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M1 ACTIVE REFERENCE MANUAL 27
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 Actives 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.)
Placement of the center M1 Active speaker
in the studio
The left, center and right speakers should all be placed in an arc so that the distance
of all drivers to the listener is identical (see page 29). The equilateral listening
triangle as described earlier for stereo (two-channel) playback should be retained,
with the center speaker placed vertically in the center, exactly between the left and
right (vertical) M1 Actives.
It does not matter what type of M1 Active is used for the center speaker (tweeter in or tweeter
out). Since M1 Actives are sold as pairs, a 5-channel system requires the purchase of a single
speaker, breaking up a pair. Some dealers may request that you purchase in pairs only; others
may be able to arrange purchase of a single unit. For the rear channels, we recommend a
mirror-image pair, although this is less critical than in the front.
As discussed above, for music-only recording the signal level to the center channel
will be attenuated over the levels of the left and right speakers. As a starting point we
would recommend that you set the center channel level –6dB below the left and right
channels. To get these levels fairly accurate a white or pink noise source (such as
from a test CD) is very helpful along with an inexpensive sound level meter (such as
the Radio Shack cat. no. 33-2050).
If you are recording audio-for-video, the center channel signal level should be the
same as left and right channels. The left, center, right speaker layout configuration
can remain the same as for an audio-only mix. With all front speakers operating at