Automatic equalizing system with discrete Fourier transform (DFT) or fast Fourier transform (FFT)

Claim 2

An arrangement for converting an electric signal into an acoustic signal, which arrangement comprises: - an input terminal for receiving an electric input signal, - an electro-acoustic transducer unit for converting an electric signal into an acoustic output signal, - an equalizing unit having a first and a second input and an output, for correcting the frequency characteristic of an electric signal applied to its first input under control of a control signal applied to its second input and for supplying the corrected electric signal on its output, - an amplifier for driving the electro-acoustic transducer unit, - pick-up means for detecting an acoustic signal and for generating an electric signal which is a measure of the acoustic signal, and - a frequency analyzing unit having a first input coupled to a point in the signal path between the input terminal and the first input of the equalizing unit, having a second input coupled to the pick-up means, and an output for supplying the control signal, which output is coupled to the second input of the equalizing unit, characterized in that the frequency analyzing unit is constructed to subject the previously digitized input signals x(t)and y(t) applied to its first and second input to a Fourier transform (for example a discrete Fourier transform or DFT or a fast Fourier transform or FFT), to provide Fourier-transformed versions F x (f) and F y(f) respectively of said signals, to calculate a transfer function from the said two versions, and to determine the control signal using the information in this transfer function.

Claim 3

An arrangement as claimed in Claim 1, characterized in that the frequency analyzing unit is constructed to ignore, in determining the control signal, the value of the transfer function for those frequencies f. for which the values of the transfer function are smaller than j times the average value of this transfer function, x being a number, for example 20 or greater.

Claim 4

An arrangement as claimed in Claim 1, characterized in that the frequency analyzing unit is further constructed to compute a coherence function γ 2 xy (f) between the two signals.

Claim 5

An arrangement as claimed in Claim 3, characterized in that the frequency analyzing unit is constructed to ignore, in determining the control signal, the values of the transfer function for those frequencies f. for which the coherence function γ 2 xy is smaller than a specific value, for example 0.8.

Claim 6

An arrangement as claimed in Claim 1, 2, 3 or 4, characterized in that the frequency analyzing unit is constructed to delay, prior to the application of the Fourier transform, the signal applied to its first input relative to the signal applied to its second input by a time interval which at least substantially corresponds to the propagation delay of the acoustic output signal of the transducer unit to the pick-up means via the optimum path.

Claim 7

An arrangement as claimed in Claim 5, characterized in that the frequency analyzing unit is further constructed to limit the length of the two signals by means of a time window, after the signal applied to its first input has been delayed and before the Fourier transform is applied, the length of the time window corresponding substantially to the time interval within which the pick-up means detect the direct acoustic output signal of the transducer unit and a plurality of early reflections of the acoustic output signal of the transducer unit.

Claim 8

An arrangement as claimed in Claim 1, characterized in that for the conversion of a stereophonic signal into an acoustic signal the arrangement is provided with a second input terminal for receiving

Claim 9

a second electric input signal, - a second electro-acoustic transducer unit for converting the second electric input signal into a second acoustic output signal, - a second amplifier for driving the second electro-acoustic transducer unit, that the equalizing unit is further provided with a third and a fourth input and a second output and is constructed to correct the frequency characteristic of the second electric signal applied to its third input under the influence of a second control signal applied to its fourth input and to supply the corrected second electric signal to its second output, that the frequency analyzing unit is provided with a third input coupled to apoint in the signal path between the second input terminal and the third input of the equalizing unit and a second output for supplying the second control signal, which second output is coupled to the fourth input of the equalizing unit, the frequency analyzing unit being moreover constructed - to perform a Fourier transform on the previously digitized input signal z(t) applied to its third input, - to provide a Fourier-transformed version F z (f) of this signal z(t), - to compute a second transfer function from the said transformed versions of the two signals z(t) and y(t), - to determine the second control signal using the information in this second transfer function, - to compute partial coherence functions γ 2 xy (f) between the signals x(t) and y( t ) and γ 2 zy.x ( f ) between the signals z(t) and y(t), and is constructed to ignore, in determining the first and the second control signal respectively, the value of the first and the second transfer function respectively for those frequencies f i for which the partial coherence function γ 2 xy.z (f) and γ 2 zy.x (f) respectively has a value smaller than a specific fixed value, said fixed value lying preferably between 0.7 and 1.0.

Claim 10

a second electric input signal, - a second electro-acoustic transducer unit for converting the second electric input signal into a second acoustic output signal, - a second amplifier for driving the second electro-acoustic transducer unit, that the equalizing unit is further provided with a third and a fourth input and a second output and is constructed to correct the frequency characteristic of the second electric signal applied to its third input under the influence of a second control signal applied to its fourth input and to supply the corrected second electric signal to its second output, that the frequency analyzing unit is provided with a third input coupled to apoint in the signal path between the second input terminal and the third input of the equalizing unit and a second output for supplying the second control signal, which second output is coupled to the fourth input of the equalizing unit, the frequency analyzing unit being moreover constructed - to perform a Fourier transform on the previously digitized input signal z(t) applied to its third input, - to provide a Fourier-transformed version F z (f) of this signal z(t), - to compute a second transfer function from the said transformed versions of the two signals z(t) and y(t), - to determine the second control signal using the information in this second transfer function, - to compute partial coherence functions γ 2 xy (f) between the signals x(t) and y( t ) and γ 2 zy.x ( f ) between the signals z(t) and y(t), and is constructed to ignore, in determining the first and the second control signal respectively, the value of the first and the second transfer function respectively for those frequencies f i for which the partial coherence function γ 2 xy.z (f) and γ 2 zy.x (f) respectively has a value smaller than a specific fixed value, said fixed value lying preferably between 0.7 and 1.0.

Claim 11

A frequency analyzing unit for use in an arrangement as claimed in anyone of the preceding Claims.