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Maybe many non-EX films would benifit from being played through an EX decoder.

Lets take a hypothetical digital film from say a few years ago (pre-EX). The mixer often had sounds mixed in both L and R surround channels. Played back in EX these sounds eminate from the back surround. Effectively giving a better effect that the person originally imagined. Or would someone argue otherwise?

Give us the question.

It's generally considered that surround coming from only the "back surround " speakers is worse than surround coming from both left and right, but of course it depends on context. This contextual dependancy is why it is hard to say "always leave it on," when that was not intended. Sometimes it's good, sometimes it's not, and where the boundary falls is the subject of many different opinions.

For example, let's say there's a point in the film's narrative where someone's voice comes over an intercom system and has to be heard all around. A normal 5.1 mix might sum the same audio across both LS and RS channels at the same level (a good example would be the MCP's voice in "Tron"). If you have EX activated, the audio will pull only to the back wall channel leaving the side walls dead. Intensity levels in the RS and LS channels would have to be intentionally varied to keep from isolating the audio element to the back wall speakers.

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This is the difference to using the matrix for the analogue formats. For EX the steering of the matrix is not always wanted.
For this reason there is some 'encoding' involved in mixing for EX.
And in fact there is an EX encoder unit the Dolby EX-EU4 (previously called SEU4-EX).
This unit is doing something different than the 4-2-4 encoder.
The LS input is send with a -45° phase shift to the LST out, RS input is send with +45° phase shift to RST out and the BS input is added with -6 dB to both outputs.
Panning a signal between the LS and RS inputs results in a fly across the cinema.
A mono signal to LS and RS inputs is reproduced by all channels. The +/- 45° phase shift disables the steering of the matrix. The matrix just does not recognize a direction anymore.
The surround voices mentioned by Bobby are mixed in the same way.
EX can be mixed without the unit. Then the phase shift is done with a plug ins or other decorrelation techniques are used.

As for any matrix mixes it is mandatory for an EX mix to monitor through an EX decoder (switchable to check 5.1 compatibility). Otherwise the mixer has no control what he is doing.

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I guess this begs the question, "How much extra work is it for sound mixers to mix films so that they sound correct in EX, even if they're not using EX features?" Perhaps, as EX equipment becomes more prevalent, we'll move to that case where every mixing room will have an EX encoder and even if the engineer chooses not to make use of the back surround channel, we can avoid having mono surround directed exclusively to the back channel by phase adjustment.

--jhawk

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With 5.1 you cannot create an all-round ambience effect with a mono signal fed to both the LS and RS channels. As I stated earlier, the Haas effect ensures only those members of the audience seated dead centre will hear this as coming from both sides. Everyone else only hears the wall nearest to them - hardly what was intended by the mixing engineer (who incidentally is usually seated very close to the centre of the mixing theatre).

For this reason nearly all 5.1 films use STEREO ambience surround tracks if an all-round effect is desired. A stereo track contains enough information which is different between left and right that even people seated well off-centre can tell the sound is coming from both sides of the room. When a stereo ambience surround track is played through an EX system it will come from all three channels (i.e even better "all-round").

Dolby's so-called EX encoder is simply a way of generating a pseudo-stereo track from a mono signal. The 90 degree phase shift applied between LS and RS for a mid-panned mono signal provides a similar de-correlation to a true stereo track and hence as Gunter says prevents the EX matrix from steering in any one direction. However Dolby's box is not required for mixing EX unless you do not have a source of stereo ambience material (like maybe if the film is being mixed in Siberia!).

The best argument for playing 5.1 films in EX is this - imagine a sound engineer trying to create an effect where the sound goes around behind the audience, such as the helicopter scene in The Matrix. With 5.1 he can only pan a mono signal from left through to right surround. However this results in the chopper going through the audience's heads not around behind them. Playing the same 5.1 track through an EX decoder produces exactly the effect the engineer was trying in vain to achieve in the first place. So the EX system often, and in fact more often than not, creates a better representation of what the engineer had in mind than playing it in 5.1

I have listened to many a 5.1 film through an EX system and only on very rare occasions has it caused the wrong thing to happen. In nearly every other instance it actually improved the directionality intended by the mixing engineer. How do I know? - I am a mixing engineer!

Also Gunter said "As for any matrix mixes it is mandatory for an EX mix to monitor through an EX decoder (switchable to check 5.1 compatibility). Otherwise the mixer has no control what he is doing".

I do not agree with this in the least! EX is very different from a fully matrixed system like Dolby Stereo. Any professional sound mixer worth his salt knows precisely what will happen when his 5.1 mix is played through an EX system. Of course it is better for him to hear his mix under all likely playback conditions, but "mandadory" - nonsense!

I think Brad has the right idea by at least giving it a try.

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Ray Derrick
President/Chief Engineer
Panalogic Corporation Sydney, Australia
Phone: 61 (0)2 9894 6655 Fax: 61 (0)2 9894 6935

quote:

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Interesting, prints of Spooky House came with a big red sticker on each reel stating "DOLBY EX".

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in my experience Dolby requires to do at least the final mix in an approved mixing facility capable of EX monitoring in order to license the movie as Dolby-EX. The more important mixing theaters are equipped with EX anyway.
So I have no idea at all why the mixing engineer could be keen to mix without EX monitoring. All the mixing engineers I work with of course use EX monitoring to do EX mixes.
Sure mixing engineers have a pretty good idea what the EX decoder will do. But sometimes the matrix is not doing what was expected. Probably you had the same experiences when mixing for the matrix that sometimes the steering of the matrix jumps between (any) channels due to some phase interaction of the various sound elements. These artefacts may happen with the matrix for 4-2-4 and the EX use. In fact there is no difference in decoding EX and the front channels of Dolby A/SR.

The surround tracks of a 5.1 mix are not allways 'true' (uncorrelated) stereo. The surround tracks usually are a mix of stereo atmos and added sound elements that are usually mono. These additional elements are not allways intended as directional sound effects and therefore often just send to both 5.1 surrounds. With the EX decoder on these sound elements will be steered to back suround. The use of the Dolby EX-encoder prevents steering if it is not wanted.
I do not agree that non directional mono elements are very rare. There is not need to move to Siberia as you mentioned, to find those 5.1 mixes. Just at the weekend I watched a French movie 'Les riveres pourpres' that did not work with EX decoding at all. E.g. they obviously used mono reverbs. This is no problem with 5.1 as it allways worked with SR mixes. When switching EX on the reverberation was steered to the back channel. Now it did not work anymore and the reverberation was annoying.

As Brad said there are 5.1 mixes that work with EX decoding and there mixes that don't work.
My point is that before using EX for 5.1 mixes you should check not only for obvious directional effects, but for subtle atmospheric effects.