Solidyne 562dsp, User Manual, Page: 36 / 44

Page 36
SOLIDYNE 562
dsp - broadcast digital processor
is compressed, by means of fast compressors.
This is obtained, additionally to the division in
bands, using attack times appreciably elevated.
This allows that very short peaks of the audio
signal arrive freely to the following stage (peak
limiter) which eliminates them, but maintaining the
psychoacoustic sensation of power associated
with the audio peaks. This is related to the
Burst
Masking effect; recently discovered (Ref. 8).
2.2.9 DENSITY ECUALIZER
This technology operates in 5 bands modifying
the density of energy (instead the level) of each
band. Is formed by complementary filters of 24
dB/octave carefully designed to obtain a response
variation smaller than 0,2 dB. This built-in
equalizer offers an enormous flexibility.
By example: in the case of FM transmission; is
well-known that the use of
peak and shelving EQ’s
at the console has an adverse effect in the sound
quality, since the more a frequency band is
emphasized, grater is the action of the audio
compressor (previous to the transmitter) for that
band. Equalize a band implies to unbalance the
entire audio spectrum. The use of equalizers
previous to multiband compressors only has
sense if they work in narrow bands.
It doesn’t happen with DENSITY EQ, since its
action is coordinated with the following stages.
The boosting of a frequency band is translated
then in a correlative modification of the multiband
compressor threshold, to carry out the new
equalization. In this form, its action extends to the
range of sounds of very high intensity,
where the conventional EQ’s are inefficient,
due the excessive compression.
2.2.10 IM CANCELLED CLIPPER
The analogic era...
One of the techniques most well-known and used
to increase the energy of an audio signal is the
use of an audio clipper. With its evolution,
the analogical processors were optimizing this
technique to reduce the undesired effects caused
by the clipping. The problem appears because
when an audio signal is clipped, it generates a
high number of harmonic components and
intermodulation (IM).
The psychoacoustic studies have demonstrated a
high tolerance of the ear towards the harmonic
components, increased by the aural masking that
the multiband processor allows to obtain. In fact,
all the musical instruments, as much as
the synthesized ones, have a high amount
of harmonics, superposed to their
fundamental notes. Therefore adding harmonics
artificially, increases the wealth of the musical
timbre.
Totally opposed is the effect produced by IM
distortion. This type of distortion DOES NOT
EXIST in the musical instruments, and its effect is
highly disagreeable and irritating for the ear of the
audience. For the exposed reasons, considerable
efforts in research have been made to obtain
audio clippers in which the intermodulation is
reduced or even better, cancelled, of being
possible. System RIMCA, developed at the
laboratories of Solidyne, begins in a multiband
clipper, was a good solution used in analogical
units.
The digital days...
In digital audio, the adverse effects derivates of
the clipping, far from disappearing, becomes more
complexes.
DSP technology, since works with digital signals,
has limitations in to manage nonlinear processes
(as it is the case of the clipping, that it generates
signals with very steep slopes). These type of
processes generates harmonic components of
very high frequency that violate the condition of
Nyquist, degrading remarkably the final audio
quality.
The conventional digital clipping
techniques, used by other marks, work resampling
the signal at greater sample rates, thus to be able
to reduce (but to never eliminate) to the distortion
caused by the alias effect.
Although the result that is reached with these
techniques is acceptable, the “collateral effects”
caused by digital clipping are diminished, but not
eradicated completely. For this reason, the
laboratories of Solidyne continued developing new
techniques.
After long investigation and numerous tests;
562dsp took a step more in the field of digital
clippers; using a new system of 4 channels with
10 MHz of bandwidth (that is equivalent to 32 bits
with 20 Megasamples), that obtains that
absolutely clean and crystalline sound; free of
spurious and stridencies caused by aliasing.
For it we used a configuration like the FIG-4 in
which we have two cut channels. The channel of
serious and average, on the one hand, does not
cause intermodulation. The channel of high
frequencies, main person in charge of the IM, has
a filter that eliminates all IM below 900 Hertz