Pulse or digital communications – Equalizers – Automatic
Reexamination Certificate
2001-02-05
2004-08-17
Tse, Young T. (Department: 2634)
Pulse or digital communications
Equalizers
Automatic
C708S323000, C381S071110, C370S290000, C379S406060
Reexamination Certificate
active
06778601
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to adaptive audio equalizer apparatuses in which effects on acoustic signals due to transmission characteristics of the signal path are automatically compensated. More particularly, the present invention relates to an adaptive audio equalizer apparatus in which ringing is prevented, and to a method of determining a filter coefficient for the adaptive audio equalizer apparatus.
2. Description of the Related Art
When sound such as music is reproduced by an audio system, the sound waves undergo reflection and absorption during transmission over the signal path. The signal path has unique transmission characteristics, which alter the magnitude and phase of the sound waves differently for various frequency components, thereby producing degenerated sound. Thus, it is desired that the magnitude and phase of the audio signals be compensated on a frequency basis in accordance with the transmission characteristics of the signal path.
Adaptive audio equalizer apparatuses have been known as devices for automatically compensating, on a frequency basis, for the effects of the signal path on the magnitude and the phase of acoustic waves.
FIG. 5
is a block diagram of a conventional adaptive audio equalizer apparatus. The adaptive audio equalizer apparatus includes an input terminal
31
, a target response output unit
32
, an adaptive filter
33
, a speaker
34
, a microphone
35
, a calculation unit
36
, and an FIR (Finite Impulse Response) filter
37
. The adaptive filter
33
includes an FIR filter unit
33
a
, and a filter coefficient setting unit
33
b
for automatically setting, in accordance with the LMS (Least Mean Square) adaptation algorithm, a filter coefficient used in the FIR filter unit
33
a.
The input terminal
31
receives an audio signal from a CD player, FM tuner, etc. The target response output unit
32
delays the audio signal that is input via the input terminal
31
for a predetermined period, and outputs a target response signal. The adaptive filter
33
adjusts on a frequency basis the magnitude and phase of the signal that is input via the input terminal
31
. The speaker
34
outputs the signal fed from the adaptive filter
33
to the acoustic space to produce the corresponding sound.
The microphone
35
is provided at a listening point in the acoustic space, detects the sound waves produced from the speaker
34
, and converts the detected sound waves into an electric signal. The calculation unit
36
calculates the difference between the target response signal that is output from the target response output unit
32
and the signal that is output from the microphone
35
, and provides the result as an error signal. The FIR filter
37
approximates the transmission characteristics of the acoustic space C with respect to the signal path from the speaker
34
to the microphone
35
, compensates the audio signal in accordance with the transmission characteristics, and provides the output to the adaptive filter
33
.
The sound waves that are output from the speaker
34
to the acoustic space are altered in accordance with the transmission characteristics of the signal path while travelling to the microphone
35
. In order to match the signal that is output from the microphone
35
and the target response signal, the adaptive filter
33
continually updates the filter coefficient so as to minimize the power of the error signal. The adaptive filter
33
adjusts the magnitude and the phase of the audio signal on a frequency basis in accordance with the filter coefficient. Accordingly, effects of the signal path on the audio signal are compensated, so that sound of desired quality is produced.
In the adaptive audio equalizer apparatus as described above, if the frequency characteristics of the signal detected by the microphone
35
exhibit a dip as shown in
FIG. 6A
, the adaptive filter
33
, with regard to the filter characteristics (frequency-gain characteristics) thereof, forms a peak corresponding to the frequency of the dip, as shown in
FIG. 6B
, in order to achieve flat frequency characteristics of the signal that is output from the microphone
35
.
However, when a very high peak is required in the frequency characteristics of the filter, the peak value may not be in a range that the construction of the filter allows. Furthermore, it is often the case that the dip is not present in the frequency characteristics of the sound waves at positions other than where the microphone
35
is disposed. In that case, over-compensated sound with an excessive peak in the frequency characteristics is generated, causing what is called ringing, which sounds uncomfortable to the listener.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide an adaptive audio equalizer and a method of determining a filter coefficient therefor, which serves to produce desired sound through compensation in accordance with the transmission characteristics of the signal path, and to inhibit excessive compensation and therefore to inhibit ringing.
To this end, according to one aspect of the present invention, there is provided an adaptive audio equalizer apparatus having an adaptive filter including a finite impulse response filter unit, and a filter coefficient setting unit for determining a filter coefficient for the finite impulse response filter unit, the adaptive filter being for compensating an input signal that is input to the finite impulse response filter unit; a target response output unit for providing a target response signal; a speaker to which a signal that is output from the adaptive filter is supplied; a microphone for detecting acoustic waves produced by the speaker and converting them into an electric signal; a first calculation unit for calculating a difference between a signal that is output from the microphone and the target response signal, and for providing the result as an error signal; a delaying and multiplying unit for delaying an output of the adaptive filter and multiplying it by a scaling factor; and a second calculation unit for adding an output of the delaying and multiplying unit and the error signal, and for providing the result to the filter coefficient setting unit of the adaptive filter.
Preferably, the adaptive filter updates the filter coefficient so as to minimize the power of the signal supplied to the filter coefficient setting unit.
It is also preferable that the signal that is input to the finite impulse response filter unit is an aperiodic signal such as a white noise signal, so that there is no correlation between the signal that is input to the finite impulse response filter unit and the signal that is input to the filter coefficient setting unit.
According to another aspect of the present invention, there is provided an adaptive audio equalizer apparatus having an adaptive filter including a finite impulse response filter unit, and a filter coefficient setting unit for determining a filter coefficient for the finite impulse response filter unit, the adaptive filter being for compensating an input signal that is input to the finite impulse response filter unit; a target response output unit for providing a target response signal; a speaker to which a signal output from the adaptive filter is supplied; a microphone for detecting acoustic waves output from the speaker and converting them into an electric signal; a first calculation unit for calculating a difference between a signal that is output from the microphone and the target response signal, and for providing the result as an error signal; a noise signal source for generating a particular noise signal having no correlation to the input signal; a finite impulse response filter, in which the same coefficient as in the adaptive filter is set, for compensating the particular noise signal input from the noise signal source; a multiplying unit for multiplying an output of the finite impulse response filter by a scaling factor; and a second calculation unit for adding an output of the multi
Ise Tomohiko
Saito Nozomu
Alpine Electronics Inc.
Tse Young T.
LandOfFree
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