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Surround sound information page

    General information

    Surround sound is a wide topic. The general idea in surround sound thatthere are multiple speakers around the listener and they are usedto form a soundfield where the listener can heard sound coming fromany direction. Surround sound is mostly used in movies. The traditional professional system (Dolby Surround) used in cinemas has four channels, Left, Right, Centre and Surround. The Centre channel is necessary in a cinema, to ensure that people sitting to one side of the auditorium do not miss the effect. It also fills the gap caused by the Left and Right speakers being so far apart. A Centre channel has not been included on this more basic home system, since the effect does not have to cover such a wide area. The surround channel is genrally played with two or more speakers to get wide not well defined sound field (so that you notice that sounds come from behind but can't accurately determine where it is from).Many newer surround sound sound systems has added the number of sound channels to more than this. Typical modern surround setup has five channels: left, right, center, surrount left and surround right. In addition to this soem systems have low freqency effect channel for low frequency effect played by large subwoofer system.There aremany surround technologies in use. Here is some informationabout most common ones you might encounter:Dolby Surround is the old surround sound system used in cinemas. It uses Left, Right, Centre and Surround. The information of all of those sound channels is encoded to two audio channels (normal stereo soudn medium) using a special analogue audio coding. Some home implementations od Dolby Sourround had only three output speaker channels: left, right and surround. Dolby Pro-Logic is a souped up version of the old Dolby Surroundused in movie industry for few dacades. It uses twoanalog stereo channels (L and R) to encode four channels of information.In the coding process of four channels to two channels some of the audioinformation is bound to be lost in the process.Dolby Digital (known in the past as AC3) is a completely digital system whichtransmits 6 independent streams of digital data. The data is compressedusing Dolby's proprietary codec algorithms. Other digital systems competing in digital surround sound field are DTS and multi channel MPEG audio. Sometimes the systems are divided to the number of channels. Here are some combinations you might see:

    • 5.1: This is the most commonly used surround sound format in DVDs. This audio format supports five separate main channels (left,center,right,rear left, rear right) fill full frequency response and one subwoofer channel (limited typically to 120 Hz and below). This audio format is supported for example with Dolby Digital and DTS.
    • 6.1: The 6.1 channel system is like 5.1 channel system but adds rear center channel. If the rear center is not present in the source (as you noted most DVD's only have 5.1) the receiver will take part of the left rear and part of the right rear and create the center rear channel. There are some DVDs with 6.1 encodings (Dolby Digital EX, DTS-ES). DTS-ES is a discrete 6.1 channel format (i.e. there is 6.1 channels of audio that is mixed specifically like that) Dolby Digital EX is a matrixed 6.1 channel format (i.e. there is 5.1 channels of discrete audio and the extra channel, the rear centre, is matrixed from the rear left and rear right channels)
    • In 7.1 set-ups the extra 2 channels to 5.1 system. Those extra speakers are are usually side speakers (placed somewhere between the front and the rear speakers, usually more towards the front speakers). The 7.1 mix is matrixed from 5.1/6.1 mixes. This matrix is usually a format made up by the manufacturer of the decoder.
    There is a lot more 6.1 DVD's being released at the moment, but it's notalways obvious as the labelling on the back of the DVD boxes is hardly evercorrect concerning 6.1 mixes. For larger 6.1 set-ups it is generally recommended that 2 rear centrespeakers are used. Each speaker is placed 5 to 10 degrees off the rearcentre axis. The same audio signal is fed to both speakers and the level ofthe signal is reduced by 6 dB to compensate for the extra rear centrespeaker.


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