Yamaha DSP-A990 - Manual (Page 9)

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English
Setting Up Your Speaker System
This unit has been designed to provide the best sound field
quality with a full seven-speaker system setup, using two extra pairs
of effect speakers to generate the sound field plus one center
speaker for dialog, when using Dolby Pro Logic Surround decoding.
We therefore recommend that you use a seven-speaker setup. A
four-speaker system using only one pair of effect speakers for the
sound field will still provide impressive ambience and effects,
however, and may be a good way to begin with this unit.You can
always upgrade to the full seven speaker system later. In the 4 or 5
speaker system, the Digital Sound Field Processing is still
performed, but the main speakers are used for both the main
channels and the front effect channels.
Use of the Center Dialog Speaker Is Recommended
With digital sound field programs No. 8 through No. 12, by using
either the Directional Enhancement circuit or the Dolby Pro Logic
decoder, decoded signals will be output from the center channel.
Therefore, if you want to maximize the performance of your
Audio/Video home theater system, it is recommended that you use a
center channel speaker.
If for some reason it is not practical to use a center speaker, it is
possible to enjoy movie viewing without it. Best results, however, are
obtained with the full system.
It is also possible to further expand your system with the addition
of a subwoofer and amplifier. You may wish to choose the
convenience of a Yamaha Active Servo Processing Subwoofer
System, which has its own built-in power amp.