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critical bands, it must also have finite temporal discrimination. When two or more versions of a sound arrive at the
ear, provided they fall within a time span of about 30 ms, they will not be treated as separate sounds, but will be
fused into one sound. Only when the time separation reaches 50-60 ms do the delayed sounds appear as echoes
from different directions. As we have evolved to function in reverberant surroundings, most reflections do not impair
our ability to locate the source of a sound.
Clearly the first version of a transient sound to reach the ears must be the one which has travelled by the shortest
path and this must be the direct sound rather than a reflection. Consequently the ear has evolved to attribute
source direction from the time of arrival difference at the two ears of the first version of a transient. This
phenomenon is known as the precedence effect. Intensity stereo, the type of signal format obtained with coincident
microphones or panpots, works purely by amplitude differences at the two loudspeakers. The two signals should be
exactly in phase. As both ears hear both speakers the result is that the space between the speakers and the ears
turns the intensity differences into time of arrival differences. These give the illusion of virtual sound sources.
A virtual sound source from a panpot has zero width and on ideal loudspeakers would appear as a virtual point
source. Figure 4.46 ( a) shows how a panpotted dry mix should appear spatially on ideal speakers whereas (b)
shows what happens when artificial stereo reverb is added. Figure 4.46 ( b) is also what is obtained with real
sources using a coincident pair of high-quality mikes. In this case the sources are the real sources and the sound
between is reverb/ambience.
Figure 4.46: Compression is less effective in stereo. In (a) is shown the spatial result of a 'dry' panpotted mix. (b)
shows the result after artificial reverberation which can also be obtained in an acoustic recording with coincident
mikes. After compression (c) the ambience and reverberation may be reduced or absent. (d) Reverberation may
also decay prematurely.
When listening on high-quality loudspeakers, audio compressors change the characteristic of Figure 4.46 ( b) to that
shown in (c). Even at high bit rates, corresponding to the smallest amount of compression, there is an audible
difference between the original and the compressed result. The dominant sound sources are reproduced fairly
accurately, but what was is most striking is that the ambience and reverb between is dramatically reduced or
absent, making the decoded sound much drier than the original. It will also be found that the rate of decay of
reverberation is accelerated as shown in (d).
These effects are heard because the reverberation exists at a relatively low level. The coder will assume that it is
inaudible due to masking and remove or attenuate it. The effect is apparent to the same extent with both MPEG
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