PLANK

Digital Audio

The world of multi-channel surround sound is both exciting and mysterious. DVD offers a variety of surround sound options to the home theater enthusiast. Dolby Digital is the standard format for delivering up to six discrete channels of surround sound on DVD. Due to Dolby Digital¡¦s flexibility, it can be enjoyed on a basic system, or for maximum impact, a sophisticated home theater system. The more channels the playback system has, the more realistic and enveloping the sonic experience will be.

Stereo,Dolby Pro-Logic, and Dolby Digital are audio formats of increasing sophistication and quality

Stereo

We all know stereo is two channels of audio (left and right) and we generally connectstereo components to receivers with a pair of 'RCA-style' or 'phono' audio cables. VHS video tapes and CDs for example have two audio tracks, one for left and the other for the right.

Dolby Pro-Logic

This clever system has, for many years now,enabled homes to have a 'surround sound' home theater system without changing VCRs or cabling between components. A surround sound system generally means having. in addition to stereo speakers, a center (mainly dialog) speaker (typically placed on top of or below the television), and rear left and right speakers, placed either side of or behind the viewing position in the room. Some people also have a 'sub woofer' to add bass and impact to movie soundtracks.

Dolby Pro-Logic is clever though because the signals indicating which sounds should come from which of the 5 or 6 speakers are 'hidden' in a regular stereo audio track and so regular stereo audio cables can be used and in fact Dolby Pro-Logic sound tracks can be recorded on VHS tapes. Many rental movie tapes are recorded with Dolby Pro-Logic 'encoding' on the stereo sound track. The Pro-Logic information indicates if a sound should come from the left, center or right front speakers, or both rear speakers - Pro-Logic can't send different signals to the left and right rear speakers, they both get the same sound. This is a compromise in the format. There's no specific sub-woofer audio signal, the receiver generally filters out low bass sounds from the other signals and sends them to the sub-woofer.

In summary, Dolby Pro-Logic encodes 4 channels of audio (left, center, right, rear) in two connections.

Dolby Digital (also called AC-3)

With the release of the Digital Video Disc (DVD), a new audio format arrived delivering far higher quality than anything before. Dolby Digital audio consists of 6 distinct signals: left, center, right, left rear, right rear, and low frequency effects (LFE or subwoofer). Dolby Digital is also indicated as a '5.1' soundtrack, meaning 5 main audio signals plus the subwoofer signal.

Each of the five main sound channels is a full 48KHz digital signal which is higher than CD even (44.1KHz).

Choosing a DVD player is tricky because it depends on the receiver you have. If you have a receiver with a built-in Dolby Digital decoder, which means it can receive the digital information for all 6 channels of audio and decode which signal is for which speaker, you can buy a less expensive DVD player without a Dolby Digital encoder in it. If you have a receiver that is described as 'Dolby Digital Ready' then it has 6 separate analog audio input connectors and you should buy a DVD with a built-in Dolby Digital decoder, which will have 6 separate analog audio output connectors to connect to the receiver. If you have a Dolby Pro-Logic receiver that is not Dolby Digital Ready or you have an older stereo receiver, buy a DVD player without the Dolby Digital decoder in it because one would not be useful to you. If you do have Dolby Pro-Logic at least, you will still get that level of surround sound from a DVD player.

If your receiver has a built-in Dolby Digital decoder however, the connection between a DVD player and the receiver is in fact simplified. Because the receiver can process the digital information, the DVD can send it directly to the receiver in a single cable, much like the way computers communicate in a network. A single digital coax or fiber optic cable is used.

DTS (Digital Theatre System)

DTS is a pioneer in multi-channel audio, DTS is featured in home theatre, car audio, PC and game console products, as well as 5.1 CD, DVD-Video, DVD-Audio and DVD-ROM software.

For home theatre and music playback in the home, DTS provides high quality 5.1-channel surround sound with many extras not offered by other consumer formats. As well as handling DTS-branded releases from a growing number of music labels and consumer software producers, DTS provides enhanced 6.1 matrix and DTS 6.1 discrete decoding that envelopes the listener in sound. DTS technology is featured in a wide cross section of receiver/pre-amplifiers, DVD players and add-on components from leading consumer audio vendors.

It¡¦s a different mixing method to supposedly give a more realistic theater experience but not really many players in the entertainment industry is pushing this format.

Hardware

The fiber optic port may be integrated with the headphone port. A special optical fiber connector that can handle either function is used. The typical connector style is a 3.5mm headphone jack. The mating fiber optic cable has a connector shaped like a 3.5mm headphone plug. The actual fiber runs through the center of, and is exposed at the tip of the plug. The type of the connector at the other end of the fiber optic cable will depend on the size and shape of the input port on the audio equipment.

Plastic Optical Cable.

POF(Plastic Optic Fiber) is widely accepted in digital audio systems e.g. AC-3 and DTS. Like glass fiber, plastic optical fiber consists of a core surrounded by cladding.The refractive index of the cladding is made smaller than the core to keep incident lightrays reflecting where the core and cladding meet, and down the fiber.When a short pulse of light is shown into the fiber, it can travel down the fiber in several different pathways or "modes" depending on the core diameter and the wavelength of the light pulse.The way light is transmitted down the fiber and switched on and off at different pulse rates is However, it is more expensive to install and maintain. In addition, it has been proven unsuitable for consumer installations. While it is highly effective for underground and vertical backbone implementations, it is too fragile for horizontal subsystems. Also, glass fiber management requires extensive technical expertise.

All plastic optical fibers commercially developed so far are of a multimode step-index (SI) type, where a homogeneous core with higher refractive index is sheathed in cladding material with lower refractive index, and their attenuation is 100-300dB/km. Step-index plastic optical fibers can be manufactured by means of a melt spinning process such as coaxial extrusion. In the case of the SI fiber, with an increase in the core diameter, the transmission bandwidth decreases due to its modal dispersion and the input pulses become broader through the fiber. Therefore, even in a short distance optical communication, the plastic SI fiber cannot cover the whole bandwidth over hundreds of MHz which will be necessary for the fast datalink and LANs in the near future.