Arbitration adjustment for acoustic reproduction systems

An acoustic production system having multiple signal sources and a power amplifier driving an acoustic transducer such as a loudspeaker includes an arbitrator in a power limiter circuit to selectively limit the input level of one of the sources to the power amp in response to an overdrive signal from the amplifier. The input level to the power amplifier is automatically reduced in response to the overdrive signal until a predetermined level of limiting has been attained. Once the predetermined level of limiting is reached, the arbitrator outputs a second overdrive signal to prompt additional limiting of the power amplifier output. Preferably, the arbitrator is included in an interior active noise cancellation module combined with an audio entertainment system for a passenger compartment of a motor vehicle. The audio system also preferably separates the audio reproduction into high frequency and low frequency branches, whereby the arbitrator acts upon the signals output in the frequency ranges of active noise cancellation and the bass output of the audio entertainment system that are most likely to overdrive an amplifier.

FIELD OF THE INVENTION 
The present invention relates generally to acoustic production systems, and 
more particularly to a DSP system arbitrating audio reproduction and 
active noise cancellation for selectively limiting power amplifier output. 
BACKGROUND ART 
To improve acoustic reproduction systems, digital signal processing (DSP) 
systems have been used to control processing of input signals to generate 
drive signals for the acoustic production emitted by a loudspeaker. 
One previously known form of acoustic production system comprises an active 
noise control (ANC) production system. Such a system may be conveniently 
employed in the passenger compartment of the motor vehicle to compensate 
for the noise of the power train as well as road noise generated as the 
vehicle travels. In such a system, a sensed signal is input to an adaptive 
filter that generates an opposing phase signal that cancels out the sensed 
primary noise signal. 
Another known production system often used in vehicles comprises an audio 
entertainment reproduction system. Since both the audio entertainment 
system and an interior active noise cancellation system operate to affect 
the sounds perceived by passengers in the passenger compartment, it would 
readily appear desirable to consolidate both systems using the same output 
transducer, for example, one or more loudspeakers, within the passenger 
compartment to perform both functions. Moreover, the structural simplicity 
of DSP components and the ease of functional changeover by software 
modification, renders the combination of noise cancellation and audio 
reproduction a most practical combination. 
Nevertheless, the use of a single power amplifier to drive the transducers 
in response to both active noise cancellation signals and audio 
reproduction signals may overdrive the power amplifier and substantially 
disrupt the signal being delivered to or emanating from the loudspeakers. 
In order to avoid distortion in the amplified signals and acoustic response 
when signal peaks of the output of the overdriven power amplifier are 
clipped, it has been known to detect the clipping of the output signal and 
then limit the input to the amplifier or the gain of the amplifier in 
order to eliminate the clipping and the resultant distortion. For example, 
U.S. Pat. Nos. 5,001,440, 5,068,620, 4,048,573 disclose apparatus for 
detecting and limiting clipping of amplifier outputs. U.S. Pat. No. 
4,912,424 also recognized that wideband input signal limitation may be 
undesirable because all frequencies may be attenuated, although only 
signals in the bass frequency range generate the clipping output. Thus the 
patent disclosed a process to limit the bass range and the wide band 
output separately through a bass limiting loop as well as the volume or 
treble limiting loop. Moreover, bass boost is generally used in an 
automotive environment to mask low frequency road and engine noise, making 
clipping more likely in the audio signal environment of a motor vehicle. 
In addition, interior active noise cancellation in motor vehicles involves 
low frequency or bass range signal generation. In such systems, it has 
been known to employ a power limiter loop. For example, Digisonix DX Model 
47 active noise controller processes a correction signal and alters it 
with a system modeling characteristics filter, including step sizing 
updates to the adaptive filter, as well as limiting the secondary noise 
signal or cancellation output to be introduced into the power amplifier. 
However, such a system is limited to the active noise cancellation circuit 
and does not limit the input that may be provided by another signal source 
input to the power amplifier. Moreover, it will be disadvantageous to 
limit both signal sources at the same rate, especially since the 
limitation of an output from an audio entertainment reproduction source 
may be subjectively more noticeable to the occupants of the motor vehicle 
than the active noise cancellation output of the transducer. 
SUMMARY OF THE INVENTION 
The present invention overcomes the above mentioned disadvantages by 
providing an arbitrator for a multiple source input for a power amplifier 
that drives acoustic production transducers. The arbitrator directs one of 
the multiple sources to deliver limited input to the power amplifier 
before limiting output of the power amplifier as driven by the input of 
the other source. Preferably, distortion produced by the clipping of the 
output signal from the power amplifier is sensed to generate a clip detect 
signal introduced to the ANC module where it is processed to limit the 
output of the ANC module to the power amplifier input. In addition, a 
return clip detect signal is preferably coupled from the ANC module to the 
power amplifier so that when the ANC module has reached a predetermined 
limit of cancellation signal input to the power amplifier, a signal for 
additional limitation of the power amplifier output due to input from 
other sources is processed. 
In the preferred embodiment, an active noise cancellation module receives 
the clip detect signal from a power amplifier employed in a bass 
reproduction system of an acoustic production system of a motor vehicle. 
Moreover, the preferred ANC module includes adaptive filters for 
generating a cancellation signal from a reference signal and for modeling 
the error path from the output of the control system to the error signal 
input, for example, through the amplifier, loudspeaker and microphone 
sensing the noise to be cancelled. Preferably, the output from the active 
noise cancellation module is generated by subtracting a noise signal from 
a control signal. The control signal is generated by continuous on-line 
modeling in the first adaptive filter. The cancellation output signal 
compensates for the system noise by the second adaptive filter that 
employs continuous on-line modeling of the errors introduced by the 
transduction path. A power limiter receives the control signal for 
filtering through a copy of the error path modeling filter, and its output 
is added to the measured error signal generated from the noise sensing 
microphone to continuously update the first adaptive filter producing the 
control signal. The power limiter includes the arbitrator, preferably 
implemented by digital signal processing in accordance with an algorithm 
implemented in the controlling software. 
Thus, the present invention provides a power limiter in a signal source 
module employing an arbitrator for limiting the module's input to a power 
amplifier serving multiple sources before limiting the power output due to 
inputs from the other sources. In addition, the present invention provides 
an active noise cancellation module which produces a limited cancellation 
signal in response to a clip detect indication from a power amplifier. The 
module also provides additional processing to automatically adjust the 
attenuation factor as periodically necessary, as well as providing a clip 
detect signal for additionally limiting the output of the power amplifier 
to the extent that a predetermined level of limitation has been imposed 
upon the active noise cancellation signal from the module. 
Furthermore, the present invention provides an acoustic production system, 
preferably including audio sources as well as an active noise cancellation 
source, in which at least a portion, such as a low frequency or bass 
reproduction portion of the audio system, is segregated as having a high 
level of distortion generating sources and including a power limiter, for 
example, in a segregated generating source. Furthermore, the combination 
of an audio reproduction system with an active noise cancellation module 
utilizing a single power amplifier according to the present invention 
restricts a subjectively annoying acoustic production by imposing a 
maximum limitation upon the active noise cancellation input to the power 
amplifier before affecting the audio entertainment signals driving the 
loudspeakers in the acoustic production system. Moreover, the system of 
the present invention enhances the stability of the ANC controller to 
improve integrity and control of the acoustic output from the 
loudspeakers. Furthermore, the system does not require substantial 
additional hardware that generally causes packaging problems generally 
associated with additional equipment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring first to FIG. 1, an acoustic production system 10 is there shown 
comprising an audio reproduction system 12 and an interior active noise 
cancellation system 14 in combination. The audio reproduction system 12 
includes a high frequency power branch 16 as well as a low frequency power 
branch 18 operating in response to an audio source 20. The audio source 20 
comprises one or more entertainment media controls, such as a radiowave 
receiver, a compact disc player, a cassette tape player unit or the like, 
providing stereo transmission of left and right signals to both high pass 
and low pass filters for entry into the high frequency power branch 16 and 
low frequency power branch 18, respectively. In addition, the low 
frequency power branch 18 includes an active noise cancellation (ANC) 
module 22. 
Nevertheless, other acoustic production system designs can use the 
arbitration scheme of the present invention. For example, as shown in 
phantom line in FIG. 1, each of the left channel signal 19 and the right 
channel signal 21 may be directed to a separate amplifier 29 and 
transducer combination without band pass filtering. In such a design, the 
ANC module 23 may provide an input to each of the separate channel 
amplifiers. As will be described in greater detail throughout the 
application, the ANC module 22 or 23 produces cancellation output 24 which 
is input with the output of the audio source 20 for introduction into a 
power amplifier, for example, the power amplifier 28 of the low frequency 
power branch 18 in the preferred embodiment, or the separate channel 
amplifiers 29 shown in phantom line in FIG. 1. The power amplifier 28 
provides a drive signal 30 to one or more speakers representing the 
transducer 32, for example, the woofer speaker for acoustic production of 
the lower frequency signals. 
The ANC module 22 interacts with the power amplifier 28 by means of a clip 
detect signal 34 generated in a conventional manner with an audio power 
amplifier I.C. such as with a TDA 7360 to introduce a binary signal 
representing the clip detect response into the ANC module 22. Similarly, 
the ANC module 22 interacts with the power amplifier 28 to signal when a 
predetermined or maximum level of signal power limitation is imposed upon 
the output 24, but without otherwise responding to the clip detect signal 
34. As will be discussed hereinafter, the ANC module 22 generates the 
secondary source signal or cancellation output 24 in response to an 
adaptively filtered primary noise signal, for example, a reference signal 
38, adjusted by an adaptively filtered and sensed error signal 40. 
As best shown in FIG. 2, the ANC module 22 produces output in response to 
the primary noise signal, such as the reference signal 38 that is 
synchronized to the engine rotation, or any other signal correlated with 
the noise to be affected by the cancellation signal to be generated by the 
module 22. Reference signal 38 is introduced into the adaptive filter 42 
used to generate an output 44 to be used in the canceling signal 24. The 
cancellation signal 24 includes the output 44 but has a noise 46 added to 
it that is utilized in the modeling of the system error path. The noise 46 
comprises a pseudo-random binary data stream that is used as a signal 
source to model the error path and obtain the error path model filter 48. 
The filter 48 creates the transfer function C(z) that represents the error 
path occurring through the amplifier 28, transducer 32 and the transducer, 
for example, one or more microphones, generating at least one measured 
error signal 40. 
Each of the adaptive filters 42 and 48 employ coefficients whose values are 
updated every sample period as may be determined to be desirable. C(z) is 
updated every sample period using the noise signal 46 and a signal 50 
representing the measured error signal 40 with the noise effects modeled 
by filter 48. These signals are processed with a predetermined step size 
to update filter 48 in appropriate empirical increments that avoid 
extremely slow filter response on one hand, and uncontrollable filter 
variation on the other hand. Filter 42 is updated in a similar manner with 
inputs comprising an error signal 60 and a filtered signal 54 resulting 
from the reference signal 38 being filtered by a copy 78 of the filter 48, 
and likewise processed according to a predetermined step size. The error 
signal 60 results from error signal 50 being added to the output of a 
power limiter 62 determining the amount of energy of control signal 44 
transmitted through the digital error path. The gain of the power limiter 
62 before filtering by a copy 68 of filter 48 is K, whereby the value of K 
determines the amount of energy of output signal 44 that goes through the 
digital error path. The control signal 44 is amplified by the gain K to 
generate a limiting control signal 74. As a result, the overall power 
amplifier output is attenuated at about 1/(K+1). 
Referring now to FIG. 3, the constant K used in determining the gain of the 
power limiting control 62 producing signal 74 is a function of the clip 
detect signal 34 (CD1). As best shown in FIG. 4, the constant K as a 
function of the clip detect signal 34 is determined by an algorithm 
logically updating whether only the cancellation output 24 from the active 
noise cancellation module 22, or both the cancellation output 24 and the 
first source signal 26 generated from the audio entertainment system 12, 
are to be affected by compensation for overdriving the power amplifier 28. 
Whenever the clip detect signal 34 is off, but the constant K is greater 
than 0, the constant K will be incrementally decreased by a predetermined 
amount .DELTA. and the clip detect output 36 (CD2) will be maintained at 
0. If the clip detect signal 34 is on, but the constant K is not yet at a 
predetermined maximum value, K will be incrementally increased by a 
predetermined amount, for example, an amount .DELTA., while the clip 
detect output 36 from the ANC module 22 to the power amplifier 28 remains 
at an off or 0 level. In either event, the amount .DELTA. that K is 
changed may also be made variable depending on the application. For 
example, .DELTA. may need to be variable to assure stability of power 
limiting near the limit of amplifier signal clipping or changed between 
increasing or decreasing K. Nevertheless, once K has reached a 
predetermined maximum level, and the amplifier 28 continues to signal that 
the clip detect signal 34 is on, the clip detect signal 36 will be 
transmitted from the ANC module 22 to the power amplifier 28. 
The clip detect signal 36 is an indication of the need for additional 
limiting of the output of the power amplifier 28. The limitation may be 
provided by limiting the first source signal 26 input to the power 
amplifier 28. An example of a circuit for limitation of the audio 
reproduction input which can be used to avoid a clipped amplifier output 
is shown in U.S. Pat. Nos. 4,912,424, 4,978,926 and 5,001,440, 
incorporated by reference in this disclosure, and diagrammatically shown 
at feedback gain loop 25 in FIG. 1. An alternative example of amplifier 
output limitation is shown in the variable-gain amplifier of U.S. Pat. No. 
5,175,508, also incorporated by reference in this disclosure. 
As a result, the acoustic production system 10 of the preferred embodiment 
improves performance by restricting the output in the frequency range most 
likely to cause clipping of the output from the power amplifier. Moreover, 
the limitation may be controlled in a manner which is least noticeable or 
objectionable to occupants of the area in which the system is employed. As 
a result, the invention is particularly well adapted for use in motor 
vehicle passenger compartments in conjunction with both audio 
entertainment and interior active noise cancellation systems. Furthermore, 
the arbitrator of the present invention is readily incorporated in the 
motor vehicle without extensive equipment additions or additional power 
source requirements. Moreover, the invention substantially enhances the 
versatility and utility of DSP components that may already be employed in 
the vehicle, for example, in a DSP audio reproduction system. 
Having thus described the present invention, many modifications thereto 
will become apparent to those skilled in the art to which it pertains, 
without departing from the scope and spirit of the present invention as 
defined in the appended claims.