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Add ARM NEON soft-clipper to prevent clipping on sharp transients 

Implements SIMD-optimized soft-clipping before Opus encoding to prevent
digital clipping distortion on sharp transient attacks (e.g., plastic cup
impacts, percussive sounds). Uses smooth saturation curve starting at
±30720 (~94% of max amplitude) to preserve audio quality while eliminating
crackles and pops.

Processes 8 samples per iteration using ARM NEON intrinsics for optimal
performance on the ARM Cortex-A7 platform.
This commit is contained in:
Alex P 2025-11-21 20:23:10 +02:00
parent 698978253b
commit 9e6ffb34e3
1 changed files with 58 additions and 0 deletions
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58
internal/audio/c/audio.c
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@ -206,6 +206,61 @@ static inline void simd_clear_samples_s16(short * __restrict__ buffer, uint32_t
}
}
/**
* Soft-clip audio samples to prevent digital clipping distortion
* Uses smooth saturation curve for transients that exceed ±30720 (~0.94 of max)
* Processes 8 samples per iteration using ARM NEON
*/
static inline void simd_soft_clip_s16(short * __restrict__ buffer, uint32_t samples) {
const int16_t threshold = 30720; // 0.9375 * 32768
const int16x8_t thresh_pos = vdupq_n_s16(threshold);
const int16x8_t thresh_neg = vdupq_n_s16(-threshold);
const int16x8_t max_val = vdupq_n_s16(32767);
const int16x8_t min_val = vdupq_n_s16(-32768);
uint32_t i = 0;
uint32_t simd_samples = samples & ~7U;
for (; i < simd_samples; i += 8) {
int16x8_t samples_vec = vld1q_s16(&buffer[i]);
// Detect samples exceeding positive threshold
uint16x8_t exceeds_pos = vcgtq_s16(samples_vec, thresh_pos);
// Detect samples below negative threshold
uint16x8_t exceeds_neg = vcltq_s16(samples_vec, thresh_neg);
// Apply soft saturation to samples exceeding thresholds
// For positive: scale down to range [threshold, max_val]
// For negative: scale up to range [min_val, -threshold]
int16x8_t clipped = samples_vec;
clipped = vbslq_s16(exceeds_pos,
vaddq_s16(thresh_pos, vshrq_n_s16(vsubq_s16(samples_vec, thresh_pos), 2)),
clipped);
clipped = vbslq_s16(exceeds_neg,
vaddq_s16(thresh_neg, vshrq_n_s16(vsubq_s16(samples_vec, thresh_neg), 2)),
clipped);
// Clamp to int16 range
clipped = vminq_s16(vmaxq_s16(clipped, min_val), max_val);
vst1q_s16(&buffer[i], clipped);
}
// Scalar: remaining samples
for (; i < samples; i++) {
int32_t sample = buffer[i];
if (sample > threshold) {
sample = threshold + ((sample - threshold) >> 2);
} else if (sample < -threshold) {
sample = -threshold + ((sample + threshold) >> 2);
}
// Clamp to int16 range
if (sample > 32767) sample = 32767;
if (sample < -32768) sample = -32768;
buffer[i] = (short)sample;
}
}
// INITIALIZATION STATE TRACKING
static volatile sig_atomic_t capture_initializing = 0;
@ -755,6 +810,9 @@ retry_read:
return -1;
}
// Apply soft-clipping to prevent digital clipping distortion on sharp transients
simd_soft_clip_s16(pcm_to_encode, opus_frame_size * capture_channels);
nb_bytes = opus_encode(enc, pcm_to_encode, opus_frame_size, out, max_packet_size);
if (__builtin_expect(nb_bytes < 0, 0)) {