Nexo GEO D - Manual (Page 68)

Page 68/97 NEXO NX242 D IGITAL C ONTROLLER FOR GEO D AND GEO SUB
3 R+L
2 L 1 R
Voice Coil Temp. ChassisTemp
Command
PHYSIO 1
Command
PEAK SIMULATION
PATCH
Disp.
Command
Voice Coil Temp.
ChassisTemp.
LF TEMPERATURE
0
INPUT VU-METER
0
A/D
EQ
A/D
DAC
MUTE
EQ
EQ
SHELVING SHELVING
A/D
EQ
Command
HF TEMPERATURE
VCA
FALLBACK AMP VOLTAGE
Signal
AMP POWER
AMP GAIN
Command
displacement
DISPLACEMENT 1
DELAY 1
ATT/REL.
5
SENSE 1
2 L 3
R+L 1 R
INPUT VU-METER
INPUT R INPUT L
OUTPUT CHANNEL 4
NX242 Digital TDcontroller Partial BLOCK DIAGRAM
Analogue path Digital Audio Digital servo & protections
HEADROOM
6.1.3 Protection
VCAs and VCEQs
Each channel has its own simulation and protection process.
Each audio channel contains a combination of controlled gain stages
(let's call them VCAs as in analogue circuitry). These VCAs are
embedded into complex composite signal chains that adapt change their
basic operation into frequency selective attenuation, similar to an analog
voltage controlled dynamic equaliser (VCEQ).
Each VCEQ and VCA is controlled via synthesis of several signals from
the various detection sections. That synthesis is in fact the envelope of
those signals, with an optimised release and attack time for each VCEQ
and VCA (depending on its frequency range and the cabinet selected).
Displacement Control
The Sense input signal is sent to a shaping filter producing a signal
whose instantaneous amplitude is proportional to the voice coil
excursion. This signal, after rectification, is compared to a preset
threshold matching the maximum usable value, as determined from
laboratory measurements. Any part of the signal exceeding the threshold
is sent to the VCEQ control buffer while the VCEQ acts as an
instantaneous (very short attack time) limiter to prevent displacement
from exceeding the maximum permissible value.
Temperature Control
Each sense signal is fed into a shaping filter (one per transducer), each
one producing a signal proportional to the instantaneous current flowing
into the voice coil of the transducer. After rectification, this signal is
integrated with attack and release time constants equivalent to the
thermal time constants of the voice coil and chassis, producing a voltage,
which is representative of the instantaneous temperature of the voice
coil.
When this voltage reaches the threshold value corresponding to the
driver’s maximum safe operating temperature, the VCA becomes active
to reduce the NX242’s output signal level until the effective temperature
falls below the maximum usable value.
In order to avoid detrimental effects induced by very long release time
constants produced by the temperature detection signal (system output
being reduced for an extended period, “pumping” effects, etc.), the
detection signal is modulated by another voltage integrated with faster
time constants that match the subjective perception of sound level. This
allows the controller to reduce the effective operating duration of the
temperature limiter and make it sound more natural, while the efficiency
of protection is fully preserved and protection thresholds are kept as high
as possible.
Physiologic Dynamic Control
The Physiologic Dynamic Control is intended to avoid unwanted effects
produced by long attack time constants. By anticipating the operation of
the temperature limiter, it prevents a high level audio signal appearing
suddenly, then being kept up for long enough to trigger the temperature
limiter. Without this, a rough and delayed gain variation would result that
would be quite noticeable and unnatural.