~Proceedings ICMCISMCI2014 14-20 September 2014, Athens, Greece
Figure 4: Direct transform plugin interface.
Figure 5: Push transform plugin interface.
and elevation. These transforms fall into two groups.
Focus and Zoom are dominance related transforms and
can be described as 'emphasising' elements in the direction
of interest. The directional response of the target planewave
(distortion angle 900) is a cardioid. This means that gain
opposite the direction of interest is reduced. Focus maintains the gain forward at 0 dB. Zoom, however, increases
gain in the direction of interest and maintains gain perpendicular to this direction.
Press and Push 22 act differently. Rather than emphasising elements in a target direction, all are 'pressed' or
'pushed' towards the direction of interest. Where the target for Focus and Zoom is a cardioid, the target for this
second group is an omnidirectional response. As a result,
the gains of all elements are maintained to a similar level
and gathered toward the direction of interest. With all four
transforms available in the same plugin it becomes easy
to explore spatial expressive quality as part of the creative
process. GUIs for two can be seen in figures 5 and 6.
The Dominance plugin increases the gain of elements in
the direction of interest while decreasing gain opposite.
With the JSFX plugin directional gain can be boosted by
up to 24 dB. While the Focus, Press, Push, and Zoom
transforms do not generally have a significant impact on
the overall sound level of the soundfield, Dominance can
result in major alterations to sound levels. For this reason, and the difference in the way the amount of soundfield
transformation is specified (gain in dB vs. distortion angle
in degrees), the Dominance transform is implemented as a
separate plugin.
4.2.4 Mirroring
The Mirror plugin mirrors the soundfield across an arbitrary plane, while the MirrorO plugin mirrors the soundfield through the origin.
22 Two novel spatial transforms, at present found only in the ATK.
4.3 Near-field effects
Proximity facilitates the introduction of the proximity effect to encoded signals. The proximity effect can be an
important contributor to perceptions of nearness. At extremes, the proximity effect introduces a strong bass boost,
requiring careful handling. Distance is described in meters, and to prevent bass boost from getting out of hand,
the lower limit is set to 10 cm (0.1 m).
Nearfield compensation (NFC) is implemented via the
NearFieldCompensation plugin, and facilitates the reduction or removal of the proximity effect from encoded signals. NFC is usually used in conjunction with decoders to
compensate for the distance of loudspeakers on playback
in loudspeaker rigs with a smaller diameter. For the artist,
NFC can also be used to reduce the proximity effect found
in nearfield natural recordings.
Proximity and NearFieldCompensation respectively apply an integrator or high pass filter to the first order components of the encoded signal.23 The Proximity filter undoes NearFieldCompensation given the same distance argument.
4.4 Audio Effects and Ambisonics
Spatial information is encoded in balance and phase relations between the four channels of a B-format signal.
Processing one or more of the B-format channels using an
audio effect which modifies gain and phase relations will
most likely disrupt or otherwise distort the encoded spatial
information. In most cases this is un-desired. The ATK
provides a remedy through the use of a pair of plugins,
BtoA and AtoB.
Native B-format signals are processed using familiar audio effects by first converting to A-format. Figure 7 illustrates the appropriate network. The BtoA plugin decodes
23 A 1st order high pass filter is the reciprocal of a 1st order integrator.
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