Chapter 21 - Audio 3D - Dolby Prologic and Dolby Digital

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The Dolby Motion Picture Matrix Encoder algorithm we have just been looking at is used in Dolby Pro Logic-type systems. However this system has some quite serious flaws. In particular the encoding process has negative repercussions, because when you encode 4 signals onto 2 you will inevitably lose something along the way. First and foremost what is lost is the compete independence of the 4 signals. In other words it will no longer be possible to have the separation we had before encoding, and consequently the sound information is degraded. The decoding algorithm we described can only extract part of the information.

To obtain better decoding we have to resort to more sophisticated algorithms. Let's use an example to get a better picture. We have seen how after the encoding operation, signal C (central) is sent equally to L and R. So, we no longer have the separation between channels L, C, and R. To minimize this inconvenience we can send a small amount of signal L to channel R, inverted in phase. This will diminish part C present on channel R. The same can be done on channel L. If the process we've just described sounds a bit confusing, read it through once again. You'll see, there's nothing complicated about it. This operation increases the separation between channels L, C and R but generates hazardous phase cancellations on channels L and R, altering the sound field sometimes quite dramatically. However, let's not go too far with these theoretical speculations. The idea was to get the gist of how decoding algorithms work. Dolby digital is one of such, and it is the most widespread surround system on the market. The system entails a decoding of the signal Lt and Rt into 5 signals plus one for low frequencies. The following are the decoded signals:

Table 21.3. Dolby Digital system signals 

TitleFull name
LLeft
CCenter
RRight
SlSurround left
SrSurround right
SubSubwoofer

The following diagram illustrates the classic positioning of loudspeakers in a Surround 5.1 system:

3D Audio - Positioning of loudspeakers in a Surround system

Positioning of loudspeakers in a Surround system

As we can see the front loudspeakers L and R are positioned at a 30o angle in relation to the central axis, whereas the posterior ones are positioned at a 110o angle. The subwoofer [Types of loudspeakers ] can be placed practically anywhere: as we know, very low frequencies don't have directional characteristics [Localization of sound sources ] and are therefore reproduced appropriately regardless of the source's position.

21.4.1. Mixing in Dolby Pro-Logic with a normal mixer

Let's now take a look at how to configure a normal mixer to carry out mixes for Surround systems. Clearly this means reproducing the aforementioned encoding. Let's refer to the following diagram:

3D Audio - Simulation of a surround mixer

Simulation of a surround mixer

What we undoubtedly need is a Dolby Pro-Logic signal decoder and the 5 loudspeakers that make up a classic surround system. A home-theatre system could suffice, even though the sound quality will be all but fantastic, but the fact that we are simulating a surround mixer with a normal one probably means that we're not aiming at absolute top quality anyway.

Having established that we will send our mixer's stereo master output to the decoder, let's see how the encoding phase takes place. Remember we have to encode the following signals: L (left), C (center), R (right), S (surround). For the stereo mix L and R we will use two groups, one of which we'll send to the master output left (panpot of the group fully turned to the left) and the other to the master output right (panpot of the group fully turned to the right). Signal C will be sent equally to the master's two outputs (centrally positioned panpot). Signal S first gets sent to a delaying [Delay ] stage (to do this we can use an aux send [Auxiliary send ] as illustrated in the diagram) and thereafter enters two separate channel of the mixers one of which gets inverted in phase. As far as the filter applied to signal S is concerned, it can be inserted at any point in the chain.










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