Alesis M1Active MKII User Manual - Page 30

Surround Sound CENTER SPEAKERS IN MUSIC MIXES In video applications, with left, center and right channels all set to deliver equal output, the ear tends to hear dialog as coming only from the center channel. (It's a psychoacoustic effect because all right and all left channel signals are fed to the center.) In Dolby Pro Logic, the center channel information is derived from sum and difference information encoded in the left and right channels. The very attributes that make this matrix-type system viable for video application make it less desirable for music-only reproduction. In practice, there is a hard center dialog channel but there is also a sort of phantom mono channel developed between the left and center channel and the right and center channel. This between-channel phantom mono image exists because so much of the information contained in each channel (left and center or right and center) is identical. Hence, mono. In the few music-only recordings encoded in Dolby Pro Logic, the derived center channel could lead to some drastic changes in spatial perception. In developing the M1 Active Mk2s for use as left, center and right front channel sources, we wanted them to be useable in both audio/video and audio-only "matrix" type applications. We discovered two, already available solutions to the large "wall-of-mono" effect produced by Dolby Pro Logic when playing audio-only material. The first is easy. Most home audio Dolby Pro Logic receivers will let you take the center channel out completely by putting the receiver in the "phantom" (no center channel) mode. It preserves the classic stereo effect of the front channels while adding rear surround. This method works well with most stereo-only recordings which have no surround encoding. But it's much better if the rear channels have true matrix (mono) surround (like applause) encoded in the mix. The second method of using Dolby Pro Logic gets the center channel back into the mix. Most of these same home audio receivers have an audio-only surround mode which significantly attenuates the output of the center channel in relation to the left and right. The audible benefit here is that the mix sounds like a stereo only recording but the center image (like a vocalist) becomes a hard center. There is no image wander, as the frequencies change, for instance. And the classic listening "sweet spot" expands dramatically, allowing for accurate soundstage and instrument placement when listening far off-axis. (This attenuating of the center channel for audio only is also employed by advanced professional matrixed decoders such as the Circle Surround, Miles Technology and Lexicon units.) 32 M1 Active Reference Manual