Solidyne 462dsp, User Manual

Page: 29 / 38

SOLIDYNE 462
dsp - broadcast digital processor Page 29
2.2.5 GATED AGC
This stage constitutes a digital compressor AGC
type (Automatic Gain Control) with its constants
(slow) based on the program signal. This
adjustment is made having in mind the
psychoacoustics criteria before explained. This is
very important since the AGC is a wide band
compressor; the ear tends to note its action due to
the modulation that bass frequencies produce in
the highs. In order to avoid it, the AGC time
constants are carefully controlled. Still more; we
cannot speak of attack or recovery times in strict
form, but a
curve of control, s ince the way of the
attack or recovery ramp is controlled according to
these criteria
Otra característica importante de este compresor
es que su recuperación es GATILLADA. Es decir
que solamente acciona en presencia de señal de
programa. Cuando el nivel de programa cae por
debajo de cierto umbral con respecto al nivel de
salida del AGC, la recuperación es cancelada y el
compresor permanece exactamente con la misma
ganancia que tenía antes de caer la señal. Esta
característica permite un funcionamiento más
natural del procesador, puesto que evita que en
las pausas entre los diálogos, por ejemplo, se
note la 'respiración' del AGC (incremento en el
ruido o soplido de fondo). O que al terminar un
tema musical, se escuche un aumento del ruido
de fondo antes de comenzar el tema musical
siguiente, etc.
2.2.6 MULTIBAND COMPRESSOR
The purpose of the multiband compressors is to
increase the perceived loudness sensation. The
human voice and music will sound more solid,
with better dynamic balance. Still more, the
increase of the average energy of the audio signal
is very considerable, increasing the coverage area
of the radio for A.M. and FM transmissions (for
more info please visit www.solidynepro.com).
Multiband technology bases on the studies of
Stevens (ref 1.2.3) about the loudness of each
band frequency and the studies of Zwicker (ref
4) About its relation with the Critics Bands of the
human ear. The integration time of the ear to
reach the maximum loudness is of the order of
200 milliseconds (ref 5). This time must carefully
be incorporated to the controls of the loudness
compressors, to obtain the desired effect. The
ear will perceive a greater loudness when the
band compressors increase the relative loudness
level.
The processor Solidyne 462dsp has digital
frequency splitters with Butterworth filters of 24
dB/octave that divides the program signal in
five frequency bands: lows; low-middle; high-
middle; highs and super-highs. The sub bands
high-middle and highs works in combination with
the 5
th
band super-highs, processing the highest
frequencies of the audio spectrum. This 5
th
band is
of great importance since take incidence over the
peaks of the signal, one of the most critical
processes carried out by any audio processor.
The controlling of the dynamic range using band
frequencies offers great advantages:
1.
To increase the total energy, by the use of fast
compressors for bass and ultra-fast for treble. If
the bands were not divided, the compressors
with so fast recovery time would produce a
disagreeable sound effect; the percussion of low
frequencies would modulate the high notes. And
the high notes of an instrument would as well
modulate the low tones, of a violoncello, for
example. The increase of the energy increases
the cover area, that is to say, the reach of a stereo
FM station. See demonstration in paper of O.
Bonello, AES Journal, New York, March 2007
2.
Allows to increase the perceived loudness. This
is because most of the modulation capacity of a
transmitter or audio amplifier in general, playing
Pop music, is generally used by low frequencies
signals, below 160 Hz. Nevertheless this
information contributes very little to the loudness
sensation, due to the reduced sensitivity of the ear
for those frequencies. Therefore is desirable to
increase the level of the mids and highs
frequencies. But this cannot be obtained by
simple equalizing, since the sound balance would
be destroyed. The compression in separated
bands allows to increase between 6 and 12 dB the
energy for high frequencies without altering the
tone balance. In fact, the frequency response
continues being flat.
3.
Processing completely eliminate the "flat sound"
sensation, perceived when a sonorous material
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