AUDIMAX 362 HD, Owner'S Manual

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3.2.3 PROCESSING STAGES
Stage 1: Peak Asymmetry Canceller
It is known that, by a particularity disposition of the
vocal cords, the sonorous emission that these
generate are asymmetric triangular pulses. The three
cavities that filter and shape these formants, to
obtain the vocal sounds, do not modify this intrinsic
characteristic of the human voice. All the spoken
word and still sung is strongly asymmetric.
This creates an important reduction of the energy of
the audio signal, particularly when it pass through a
compressor, because the compressor sets its
compression level for the greater peak, does not
concern its polarity. In this way, when a polarity is fit
to the 100%, the opposite polarity hardly surpasses
50%, due to the asymmetry. The fact that the music
sounds louder than the human voice, after pass
through a compressor is a phenomenon well known.
This is because the musical sounds are symmetrical,
whereas the human voice is not.
In order to correct this problem,
WITHOUT
INTRODUCING ANY ALTERATION AT THE SOUND QUALITY
,
peak asymmetry canceller is used.
This technique, based in a discovery of the Dr
Leonard Kahn, acquires international validity with the
work of Oscar Bonello, published at the Journal of
AES, Vol.24,5 in which it is described, for the first
time, the theory of its operation.
The peak asymmetry canceller is in essence an all-
pass network, a class of not minimum phase network.
That is: a network whose transference function has
zeros in the right semi plane. This network has a full
flat response to frequency; only its phase response is
function of the frequency. This phase rotation, which
must compliment very particular conditions, is
responsible of the peak symmetry of the audio
signals. Signals that by their nature are symmetrical
(like most of the musical instruments), are not
modified by this processor.
This processor, by itself, allows to increase between 3
and 5 dB the final power broadcast by your
transmitter (it is to say that it increases by a factor of
TWO the average power transmitted). Numerous
tests have been made in different countries, to verify,
in real conditions, these results.
Stage 2: Input Expander
The expander, previous to the compression
process, is an excellent resource to increase the
signal/noise ratio of the original program. This is
advisable, since the compression process, when
reducing the high level passages, consequently
increases the relative level of the passages of low
level, and therefore the noise. This is a forced
consequence of the compression process that has
particular effect in the increasing of the ambient
noise of the microphones. To avoid this, Solidyne
processors incorporate a linear expander, previous
to the compressor stage.
The concept of linear expander implies an expander
that works within a very wide range of signals,
below a threshold value. That means it always
expands within that range, for any level of signal.
That is to say that their curve of transference,
based on the input level, is a straight line (from
there the "linear" name). This implies that by each
10 dB that the input level reduces, the expander will
reduce, for example, 13 dB. This happens for any
input value, below the threshold. Then if the input is
reduced in 30 dB, the output will do it in 39 dB; that
is to say that the background noise has been
reduced in 9 dB. This way, the expander
compensates the increase of the noise that the
compressor, like undesired effect, will increase.
At this point, maybe you will be thinking that DOES
NOT HAVE SENSE to make an expander of the
signal and soon to compress it. You will think,
perhaps, that an effect cancels to the other. But it’s
not true for two reasons. First: the different attack
and recovery times. Second: multiband
compressors have elevated threshold, whereas the
linear expander has a very low threshold and a
linear behavior below the threshold. It means that
the actions do not cancel, because both processes
are not complementary .
The linear expander, to optimize its behavior, has
instantaneous attack and a fast recovery times.
Here is where the psychoacoustic concept “post-
pulse hearing inhibition” is used. This allows using
an expander with a quick recovery time, so that the
ear does not perceive it. The broadband
compressor that follows the expander has a very
slow recovery time. Therefore, with impulsive
signals, as the audio program, does not exist any
cancellation effect.
Another advantage of using a linear expander
previous to the processing is that an excellent
audible sensation of dynamic range is obtained. In
fact, recent studies have demonstrated that the
audible sensation produced by the level variations
of an audio signal, is related to the changes
happened in the first 50 milliseconds, and is little
dependent of the reached final value. This implies
that an expander in the short term is perceived like
a great dynamic range, whereas the power
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