T+A DD 820 M, User Manual

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63
The     surround system
Multi-channel reproduction based on surround
technology does indeed open up new worlds of
experience, but the technical implementation of a multi-
channel surround system does involve a number of
potential pitfalls. Many existing surround systems have
gained a reputation for no more than moderate sound
quality, especially when used in normal stereo mode.
The reason for this lies in the tiny magnitude of the
sensitive analogue stereo signals. These signals are
measured in the range of just a few Millivolts
(thousandths of a Volt), and unwanted interference to
these signals can be perceived by the human ear even
when a thousand times smaller than that. Conventional
surround amplifiers feature a single case in which digital
signals and high-frequency video signals live in the same
space as the sensitive analogue signals, and this
represents an open invitation for interference between
the various signals.
This compromise solution is entirely unacceptable to us
at
 , so for our surround system we developed an
arrangement in which the various signal processing
sections were kept entirely apart. The system therefore
comprises two specialised devices: an analogue two-
channel stereo amplifier is used to process high-quality
stereo signals, while the digital surround signals are
processed in the surround decoder which is an
independent machine.
In  systems the analogue and digital sub-assemblies
are kept strictly separate from each other. In stereo
mode the decoder is switched completely out of circuit,
and removed entirely from the signal path. This
eliminates any danger of influencing the sensitive
analogue signals, and therefore ensures that sound
quality in analogue stereo mode is as high as possible.
To ensure that the sound from the whole system is as
balanced and harmonious as possible, the output stages
built into the decoder are identical in circuit design to our
stereo output stages. This design philosophy ensures
identical frequency, phase and transient behaviour on all
channels, combined with perfectly balanced dynamic
characteristics.
The  surround philosophy also provides further
advantages: two separate pre-amplifier circuits are
present, each with its own volume and tone control
functions: a stereo pre-amplifier and an independent six-
channel pre-amplifier in the surround decoder.
It is therefore possible to adjust the levels and tone
settings separately to meet the different requirements for
stereo and surround modes. When you switch mode, the
correct settings automatically take effect.