Ambience Retrieval

This section is devoted to encourage the use of passive matrix systems creating more convincing spatiality by extracting the recorded ambience information inherent in two channel stereo recordings.

The active matrixes like ProLogic kill the reverberation time and hence the characteristic of the recorded acoustics for music reproduction. A simple and good solution, a Hafler type matrix with added delay for the rear channels, resembling the "Classical" mode in the now discontinued Fosgate Audionics processor range, should be easy to implement in any surround processor. For the ambitious manufacturer, inspiration may be achieved by looking into the even more advanced developments like the Ambience retrieval modes in the Meridian and Lexicon surround processors and the Circle Surround by Rocktron, also used in digital format by the Theta processor. Licence cost, if any, would probably not be a big issue, not to the extent of a Dolby licence anyway. And why not invest for music, not just movie sounds.
Only High End surround processor manufacturers have so far bothered including a useful ambience retrieval mode (with the notable exception of the bigger Harman Kardon AV receivers), while such a facility really should be standard in every multi-channel processor that the manufacturer claims to be useful for reproducing musical recordings.

Why do not the bread and butter home entertainment manufacturers understand what this is all about and add some useful ambience retrieval music surround modes instead of their boring artificial reverb modes?

Since I first wrote this, Denon for one has included a "Matrix" mode in their receivers. The Dolby ProLogic II also may show good potential for music listening. The future for ambience retrieval from two channel material now (2001) looks brighter than I could expect only a year ago.

More speakers are needed for the surround channels!

In a stereo set-up the front speakers cover an angle of about 60 degrees in front of the listener. This leaves 300 degrees to cover with only two surround loudspeakers…

Expecting a seamless ambience sound field from only two surround speakers is obviously not possible. When you turn your head even slightly, the rear loudspeaker in that direction will be heard with a completely different timbre because of the function of the ear.

A much smaller percentage of the total ambience sound pressure must be heard from each speaker at the side wall, or behind the listeners head, or the surround speakers will be readily discernible as point sounds. Adding more surround speakers will produce a sound field where each separate loudspeaker has so much less output that identification of individual loudspeakers will be eliminated.

The concept of side placed dipole surround speakers for home surround systems, as suggested by THX is proof enough that two normal rear speakers are problematic.

Dipoles are not a perfect solution though, and much criticism has been raised against their sound in other aspects.

Will multiple surround loudspeakers be useful only for music and not for 5.1 movie sound? Look at the movie theatres, do they have a single pair of surround speakers even for Dolby Digital or DTS sound? No, and movie sound will therefore be continued to be produced for multiple surround speakers, and will be best presented that way in your home too. This is valid for Dolby Pro-Logic or 5.1 discrete systems as well as for ambience retrieval surround systems.

The question is, how many surround loudspeakers are really needed? A standard Dolby Stereo set-up for Movie Theatres has four per channel. One may think that in smaller listening rooms fewer will suffice, but for the most demanding THX specified movie sound studios, even more surround speakers are used.

Well, for living rooms eight or more surround speakers are not wanted for several reasons, but already the addition of a second pair will give a great improvement.

Two of the multiple surround speakers should be placed on each side of, and just a little in front of the listener to recreate a side wall reflected ambience. When using four surround speakers, it will be advantageous to have less output from the side speakers than from the rear pair, but when using six or more surround speakers, individual level adjustments are no longer necessary.

Adding more pairs will further improve the spatial impression but with gradually decreased noticeable improvements for each pair added.


© Per Arne Almeflo

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