Digital filters

Claim 2

An adaptive digital filter comprising means (10) for receiving and storing input samples (x), means (11) for forming during each sample period a plurality of convolution products which are accumulated to produce an output signal (r), and means (12) for adjusting the filter coefficients according to a predetermined algorithm, the filter being arranged, in operation to form the said convolution products during a first part of each sample period and perform at least a part of the filter coefficient adjustment process during a second part of each sample period which is subsequent to said first part.

Claim 3

A digital filter as claimed in claim 1 and comprising a memory stage (10) for storing the input samples (x) and the filter coefficients (h), an arithmetic stage (11) for forming convolution products of the input samples and the said coefficients, and accumulating the products to provide an output signal (r), and a third stage (12) arranged to form an update signal ( h) for adapting the filter coefficients according to a predetermined algorithm.

Claim 4

A digital filter as claimed in claim 2 wherein said memory stage (10) comprises a first section (30) for storing values of input samples (x), a second section (part of 33) for storing values of filter coefficients (ha) which can be adapted, and a third section (part of 33) for storing values of filter coefficients (hs) which are not adaptable.

Claim 5

A digital filter as claimed in claim 3 including means for transferring the contents of the second memory section to the third memory section.

Claim 6

A digital filter as claimed in claim 3 or 4 wherein the arithmetic stage includes means (35) for forming convolution products between values of the input signal (x) and filter coefficients (ha) from the second memory section, means (35) for forming convolution products between values of the input signal (x) and filter coefficients (hs) from the third memory section, and means (136) for accumulating said convolution products to form respective output signals.

Claim 7

A digital filter as claimed in any one of claims 2 to 6 including bus means coupling said stages to permit data transfer between them, and control means (18,20) for providing timing signals to control operation of said stages and transfer of data between said stages.

Claim 8

A digital filter as claimed in claim 6 wherein the input samples (x) and coefficient values (h) are multiplexed onto the bus means (14,15) for transfer from the memory stage (10) to the arithmetic stage (11) under the control of clock signals from said control means (18,20).

Claim 9

A filter as claimed in any one of claims 2 to 7 in which, in the said predetermined algorithm the update values for the filter coefficients are obtained by adding a correction equal to the product of a scaling factor, an error representing the difference between the filter output and a desired signal, and a respective one of the last W input samples, where W is the number of coefficients; the multiplication by the input samples and the addition of the product being carried out in the first part of the sample period and the derivation of the scaling factor and error and their multiplication being carried out in the second part of the preceding sample period.

Claim 10

A digital filter as claimed in claim 8 wherein said arithmetic stage includes means (41,43) arranged during each sample period to form from sample values x values of x , accumulated over a predetermined number of samples, which are fed to the third stage (12) for use in forming said update values, and 8 wherein the said predetermined algorithm for updating the filter coefficients H can be expressed as H new = H old + f(2u).e.x. where e is an error signal equal to the difference between the filter output (r) and a reference signal (y), x is the input sample and f(2u) is a factor derived by decoding in said third stage (12) the said accumulated values of x 2 .

Claim 11

A filter according to claim 9 in which, (a)in the second part of the (n-I)th sample period, the scaling factor f(2u) is derived from the accumulated value of x 2 ; a new sample (x) is received, and the square of this new value is added to the accumulated value of x 2 , and the error between the filter output and the derived value is obtained and its product with f(2u) obtained, (b) in the first part of the nth sample period, a plurality W of process cycles occur in each of which a respective one of the stored sample values x, x n-1 .....x n-W-1 is multiplied the said product and added to the respective one of the filter coefficients; and the updated value of the coefficient is multiplied by the corresponding one of the stored sample values and accumulated, whereby at the end of the first part of the nth sample period an accumulated filter output is obtained using the error obtained from the immediately preceding coefficient update. (c) in the second part of the nth sample period the sample value x n-W is squared and subtracted from the accumulated x value.

Claim 12

A digital filter as claimed in any one of claims 2 to 10 wherein the step size provided by said third stage can be adjusted.

Claim 13

A digital filter which comprises a plurality of filters according to any preceding claim arranged in cascade.

Claim 14

An adaptive digital filter comprising a memory stage for storing input samples and filter coefficients, an arithmetic stage for forming convolution products of said input samples and filter coefficients, an arithmetic stage for forming convolution products of said input samples and said coefficients and accumulating said products to provide an output signal, a third stage arranged to form an update signal for adapting said filter coefficients according to a predetermined algorithm, bus means coupling said stages to permit data transfer therebetween, and control means for providing timing signals to control operation of said stages and transfer of data between said stages.

Claim 15

An adaptive digital filter comprising a memory stage for storing input samples and filter coefficients, an arithmetic stage for forming convolution products of said input samples and filter coefficients, an arithmetic stage for forming convolution products of said input samples and said coefficients and accumulating said products to provide an output signal, a third stage arranged to form an update signal for adapting said filter coefficients according to a predetermined algorithm, bus means coupling said stages to permit data transfer therebetween, and control means for providing timing signals to control operation of said stages and transfer of data between said stages.

Claim 16

A loudspeaking telephone incorporating a digital filter as claimed in any one of claims 1 to 13.