2.Operating Modes; 2.1. Pressure Model Mode; 2.2. Virtual Speaker Mode - powersoft IpalMod User Manual

15 : 2.Operating modes

The DPC technology can be exploited in two operating
modes which differ in signal processing path and applica-
tion scenarios.

15 : 2.1. Pressure Model Mode

The amplifier's output signal is modified in real time as
a result of the combination of the input audio signal and the
feedback loop.
The alteration is applied to the input stage so to minimize
the difference between the desired output and that meas-
ured and reported by interpolating two the feedback loops.
15 : 2.1.1. Pressure control (pressure feedback loop)
This loop brings the pressure level measured at the readi-
ating sound field, back to the input stage. This is achieved
by feeding the pressure sensor measurement through a first
order low pass filter (LPF) and a linear amplifier.
The parameters that can be set by the user for this feed-
back loop therefore are:
f the loop gain, expressed in dB and limited by the sys- f
tem's stability, defining the strength of the feedback
effect;
f the LPF bandwidth, defining the range of frequency at
f
which this feedback loop is active.
It is important to set the loop gain carefully, as too high
a value will bring the system to instability. This results in a
loud "ship's horn" sound which can potentially damage the
system if not silenced after a short time. This can be done
by lowering the feedback loop gain until the sound stops.
The entire pressure feedback loop can be bypassed by
entering a loop gain value of -100 dB and an Re value of 0
(see below for Re): with these value the loop's effect will be
disabled.
15 : 2.1.2. Impedance cotrol (current feedback loop)
This loop brings back a voltage signal proportional to
the current present at the output stage to the system's
input. The signal taken from
the output is first filtered by
a first order low pass filter.
Following this step, the
current/voltage translation
is carried out by a virtual
resistor named "Re".
The importance of this
series resistor relies in the
difference between tradi-
tional transducers and IPAL
compatible speakers. IPAL
speakers have an extremely
low impedance. This means
Input
Input
DSP
signal
gain
FIGURE 24: Pressure model feedback loop.
that they manifest a violent, extremely high Q resonant
peak which can result in an unnatural sound reproduction.
By adding a virtual series resistor Re, the entire system's
resonant peak will be slightly flattened and widened in a
way that mimic's a traditional speaker's lower Q resonance.
This resistor will however not affect the power transfer
from input to output as this is not a "real" physical resistor
subject to ohmic heating.
The parameters the user can set for this feedback loop
therefore are:
f the "added Re" value, defining the additional series f
resistor inserted to simulate a traditional speaker be-
havior. This is usually a small number, which aims to
return the apparent loudspeaker resistance to a tradi-
tional 2-4 Ohm value;
f the LPF bandwidth, defining the range of frequency at f
which this feedback loop is active.

15 : 2.2. Virtual Speaker Mode

The virtual speaker mode allows the user to enter a set
of Thiele Small parameters that describe a transducer: this
can be either a physically existing loudspeaker the user
would like to emulate, or a non-existing, ideal transducer.
The DSP uses these parameters to drive the amp module
so that the entire system performs exactly as if the cabinet
houses the wanted transducer.
The system automatically adapts its internal process-
ing using information from the pressure sensor to virtually
reproduce the behavior of the user defined transducer. The
way this is achieved is practically user-transparent, meaning
that the user can "interact" with the system only by chang-
ing the virtual speaker's parameters and not the system's
internal loop parameters.
Setting the Differential Pressure Control | 17
Current feedback loop
Re LPF
Transducer
Output
stage
Pressure
sensor
Feedback
LPF
gain
Pressure feedback loop
Radiating
sound field