consider an n channel system x ( ω ) which is excited with unit mean power white noise bandlimited to b hz at all inputs . if the input signals are all uncorrelated , it can be shown that the mean power output is given by p out = 1 2 ⁢ b ⁢ ∫ - b b ⁢  x ⁡ ( ω )  2 ⁢ ⁢ ⅆ ω ( 5 ) where the integrand is the ( frobenius ) norm squared of the transfer function matrix frequency ω . thus the power gain , p out / n , is governed by the average squared norm . in order to study the power gain of the system , at a single frequency , we consider exciting the system with a vector signal consisting of unit power complex sinusoids at all n inputs ; s ( t )= exp ( jω o t ) u , where a =( α jφ1 , c jφ2 , . . . c jφn ) r ( 6 ) if the phases of the sinusoids are samples from an underlying probability distribution p ( φ 1 , φ 2 , . . . φ n ) in which the phase random variables are independent then it can be shown that the sum of the variances at the n outputs is given by thus the squared norm of x ( ω ) divided by n is the ‘ phase avenged ’ power gain . this function allows a definition of ‘ low fluctuation ’ of the response of an n channel system . an ideal multichannel reverberator for use in a non - in - line assisted reverberation system will have a constant norm for all frequencies . in the multichannel case , such a reverberator may be loosely termed allpass . where † denotes the conjugate transpose . the rows ( and columns ) of x are thus orthonormal vectors . the norm squared of x is given by  x  2 = ∑ k = 1 n ⁢ ⁢ [ x + ⁢ x ] kk = ∑ k = 1 n ⁢ ⁢ i kk = n ( 9 ) hence may unitary matrix has a norm squared equal to the matrix dimension . the wideband or phase averaged power gain is thus equal to core . it will now be shown that for the unitary case the power gain is also one for constant sinusoids at one frequency applied to all inputs . suppose the matrix x represents the value of a transfer function at frequency ω o − y ( t )= exp ( jω o t ) xu ( see equation 6 ) and the total power in the output is where the time dependence cancels . thus the output power is equal to n for constant sinusoidal excitation and is independent of the input phases . the power gain is thus unity at all frequencies . hence : a linear multichannel system may be termed unitary if its transfer function matrix is unitary at all frequencies . a unitary system has a constant norm and unit power gain for all frequencies . a unitary system is ideal for use in the vra system since it has the same power gain at all frequencies and thus will not increase the colouration . it may also be inserted in ( o an mcr system without altering the loop gain . the power gain of the vra system with a unitary reverberator is given by p vra unitary 1 = 1 1 - α var 2 ⁢ n ( 11 ) most digital reverberators are based on the comb filter , shown in fig2 [ 8 - 10 ]. this circuit produces an impulse response that is in exponentially decaying sequence of delta functions occurring at multiplies of the delay time r = l / f , where f g is the sample rate . the output may be taken from the summer , the delay or the multiplier outputs . the transfer function for the comb filter with output taken from the delay output is x ⁡ ( z ) = z - l 1 + μ ⁢ ⁢ z - l ( 12 ) the single channel comb filter can be made to have a constant magnitude verses frequency response ( termed an allpass response ) by incorporating a feedforward section into the circuit . an efficient one multiplier form of the allpass form is shown in fig3 [ 8 , 9 ]. the transfer function is given by x ( z ) = μ + z - l 1 + μ ⁢ ⁢ z - l ( 13 )  x ⁡ ( e j ⁢ ⁢ θ )  2 = 1 + μ 2 + 2 ⁢ cos ⁡ ( l ⁢ ⁢ θ ) 1 + μ 2 + 2 ⁢ cos ⁡ ( l ⁢ ⁢ θ ) ( 14 ) early forms of reverberator were constructed using a number of comb filters in parallel , with the summed outputs being fed into a number of allpass sections to increase the echo density [ 8 ]. a more recent structure for multichannel reverberators is as shown in fig4 [ 9 , 10 ]. this structure is an extension of the single channel comb filter which achieves a high echo density by the cross coupling of a number of single channel comb filters , via the cross coupling matrix g . subsequent allpass sections are not required . the response of the vector comb filter may be determined by assuming that the unput is a vector of discrete signals u f ( n ) with a vector spectrum u ( z )=( u 1 ( z ), u 2 ( z ), . . . , u n ( z )| t ( 15 ) the vector spectrum at the output of the adders is given by solving for v +( z ) allows the output vector spectrum v ( z ) to be found ; it can be shown [ 9 , 10 ] that if the gain matrix g is orthonormal , ie g t g = i , then the system is stable for μ & lt ; 1 . the poles of the system are distributed in the z plane around a circle with radius less than unit . the multichannel reverberator circuit can be made to have allpass properties if a feedforward action is incorporated into the circuit , as in the one dimensional case . an efficient form with a single vector gain element ( μ ) and single cross coupling matrix g , is shown in fig5 . the output vector spectrum is given by the order of the gain and delay matrices may be reversed without altering the allpass properties of the circuit . it may be verified that the transfer function maxtrix is equation 19 is unitary at all frequencies as follows : at any given frequency ω , the matrix transfer function has the form where d = diag [ exp ( jφ 1 ), exp ( jφ 2 ), . . . , exp ( jφ n ]. now the product gd is a unitary matrix since where q is a unitary matrix and λ is a diagonal matrix of eigenvalues . since gd is unitary the eigenvalues have unit magnitude , ie a = diag [ e jα 1 , e jα 2 , . . . e jα n ] ( 23 ) x =[ μl + qλq † ][ 1 + μqλq † ] − 1 =[ q ( μl + λ ) q † ][ q ( 1 + μλ ) q † ] − 1 = q ( μi + λ ) q 554 q ( i + μλ ) 1 q † = a ( μi + λ )( i + μλ ) 1 q † = qλq † ( 24 ) a = diag ⁡ [ μ + ⅇ jα 1 1 + μⅇ jα 1 , μ + ⅇ jα 2 1 + μⅇ jα 2 , … ⁢ ⁢ μ + ⅇ jα n 1 + μⅇ jα n ] . ( 25 ) hence the transfer matrix x is unitary at all frequencies . the unitary system is formed from a set of n independent single dimensional allpass filters with a precoupling matrix q † ( ω ) and a post coupling matrix q ( ω ). the foregoing describes the invention including preferred forms thereof . alterations and modifications as will be obvious to those skilled in the art are intended to be incorporated in the scope hereof . 1 ) p . h . parkin and k . morgan , “ assisted resonance in the royal festival hall ,” j . acoust . soc . amer .. vol . 48 , pp 1025 - 1035 , 1970 . 2 ) de koning , s . h ., “ the mcr system - multiple channel amplification of reverberation .” philips tech . rev ., vol . 41 , pp 12 - 23 , 1983 / 84 . 3 ) a krokstad , electroacoustic means of controlling auditorium acoustics , applied acoustics vol . 24 , pp 275 - 288 , 1988 . 4 ) nz93 / 00041 , “ wideband assisted reverberation system .” industrial research ltd . may 20 , 1993 . 5 ) m . a . poletti , “ on controlling the apparent absorption and volume in assisted reverberation systems ,” acustica , vol . 78 . p 61 - 73 . 1993 6 ) m . a . poletti , “ an improved assisted reverberation system ,” proceedings of the 12th biennial conference of the new zealand acoustical society , 2nd and 3rd september , 1993 , pp . 107 - 115 8 ) m . r . schroeder , “ natural sounding artificial reverberation ,” j . audio eng . soc ., vol . 10 , no . 3 , pp 219 - 223 , july 1962 9 ) jot , “ digital delay networks for designing artificial reverberators ,” 90th convention of the audio engineering society . feb . 19 - 22 , paris 1991 , preprint 3030 . 10 ) stautner and m . puckette , “ designing multi channel reverberators .” computer music journal , vol . 6 , no . 1 , pp 52 - 65 , 1982 .