Formant filter bank

pulp::signal::FormantFilterBankT is a fixed-capacity audio filter for vowel and other source-filter colour. It is deliberately separate from formant analysis: it does not estimate resonances from speech, track a voice, or shift an STFT spectral envelope. The caller supplies one to five ordered resonant peaks.

#include <pulp/signal/formant_filter_bank.hpp>

pulp::signal::FormantFilterBank filter;
filter.prepare(48000.0);
filter.set_transition_samples(128);
filter.configure_vowel_morph(pulp::signal::FormantVowel::a,
                              pulp::signal::FormantVowel::u, 0.35);
filter.process_block(samples, frame_count); // in place

Each FormantSpec contains frequency, bandwidth, and relative gain. A complete recipe is rejected unless every value is finite and bounded, frequencies are strictly increasing, and Q = frequency / bandwidth lies in the documented range. Rejection leaves the requested and processing state unchanged.

Retunes crossfade two complete banks, so coefficients never pass through an unstable interpolated state. The default fade is 64 samples; set_transition_samples(0) explicitly opts into immediate block-boundary replacement. The built-in A/E/I/O/U recipes use five peaks and configure_vowel_morph() interpolates their frequencies geometrically. The filter has zero latency. tail_seconds() reports the slowest section's ideal -60 dB amplitude decay estimate.

The output uses a conservative normalization: linear formant weights sum to the selected headroom gain (3 dB by default). This bounds the bank's steady-state frequency response without clipping or adding a nonlinear limiter. One object contains one recursive path; use one object per channel.