ms. Dolby Surround Pro Logic Decoders feature in addition high-separation, active matrix decoding for enhanced directionality, four output channels: left, centre, right and surround, automatic input balance control, test noise sequencer and level adjustments to balance all four channels.

The centre channel mode control gives Normal, Phantom and optional Wide modes. The Left and Right channels are the carriers of the other two. That is, nothing special is done to these two input stems. They are the basis for what we will call LeftTotal and RightTotal, once the center and surround information have been added to them. This system is called a Matrix. The Center channel stem is added equally to the Left and Right, but reduced by 3 dB so as to maintain constant total power when being carried by two channels as it is.

The surround channel is also input equally to both the Left and Right channels, but each half gets a 900 phase shift opposite from each other. The surround channel information that is in Left and Right therefore ends up being 1800 to each other. In other words, they are totally out of phase. Before getting added to the soundtrack, it gets a -3 dB reduction (for the same reason as the center), is then filtered to the frequency range of 100 Hz to 7 kHz, and undergoes a form of Dolby B-Type noise reduction. Dolby Digital

Dolby Labs introduced Dolby Digital in the movie theatres in 1992 with the Batman Returns movie and it became available in the home shortly afterwards. It is also known as 5. 1 or AC-3. Dolby Digital has many advantages over the older Pro Logic. Dolby Digital offers six discrete channels of sound. There are five full range channels of sound (front left, front centre, front right, surround left and surround right) plus a special sixth channel known as the Low Frequency Effects channel. While Dolby Digital is capable of the six separate sound channels, that does not mean that all will be used.

An older film with a mono soundtrack may still be encoded in Dolby Digital, but will still have mono sound. Unlike Pro Logic, the five main channels are full range, which means that they handle the complete range of sounds from 20Hz (very low bass) to 20kHz (very high treble). The surround channels also differ from Pro Logic in being fully discrete or separate. This means that each surround channel can have differing sounds in it, allowing the sound to move to each channel independently. This can heighten surround effects tremendously. The sixth channel (Low Frequency Effects or . ) causes much confusion. Many people mistakenly think the the LFE channel is the “bass” channel, which is incorrect. Some movie theatres have large bass-only speakers, designed for reproducing huge amounts of bass, taking the strain off the main speakers in the movie theatre. When making the film, a sound engineer may choose to put bass heavy sounds into the LFE channel, for use by those movie theatres described. The confusion arises because many people believe that the LFE channel equates to the frequencies that an AV receiver sends to a dedicated subwoofer.

This is not so. Dolby Digital uses a means of compression to fit all six sound channels on to a DVD. This compression is “lossy” which means that some information is thrown away. Compare this to the computer Zip (PC and Stuffit (Macintosh) compression formats, which are “lossless”. With Dolby Digital an encoder chooses the sound most likely to be heard, based on a pyschoacoustic model of human hearing. Dolby Digital pros: full range discrete channels, becoming ubiquitous Dolby Digital cons: use restricted to digital formats

Current uses: Dolby Digital can only be found on DVDs (and a few laserdiscs). The digital soundtrack can not be used on VHS videotapes or on stereo broadcast TV. Dolby Digital / AC-3 Dolby Digital delivers six totally separate (discrete) channels of sound. Like Dolby Surround Pro Logic, it includes left, centre and right channels across the front of the room. Dolby Surround Pro Logic provides a single limited bandwidth (100Hz to 7,000Hz) surround channel which is typically played back in the home through two channels of amplification and two speakers.

In comparison, Dolby Digital provides separate (discrete) left surround and right surround channels for more precise localisation of sounds and a more convincing, realistic ambience. With Dolby Digital, all five main channels are full range (3Hz to 20,000Hz). A subwoofer could be added to each channel if desired. The sixth channel, the Low Frequency Effects Channel, will, at times, contain additional bass information to maximise the impact of scenes such as explosions, crashes, etc. Because this channel has only a limited frequency response (3Hz to 120Hz), it is sometimes referred to as the “. ” channel. When added to the five full range channels, the Dolby Digital system is sometimes referred to as having “5. 1” channels. All six channels in a Dolby Digital system are digital, which means that they can be transferred without loss from the producer’s mixing console to your home playback system. The Dolby Digital System packs all six channels into less space than a single channel on a compact disc. This advanced approach to handling the digital audio data is what makes it possible for Dolby Digital soundtracks to be added to DVD discs, and to a whole host of other sources.

Dolby Digital has been used to bringing you Dolby Digital movies for several years, and now, due to the advent of Dolby Digital DVD discs, you are able to enjoy this unique performance in your own home. Dolby Digital can deliver 5. 1 channel sound from DVD Video Discs, DVD ROM discs, laser discs, digital TV broadcasts, satellite transmissions and cable systems. It differs from Dolby Surround by providing two separate surround channels for greater realism and an additional “. 1” or “LFE” channel for Low-Frequency (bass) special sound Effects.

The LFE channel is sometimes incorrectly identified as the “subwoofer” channel. While a subwoofer can be helpful in reproducing low bass in general, it is not necessary to have one if one or more of the speakers in the playback system have extended bass response. Without a subwoofer, the LFE channel’s bass effects can be directed to those speakers with extended bass. Furthermore, not all multichannel soundtracks have the “. 1” channel as it is used primarily for explosions and rumbles on movie soundtracks, and is not present on all programs.


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