From 999e1deda0baf3a277e3709910d90bf01d21faff Mon Sep 17 00:00:00 2001 From: minjaesong Date: Wed, 29 Oct 2025 02:52:09 +0900 Subject: [PATCH] TAD: coefficient dithering --- video_encoder/decoder_tad.c | 9 +++-- video_encoder/encoder_tad.c | 68 +++++++++++++++++++++++++++++++++---- 2 files changed, 69 insertions(+), 8 deletions(-) diff --git a/video_encoder/decoder_tad.c b/video_encoder/decoder_tad.c index a41ed81..2880089 100644 --- a/video_encoder/decoder_tad.c +++ b/video_encoder/decoder_tad.c @@ -342,10 +342,15 @@ static void pcm32f_to_pcm8(const float *fleft, const float *fright, uint8_t *lef const float scale = 127.5f; const float bias = 128.0f; + // Reduced dither amplitude to coordinate with coefficient-domain dithering + // The encoder now adds TPDF dither in coefficient domain, so we reduce + // sample-domain dither by ~60% to avoid doubling the noise floor + const float dither_scale = 0.2f; // Reduced from 0.5 (was ±0.5 LSB, now ±0.2 LSB) + for (size_t i = 0; i < count; i++) { // --- LEFT channel --- float feedbackL = b1 * dither_error[0][0] + b2 * dither_error[0][1]; - float ditherL = 0.5f * tpdf1(); // ±0.5 LSB TPDF + float ditherL = dither_scale * tpdf1(); // Reduced TPDF dither float shapedL = fleft[i] + feedbackL + ditherL / scale; shapedL = FCLAMP(shapedL, -1.0f, 1.0f); @@ -360,7 +365,7 @@ static void pcm32f_to_pcm8(const float *fleft, const float *fright, uint8_t *lef // --- RIGHT channel --- float feedbackR = b1 * dither_error[1][0] + b2 * dither_error[1][1]; - float ditherR = 0.5f * tpdf1(); + float ditherR = dither_scale * tpdf1(); // Reduced TPDF dither float shapedR = fright[i] + feedbackR + ditherR / scale; shapedR = FCLAMP(shapedR, -1.0f, 1.0f); diff --git a/video_encoder/encoder_tad.c b/video_encoder/encoder_tad.c index be6fdf8..ea9deb8 100644 --- a/video_encoder/encoder_tad.c +++ b/video_encoder/encoder_tad.c @@ -37,13 +37,43 @@ static const float BASE_QUANTISER_WEIGHTS[] = { // Forward declarations for internal functions static void dwt_dd4_forward_1d(float *data, int length); static void dwt_dd4_forward_multilevel(float *data, int length, int levels); -static void quantize_dwt_coefficients(const float *coeffs, int8_t *quantized, size_t count, int apply_deadzone, int chunk_size, int dwt_levels, int quant_bits, int *current_subband_index, float quantiser_scale); +static void quantize_dwt_coefficients(const float *coeffs, int8_t *quantized, size_t count, int apply_deadzone, int chunk_size, int dwt_levels, int quant_bits, int *current_subband_index, float quantiser_scale, uint32_t *dither_seed); static size_t encode_twobitmap(const int8_t *values, size_t count, uint8_t *output); static inline float FCLAMP(float x, float min, float max) { return x < min ? min : (x > max ? max : x); } +//============================================================================= +// Deterministic PRNG for Coefficient-Domain Dithering +//============================================================================= + +// Simple LCG for reproducible dithering +static inline uint32_t lcg_next(uint32_t *seed) { + *seed = (*seed * 1664525u) + 1013904223u; + return *seed; +} + +// Uniform random in [0, 1) +static inline float uniform_01(uint32_t *seed) { + return (lcg_next(seed) & 0xFFFFFF) / 16777216.0f; +} + +// TPDF (Triangular Probability Distribution Function) dither in range (-1, 1) +static inline float tpdf_dither(uint32_t *seed) { + float u1 = uniform_01(seed) - 0.5f; // [-0.5, 0.5) + float u2 = uniform_01(seed) - 0.5f; // [-0.5, 0.5) + return u1 - u2; // Triangular distribution in (-1, 1) +} + +// Calculate per-subband dither scaling factor +// alpha = 0.0 → flat per-band noise +// alpha = 0.5 → pinkish noise (default) +// alpha = 1.0 → more noise in low bands +static inline float subband_dither_scale(int level, float alpha, float scale) { + return (powf(alpha, level / 10.0f) - 1.0f) / alpha * scale; +} + // Calculate DWT levels from chunk size static int calculate_dwt_levels(int chunk_size) { /*if (chunk_size < TAD32_MIN_CHUNK_SIZE) { @@ -280,7 +310,7 @@ static int8_t lambda_companding(float val, int max_index) { return (int8_t)(sign * index); } -static void quantize_dwt_coefficients(const float *coeffs, int8_t *quantized, size_t count, int apply_deadzone, int chunk_size, int dwt_levels, int max_index, int *current_subband_index, float quantiser_scale) { +static void quantize_dwt_coefficients(const float *coeffs, int8_t *quantized, size_t count, int apply_deadzone, int chunk_size, int dwt_levels, int max_index, int *current_subband_index, float quantiser_scale, uint32_t *dither_seed) { int first_band_size = chunk_size >> dwt_levels; int *sideband_starts = malloc((dwt_levels + 2) * sizeof(int)); @@ -290,6 +320,10 @@ static void quantize_dwt_coefficients(const float *coeffs, int8_t *quantized, si sideband_starts[i] = sideband_starts[i-1] + (first_band_size << (i-2)); } + // Coefficient-domain dithering parameters + const float dither_k = 0.5f; // Amplitude factor (0.5 × Q_level) + const float dither_alpha = 78.0f; // Subband scaling exponent (0.5 = pinkish) + for (size_t i = 0; i < count; i++) { int sideband = dwt_levels; for (int s = 0; s <= dwt_levels; s++) { @@ -306,7 +340,22 @@ static void quantize_dwt_coefficients(const float *coeffs, int8_t *quantized, si // Apply base weight and quantiser scaling float weight = BASE_QUANTISER_WEIGHTS[sideband] * quantiser_scale; - float val = (coeffs[i] / (TAD32_COEFF_SCALARS[sideband] * weight)); // val is normalised to [-1,1] + float scalar = TAD32_COEFF_SCALARS[sideband] * weight; + + // Calculate quantization step size Q in coefficient domain + // Q represents the spacing between quantization levels + float Q = scalar / max_index; + + // Per-subband dither scaling: lower levels get more dither energy + float s_level = subband_dither_scale(sideband, dither_alpha, 0.3f); + + // TPDF dithering in coefficient domain + float tpdf = tpdf_dither(dither_seed); + float dither_amplitude = dither_k * Q * s_level; + float dithered_coeff = coeffs[i] + (tpdf * dither_amplitude); + + // Normalize dithered coefficient to [-1, 1] range for quantization + float val = dithered_coeff / scalar; int8_t quant_val = lambda_companding(val, max_index); quantized[i] = quant_val; @@ -834,9 +883,16 @@ size_t tad32_encode_chunk(const float *pcm32_stereo, size_t num_samples, accumulate_coefficients(dwt_side, dwt_levels, num_samples, side_accumulators); } - // Step 4: Quantize with frequency-dependent weights and quantiser scaling - quantize_dwt_coefficients(dwt_mid, quant_mid, num_samples, 1, num_samples, dwt_levels, max_index, NULL, quantiser_scale); - quantize_dwt_coefficients(dwt_side, quant_side, num_samples, 1, num_samples, dwt_levels, max_index, NULL, quantiser_scale); + // Step 3.75: Initialize deterministic dither seed for coefficient-domain dithering + // Using a static counter ensures reproducible dithering per chunk + static uint32_t chunk_counter = 0; + uint32_t dither_seed_mid = 0x12345678u ^ (chunk_counter * 2); // Seed for Mid channel + uint32_t dither_seed_side = 0x87654321u ^ (chunk_counter * 2 + 1); // Seed for Side channel + chunk_counter++; + + // Step 4: Quantize with frequency-dependent weights, quantiser scaling, and coefficient-domain dithering + quantize_dwt_coefficients(dwt_mid, quant_mid, num_samples, 1, num_samples, dwt_levels, max_index, NULL, quantiser_scale, &dither_seed_mid); + quantize_dwt_coefficients(dwt_side, quant_side, num_samples, 1, num_samples, dwt_levels, max_index, NULL, quantiser_scale, &dither_seed_side); // Step 4.5: Accumulate quantized coefficient statistics if enabled if (stats_enabled) {