Nexo GEO T Series CD18, User Manual

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Page 38/63 NEXO NX242 D
IGITAL
C
ONTROLLER FOR
GEO T
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.
The Physio control voltage acts independently on the VCA with its operation threshold slightly (3 dB)
above that of the temperature limiter and a low compression ratio; its optimised attack time constant
allows it to start operating without any subjectively unpleasant transient effects.
Peak Limiter
The peak limiter primary function is to avoid massive clipping of the amplifier, which can have some
very audible artefacts and in some cases may be damaging to the loudspeakers. Modulation of the
amplifier’s supply voltage rails can create very low frequencies or high-frequency, high level harmonics.
Since these occur after the NX242 in the signal path, they are not filtered by the TDcontroller circuitry.
The threshold of the peak limiter is set by the user to match the amplifier’s clipping point.
The second function of the peak limiter is to avoid huge amounts of power being sent to a driver. Each
driver is protected against overheating and overexcursion, but there could be other failure modes that
cannot be predicted by simulation (especially mechanical damage to the cone). Each driver is specified
for a certain power handling and a factory set peak limiter threshold is tuned to avoid any abuse.
4.2 Cardioid LF and VLF
Cardioid LF and VLF is one of the key advances of the GEO T/CD18 system. This feature is only
achievable with a total control of the DSP chain. Although the basic concept as presented in audio text
books is relatively simple, producing a cardioid pattern from a loudspeaker without a major loss of
efficiency requires advanced DSP processing.