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Re: 7.1 versus 5.1 (long.....)

203.117.146.139

Posted by jeme on November 2, 2005 at 01:55:58

In Reply to: 7.1 versus 5.1 posted by junkfoodjunky on October 17, 2005 at 06:24:07:

Maybe a little history first, to reel you in? You may well know that Dolby Digital Surround EX was first introduced in cinemas with the release of Star Wars: Episode One—The Phantom Menace. Dolby Digital Surround EX (theatrical name) adds a monaural surround back channel to the traditional complement of five full-range and one bandwidth limited channels of a 5.1 channel system. The Surround EX third surround channel is playback at the cinema's option over surround speakers located behind the audience, while the left and right surround channels are reproduced by surround speakers to the sides. At this point, are you surprised to read that Dolby Digital Surround EX is a 6.1-channel format? (Although to be more precise, it should be termed as 5.1 + 1 channel)

To maintain compatibility with the mainstream 5.1 channel platform, the back surround channel is matrix-encoded onto the left and right surround channels of the 5.1 mix so that no information is lost when the film is played in conventional 5.1 channel system. This is achieved by matrix encoding the surround back (SB) channel into the two existing discrete surround channels, surround left (SL) and surround right (SR). During playback, a Dolby Pro Logic-style matrix decoder, called the SA-10 is used in the theatre to extract the additional non-discrete channel upon playback.

SR + SL = SB sum

This is similar to the way in which a Pro-logic® system extracts a centre channel

OK, so what good is this extra surround back channel for?

In the movie theatres, the benefits of Surround EX include:

1. More realistic fly over and fly-around effects
2. More stable sonic imaging for atmospheres and music
3. More consistent surround effect throughout the auditorium.

In a home theatre, using a Dolby Digital EX decoder (the term for home consumer version), the new 6.1 channel surround sound format adds an additional surround back in additional to the surround left and right channels. As can be experienced in the theatres, with the additional surround back channel, comes with the dramatic increase in sonic advancement and creative possibilities.

You see, film-makers and sound designers have always wanted to be able to pan sound effects in a direction that alerts the audience to events happening around them. This new channel configuration adds unprecedented dimension to the surround. With the current stereo surround in 5.1 channel configuration, it is difficult to create the perception of sound coming from directly behind the audience. The current 5.1 channel surround format does not have the ability to send sounds behind the listener. On occasions when sound is actually panned to the position midway behind the listener, it tends to sound as if it is coming from above the listener instead.

With this 6.1 channel format, “behind-the head” sound effects are now possible.

Listeners who have heard this new surround format, all felt that the three surround channels offer a greater sense of spaciousness and that the surround sound are wrapping around them more completely. There’s a lot of opportunities for sound being behind the listener in that can be very effective for things that are scary, or things that are atmospheric, to really put him in the middle of the story. Left and right surround sounds much more like coming from left and right to the side of the listener. There is far greater sense of spaciousness. Surround sound are wrapping around the listener more completely. Sound effects can be effectively placed behind the listener.

The number of sound panning possibilities are increased with 6.1 channel configuration. I wish I can show you the diagrams that show the number of possible channel pairs for sound panning to make you understand better. But here goes:

2 channel = 5 panning possibilities
5 channel = 12 panning possibilities
6 channel = 17 panning possibilities

It is thus quite remarkable that the additional of just one channel over 5.1 digital sound results in an increase in the possibilities for directional panning.

So what is the thing with the numbers - 5.1, 6.1, and 7.1?

To understand it better, you need to distinguish between the number of channels in a surround soundtrack and the channel configuration of a home cinema playback system.

There are no 7.1 channel surround sound tracks. We just learnt that 6.1 (or 6.1+1) is as far as it goes. Wheras a home cinema system can comes as 5.1 channel configuration, 6.1 channel configuration or 7.1 channel configuration.

A Dolby Digital EX home theatre system closely resembles a conventional 5.1-channel system. The differences are the addition of EX decoding, one or two more surround speakers at the rear, and one or two more amplifier channels.

With receivers and preamp/processors equipped for Dolby Digital EX, the additional decoding is built in along with conventional Dolby Digital 5.1 decoding. Some receivers also build in extra amplification for the third surround channel, while with others you need an extra, standalone amplifier.

If you have a preamp/processor, you can either add another single- or two-channel amplifier to your existing setup, or opt for a six- or seven-channel amplifier to consolidate all amplification in one unit. If you add an extra amplifier, it should be of similar design and power rating to your existing amplifier or receiver.

In a 6.1 channel playback system, you would have a 6 channel amplifier, 3 front speakers, and 3 surround speakers.

In a 7.1 channel playback system, you would have a 7 channel amplifier, 3 front speakers, and 4 surround speakers. Take note that the 2 surround back speakers are monaural.

The number (i.e., 5.1) describing the soundtrack does not have to match the number applied to the speaker system.

A 5.1-channel soundtrack can be playback on a 6.1-channel speaker system. . In this case, the two surround signals on the 5.1 soundtrack are spread across the three surround speakers of a Dolby Digital EX decoder equipped system. Conversely, a 6.1-channel soundtrack can be playback on a 5.1-channel speaker system. In this case the surround back channel is down-mixed into the surround left and surround channels.

A 5.1-channel soundtrack can also be playback on a 7.1-channel speaker system. In this case, the two surround signals on the 5.1 soundtrack are spread across the four surround speakers a Dolby Digital EX decoder equipped system. And so can a 6.1-channel soundtrack can be playback on a 7.1-channel speaker system. The monaural surround back channel is simply playback via the 2 surround back speakers.

So you see, the surround format and the speaker configuration are independent, and it is the decoder's job to bridge them effectively.

So why the hell do we need 7 speakers (or 2 surround back speakers)?

Well, here’s the dirty secret:

Some THX certified playback systems use dipole speakers and these system will have tremendous phase cancellation problems if only 3 surround speakers are used. Dipole speakers are like Bi-pole speakers in that they have speaker drivers mounted on opposing sides and their output are meant to be radiated at wall surfaces to create sound reflections of these walls to create some form of “surround sound”. Their output are never meant to be radiated towards the listener. The listener sits in the null point (the point where output is weakest between the 2 speaker drivers). But unlike Bi-pole speakers, where the opposing speaker drivers are “pushing” (firing outwards) and “pulling” (firing inwards) simultaneously, the opposing speaker drivers of a Di-pole design push and pull simultaneously. In a 3 surround speaker configuration, there will be severe phase cancellation at the spot between either one of the surround-left/surround back or the surround right/surround back channels. Bi-pole speakers used singularly for surround back channel don’t have this problem. To solve the phase cancellation problem with Di-pole speakers, one needs to use 2 speakers for the surround back channels. Then orienting the radiating patterns correctly, one can avoid the phase cancellation problem.

Then of course, there is DTS Extended Surround (DTS ES), which is a competing but compatible format. The name "DTS ES" applies to Digital Theater System's theatrical EX adapter, the competitor to Dolby's SA-10. When an additional surround back channel is delivered by DTS, it is done so by using the technology developed by Dolby and DTS pays a license fee accordingly. Therefore, the correct name for an EX-encoded soundtrack delivered by DTS is: DTS Digital Surround Dolby Digital Surround EX.

Bear in mind this is still technically a 5.1 + 1 channel system, and this is not to be confused with DTS ES 6.1 Discrete, an entirely different format which is indeed correctly described using the term "6.1" as it does deliver six discrete channels of full-range information plus an LFE channel. DTS has expanded on this technology by allowing movie soundtracks and music material to be mixed and presented in the home utilizing a discrete back surround channel, thus creating a true discrete 6.1-channel format - DTS-ES Discrete 6.1.

The discrete back surround channel is encoded as an extension file, encoded to the core 5.1-channel DTS bitstream. The extension file data is invisible to existing DTS decoders, which will simply play back the core DTS 5.1 stream. It is important to note that this new format is backward compatible with existing 5.1-channel DTS decoders, and that DTS-ES Discrete 6.1 soundtracks will also carry a matrix encoded back surround channel in the 5.1-channel bit-stream insuring compatibility with existing 5.1-channel Surround EX-capable matrix decoders.

So should you get a 7.1 channel system when you only intend to set up maximum of 5 speakers and a sub?

Well only the higher end receivers or pre/pro are equipped with the more advanced decoders to decode and playback the 6.1 channel sound tracks. These more advanced decoders often use more higher capacity DSP devices that perform more precisely than conventional 5.1 channel capable DSPs. The higher calculation capacities of these more advanced DSPs also includes a host of functions like bass re-direction and distance compensation that are not available or simply more inferior on the lesser DSPs. So simply for better 5.1 channel performance, it is worth the while to go for one that can decode and playback up to 7.1 channels.

Hope that helps.

This post is made possible by the generous support of people like you and our sponsors:

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