dc_blocker.hpp

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// sbl/dsp/dc_blocker.hpp — DC offset removal filter
//
// First-order high-pass IIR that rejects DC while passing audio.
// Essential for feedback paths (reverb, delay, waveshaping) where
// DC offset accumulates and eats headroom.
//
// Transfer function: H(z) = (1 - z^-1) / (1 - R * z^-1)
// Default R = 0.995 gives ~16 Hz cutoff at 48 kHz (-3 dB).
//
// Usage:
//   sbl::dsp::DcBlocker dc;
//   for (auto& s : buffer) s = dc.process(s);
 
#pragma once
 
#include <cstdint>
 
namespace sbl::dsp {
 
class DcBlocker {
public:
    /// @note All public methods are ISR-safe — bounded computation, no I/O.
 
    /**
     * @brief Process one sample through the DC blocker
     * @param x Input sample
     * @return Output with DC removed
     */
    float process(float x) {
        float y = x - x1_ + r_ * y1_;
        x1_ = x;
        y1_ = y;
        return y;
    }
 
    /**
     * @brief Process a block of float samples in-place
     * @param buf Sample buffer
     * @param frames Number of samples
     */
    void process(float* buf, uint16_t frames) {
        for (uint16_t i = 0; i < frames; ++i) {
            buf[i] = process(buf[i]);
        }
    }
 
    /**
     * @brief Set the pole coefficient
     *
     * Higher values (closer to 1.0) = lower cutoff frequency.
     * Default 0.995 ≈ 16 Hz at 48 kHz sample rate.
     *
     * @param r Pole coefficient, typically 0.99-0.999
     */
    void set_coefficient(float r) { r_ = r; }
 
    /** @brief Reset filter state to zero */
    void reset() { x1_ = 0.0f; y1_ = 0.0f; }
 
private:
    float r_ = 0.995f;
    float x1_ = 0.0f;
    float y1_ = 0.0f;
};
 
} // namespace sbl::dsp