Nexo NX241, Руководство пользователя

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PROTECTION INFORMATION NOTES
USER MANUAL LOAD1_80
DATE: 30/08/00 12:48
Protection Information Notes
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 VCA’s as in
our analogue circuitry). These VCA's are embedded into complex composite structures in order to
change their basic operation into frequency selective attenuation. This operation is similar to that of
a voltage controlled dynamic equaliser (VCEQ).
Each VCEQ and VCA is controlled by the synthesis of several signals issued 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 limiter (very short attack time) to prevent displacement from overriding
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 maximum safe operation
temperature, the VCA becomes active to reduce the Audio signal level and limit the effective
temperature to fall under the maximum usable value.
In order to avoid detrimental effects induced by very long release time constants coming from the
temperature detection signal (level being reduced for an extended period, « pumping » effects...),
the detection signal is modulated by another voltage integrated with faster time constants matching
the sound level subjective perception. This allows the controller to reduce the effective operation
duration of the temperature limiter and make it sound more natural, while the efficiency of
protection is fully preserved and operation thresholds are unaffected (kept as high as possible).