fdn.h

effects.h's Reverb is Schroeder/Moorer: four combs into two allpasses, the 1962 topology, and it sounds like it -- a comb bank has a modal density set by four delay lengths and the tail carries their signature. Jot's feedback delay network (1991) is the modern answer: N delays feeding back through a UNITARY mixing matrix, so every line hears every other and the modal density grows with N² instead of N. Eight lines of it are the substrate of every plate, hall and shimmer written since.

What is decided here, and why:

The matrix is a Hadamard, applied as a fast Walsh-Hadamard transform:
N log N adds, no multiplies, and unitary by construction (scaled by
1/sqrt(N)), which is the property that makes the network lossless before
the per-line gains are applied -- the decay then comes ONLY from those
gains, which is what lets the decay time be set exactly.

Per-line gain from the delay length and the decay time, Jot's formula:
g = 10^(-3 L / (T60 sr)). Every path through the network loses 60 dB in
T60 seconds whichever lines it passes through, so the declared decay is
the measured one. The unit test checks this against the Schroeder
backward integral of the impulse response.

Delay lengths are distinct PRIMES, picked from a logarithmic spread and
rounded to the nearest prime at prepare(): distinct primes are coprime by
definition, and coprime lengths are what stop the tail from ringing on
the pitch of a common factor. Scaled by the sample rate, so the room
keeps its size -- up to MaxDelay: the longest line is at most
MaxDelay / sr seconds (42 ms at 192 kHz for the default 8192), and past
that the whole spread shrinks together rather than the long lines piling
up at the ceiling. A plugin that wants a full-size hall at 192 kHz asks
for FdnReverb<8, 32768>.

Damping is a one-pole lowpass inside each line's loop, so high
frequencies decay faster than low ones the way air makes them.

Modulation moves each line's read point by a fraction of a sample on its
own slow LFO. With static lengths a network of this size still has a
faint metallic signature at the lengths' beat frequencies; a few samples
of drift smears it out. Read with readCubic(), because a delay that
sweeps must -- see DelayLine.

Included by dsp.h.