A schematic diagram of a stereo multiple-source simulation is shown in Fig.5.6. To simplify the layout, the input and output signals are all on the right in the diagram. For further simplicity, only one input source is shown. Additional input sources are handled identically, summing into the same delay lines in the same way.
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The input source signal first passes through filter
, which
provides time-invariant filtering common to all propagation
paths. The left- and right-channel filters
and
are typically low-order, linear, time-varying
filters implementing the time-varying characteristics of the shortest
(time-varying) propagation path from the source to each listener.
(The
superscript here indicates a time-varying filter.) These
filter outputs sum into the delay lines at arbitrary
(time-varying) locations using interpolating writes.
The zero signals entering each delay line on the
left can be omitted if the left-most filter overwrites delay memory
instead of summing into it.
The outputs of
and
in Fig.5.6
correspond to the ``direct signal'' from the moving source, when a
direct signal exists. These filters may incorporate modulation of
losses due to the changing propagation distance from the moving source
to each listener, and they may include dynamic equalization
corresponding to the changing radiation strength in different
directions from the moving (and possibly turning) source toward each
listener.
The next trio of filters in Fig.5.6,
,
,
and
, correspond to the next-to-shortest acoustic propagation
path, typically the ``first reflection,'' such as from a wall close to
the source. Since a reflection path is longer than the direct path, and since a reflection
itself can attenuate (or scatter) an incident sound ray, there is
generally more filtering required relative to the direct signal. This
additional filtering can be decomposed into its fixed component
and time-varying components
and
.
Note that acceptable results may be obtained without implementing all
of the filters indicated in Fig.5.6. Furthermore, it can be
convenient to incorporate
into
and
when doing so does not increase their orders
significantly.
Note also that the source-filters
and
may include HRTF filtering [57,549]
in order to impart illusory angles of arrival in 3D space.