Allpass feedback loop

There are many possible reverberation algorithms that can be constructed by adding absorptive losses to allpass feedback loops, and these reverberators can sound very good. However, the design of these reverberators has to date been entirely empirical. There is no way to specify in advance a particular reverberation time function A(co), nor is there a deterministic method for choosing the filter parameters to eliminate tonal coloration. 

Reverberator formed by adding absorptive losses to an allpass feedback loop 115... 

Dattorro s plate reverberator based on an allpass feedback loop 117... 

Figure 3.21 Dattorro s plate reverberator based on an allpass feedback loop, intended for 29.8 kHz sampling rate [Dattorro, 1997]. //i(z)and H2(z) are low-pass filters described in figure 3.11 H (z) controls the bandwidth of signals entering the reverberator, and H2(z) controls the frequency dependent decay. Stereo outputs yL and yR are formed from taps taken from labelled delays as follows yL = a [266] + a[2974] - [1913] + c[1996] - < [1990] - e[187] - f[ 066], yR = < [353] + < [3627] - e[1228] + /[2673] - a[2111] - >[335] - c[121]. In practice, the input is also mixed with each output to achieve a desired reverberation level. The time varying functions u(t) and v(t) are low frequency (= 1 Hz) sinusoids that span 16 samples peak to peak. Typical coefficients values are gj = 0.75, g2 = 0.625, g3 = 0.7, g4= 0.5, g5= 0.9.

Vary the coefficients of the feedback matrix in the reference filter while maintaining the energy conserving property, or similarly vary the allpass gains of an allpass feedback loop reverberator. 

Modulate the output tap gains of an allpass feedback loop structure such as in figure 3.20, or similarly vary the mixing matrix shown in equation 3.28. 

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