AUDIMAX 362 HD, Owner'S Manual, Page: 13 / 19

Chapter 3
Chapter 3
Theory of audio
Theory of audio
processors
processors
NOTE: in order to complement the study of this subject is
recommended to visit our WEB (www.solidynepro.com). In
the DEMO section there are a Power Point presentation
called Audio Processors. It has a complete Technical
Appendix that it analyzes how the audio processing
increases the coverage area of the FM stereo transmission.
3.1 A brief story...
From mid of the 1930 decade, when appears the
first compressors and expanders units, to the
present time, all chains of audio for broadcasting
incorporate devices whose function is to alter the
dynamic range of the sound. The advance of the
technology improves these devices during the ‘70s.
The compressors, expanders and audio limiters
were gaining in efficiency and complexity. In the
beginning, its main parameters (attack and recovery
times, thresholds, etc.) were fixed by design or by
the operator, through the device’s controls. In the
'70s, these functions begin to be automatic, based
on the characteristics of the audio signal, but having
at the same time a control on their action to be able
to customize the sound.
When five or more devices are grouped in a same
equipment, they begin to be denominated: AUDIO
PROCESSORS.
Since 1970, Solidyne introduces important
advances in this field, like the invention of a control
technique based on FET’s with guided gate (see
publication in Rev. Tel. Electrónica, September/70).
They follow diverse publications, having particular
international relevance the work published in
June/76 at the Journal of the Audio Engineering
Society , New York, U.S.A. where a new concept
was introduced, which persist to the present time:
PHSICOACUSTIC PROCESSING .
This new technique is the base for all the modern
audio processors for broadcasting use. The
necessity to process the phase to make
symmetrical the human voice waveform is another
one of the techniques that Solidyne has introduced
internationally (see mentioned article AES). Today,
Orban, Omnia, Aphex, etc use our ideas.
The concept of psychoacoustic processing is simple
in essence, although of complex accomplishment.
It consists of analyzing the way in which the sound
is perceived by our ear, considering diverse
investigations and developed acoustic models.
The brain uses to process audio data, the
information that arrives through 30,000 nervous
fibers, originating at the Basilar membrane. Then, it
will be possible to be computed the auditory
reactions and to be governed all the aspects of the
audio processing. This way, the electronic system
works transforming the original signal into another
one, of greater energy and greater quality of sound.
Then, it will be possible to reduce the dynamic
range of the audio signals, to eliminate the peaks,
and even, to clip them partially to increase its
energy.
If this were made directly, obeying to purely
electronic concepts of efficiency, the quality would
be degraded and the sound would be very poor. If,
however, the psycho acoustic concepts are applied,
and factors like the aural masking, the pre and post
pulse inhibitions, the Hass effect, the reflections at
the ear pinna, the aural models of Dr Karjalainen,
etc; it will be possible to create a new generation of
processors that allow to important increases of
energy, increasing at the same time the sensation
of “Perceived Sound Quality”.
At the light of these discoveries the
psychoacoustics processing was defined in these
terms:
PHSYCO ACOÚSTIC PROCESSING is the
technique that allows to increase the range of
AM or stereo FM transmission, by increasing
the energy of the audio signal, and also
increasing the “quality of sound” perceived by
the listener.
Nevertheless, it is fundamental throughout this
process, to maintain very low the audio distortion
produced by harmonic and IM components. This
happens because the psychoacoustic processing
MODIFIES the waveform of the complex signal of
audio, but IT DOES NOT DISTORT IT. Since the
distortion concept, in this context, implies the
existence of a sound that offends the ear, sounding
unnatural.
This is because the psychoacoustic processing
obtains that the ear accepts like of better quality
than the original one, to certain modifications of the
waveform. But it does not mean an "anesthesia" to
the ear, to avoid perceiving the distortions due to
deficiencies in the quality of the electronic circuits of
the processors.
Considering that to obtain an excellent processing
is necessary, at the moment, to use between 7 and
10 stages of processors, the distortion of each
stage must be smaller than 0.01%. Greater
distortion values, will lead inexorably to a
degradation of the sound quality. You must
remember that has been demonstrated (Journal of
AES, Vol. 29,4,p.243), that is possible to measure
distortions of 0.05% through a common
loudspeaker (distortion bigger than 3%). This
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