Feedback Delay Networks (FDN)

**Figure 12:** A feedback delay network structure proposed for artificial reverberation by Jot (1992).
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Figure 12 illustrates an example FDN reverberator using three delay lines proposed by Jot (1992).
An FDN can be seen as a vector feedback comb filter, with feedback “channels” (=3 in Fig. 12).
The “mixing matrix” provides diffusion by “scattering” energy amongst the channels. Assuming decay control is handled by the $g_{i}$ coefficients, this matrix should be “lossless”.
To achieve frequency-dependent decay control, the $g_{i}$ coefficients can be replaced by low-order digital filters.
The “tonal correction” filter is a low-order filter which serves to equalize modal energy amongst the three bands.
The delay-line lengths are generally chosen to be mutually prime. System “tuning” remains a manual, trial and error process.
A relatively intuitive approach to the building of an FDN reverberator is provided by Geraint Luff.
A Matlab Toolbox for developing FDN reverberators is provided by Sebastian Schlecht.