HK Audio CDR 210 F - Manual (Page 62)

Manual
COHEDRA
® & COHEDRA ® Compact
1 The COHEDRA ®
Controller Concept
The performance of a conventional controller is
confined to providing:
• crossover functions
• equalization
• time alignment
• limiting functions protecting against power amp
and speaker overloading
Current digital controllers compute frequency equa-
lization using IIR filters, which are however unable
to equalize phases. At present, some line arrays use
purely mechanical time alignment for high frequen-
cy drivers, but not for midrange woofers.
An example of filtering using an IIR controller or
analog EQ follows:
When equalizing frequency response as shown in
the example above, the 100 Hz frequency suffers a
delay of 13.1 ms in comparison to the first overtone
at 200 Hz! (see Fig. 2). If this frequency is rende-
red by an 18" woofer, delay time increases again
before the signal reaches the ear! This means the
fundamental and its overtone are no longer in sync,
causing dynamic distortion that degrades the sonic
image's natural homogeneous sound.
To ensure natural response, the speaker system
must be processed with phase and time alignment
as well as real phase equalization.
The functions of the HK AUDIO ® Digital Field
Controller (DFC) go far beyond those of a standard
controller:
1.1 Frequency and Phase Equa-
lization Using FIR Filter Tech-
nology
FIR filter technology lets you equalize a sound
systems' phase and frequency response of
(loudspeakers and power amp!) separately. Unlike
IIR filters, FIR filters do not consist of a specific
number of separately computed filter elements. In-
stead, they contain a complete sampled copy of the
function required for equalization.
The entire filter is recomputed every time a filter
setting is modified.
The DFC's controller concept is geared specifi-
cally to avoid unsatisfactory group time results
of conventional digital controllers using IIR filter
technology such as shown in the above example.
Figure 1: Equalization at 100 Hz and 1 kHz
Figure 2: Resultant group time of IIR filter processing
Figure 3: Phase characteristic with and without
phase equalization
Figure 4: The Overshoot Limiter's mode of operation
1.2 3-Way Virtual Crossover
The crossover splits the input signal into three
frequency bands. The selected filter equalizes the
entire sound system's frequency and phase response.
This includes all components following the DFC in
the signal chain such as amps, passive crossovers,
and speakers.
1.3 The DFC Limiter in
Combination with the VX 2400
RMS/ Peak Limiter and Thermo Limiter
The DFC is equipped with temperature and RMS
limiters for all three frequency bands. These forward-
looking features anticipate the amount of power
routed to the amp's outputs. In the event of im-
pending electrical, mechanical or thermal overload,
they cut output power to allowable levels for the
connected speaker systems.
Overshoot Limiter
The VX 2400 power amp can produce intermittent
peak output levels ranging up to 2,000 W per channel.
For this reason, the overshoot limiter also takes the
duration of an impending overload into account
alongside its amplitude. This ensures that the DFC
limiters exert little or no influence on the amplitude
and duration of brief percussive impulses with high
amplitude but very brief durations. This, in turn,
clearly extends the sound system's useful dynamic
range. Beyond that, the rendered audio signal
retains its natural characteristics despite the use
of limiters.
1.4 Specific Speaker Filters
The DFC features an extendible database archiving
functions for equalization, phase correction, and
limiting different HK AUDIO
® speakers and sound
reinforcement systems in combination with the
VX 2400 power amp (see the chapter entitled
Controllers and Controller Software).