diff --git a/src/rtl_sdr_fm.cpp b/src/rtl_sdr_fm.cpp index 6c1ac19..5b8e883 100644 --- a/src/rtl_sdr_fm.cpp +++ b/src/rtl_sdr_fm.cpp @@ -146,6 +146,18 @@ static inline void widen_u8_to_s16_bias127_scalar(const unsigned char * DSD_FME_ } } +/* Scalar widening that subtracts 128 instead of 127. + Used to pair with legacy byte-wise rotate_90(u8) which performs 255 - x negation, + so that the overall effect equals correct centered negation (127 - x). */ +static inline void widen_u8_to_s16_bias128_scalar(const unsigned char * DSD_FME_RESTRICT src, + int16_t * DSD_FME_RESTRICT dst, uint32_t len) +{ + uint32_t i = 0; + for (; i < len; i++) { + dst[i] = (int16_t)src[i] - 128; + } +} + /* ===================== Combined rotate_90 (1, j, -1, -j) + widen (u8->s16 centered at 127) Processes 4 IQ samples per iteration to avoid branches. @@ -162,18 +174,18 @@ static inline void widen_rotate90_u8_to_s16_bias127_scalar(const unsigned char * int16_t i1 = (int16_t)src[i + 2] - 127; int16_t q1 = (int16_t)src[i + 3] - 127; - dst[i + 2] = (int16_t)(1 - q1); + dst[i + 2] = (int16_t)(-q1); dst[i + 3] = i1; int16_t i2 = (int16_t)src[i + 4] - 127; int16_t q2 = (int16_t)src[i + 5] - 127; - dst[i + 4] = (int16_t)(1 - i2); - dst[i + 5] = (int16_t)(1 - q2); + dst[i + 4] = (int16_t)(-i2); + dst[i + 5] = (int16_t)(-q2); int16_t i3 = (int16_t)src[i + 6] - 127; int16_t q3 = (int16_t)src[i + 7] - 127; dst[i + 6] = q3; - dst[i + 7] = (int16_t)(1 - i3); + dst[i + 7] = (int16_t)(-i3); } /* Tail: apply rotation pattern for remaining up to 6 samples */ if (i < len) { @@ -188,14 +200,14 @@ static inline void widen_rotate90_u8_to_s16_bias127_scalar(const unsigned char * if (rem >= 4) { int16_t i1 = (int16_t)src[base + 2] - 127; int16_t q1 = (int16_t)src[base + 3] - 127; - dst[base + 2] = (int16_t)(1 - q1); + dst[base + 2] = (int16_t)(-q1); dst[base + 3] = i1; } if (rem >= 6) { int16_t i2 = (int16_t)src[base + 4] - 127; int16_t q2 = (int16_t)src[base + 5] - 127; - dst[base + 4] = (int16_t)(1 - i2); - dst[base + 5] = (int16_t)(1 - q2); + dst[base + 4] = (int16_t)(-i2); + dst[base + 5] = (int16_t)(-q2); } } } @@ -237,7 +249,6 @@ static void DSD_FME_TARGET_ATTR("avx2") widen_rotate90_u8_to_s16_bias127_avx2(co 0x0000,0x0000,0xFFFF,0x0000,0xFFFF,0xFFFF,0x0000,0xFFFF, 0x0000,0x0000,0xFFFF,0x0000,0xFFFF,0xFFFF,0x0000,0xFFFF); const __m256i c127 = _mm256_set1_epi16(127); - const __m256i c128 = _mm256_set1_epi16(128); uint32_t i = 0; for (; i + 32 <= len; i += 32) { __m256i v8 = _mm256_loadu_si256((const __m256i*)(src + i)); @@ -247,9 +258,9 @@ static void DSD_FME_TARGET_ATTR("avx2") widen_rotate90_u8_to_s16_bias127_avx2(co __m256i v16_lo = _mm256_cvtepu8_epi16(sh_lo); __m256i v16_hi = _mm256_cvtepu8_epi16(sh_hi); __m256i bs_lo = _mm256_sub_epi16(v16_lo, c127); - __m256i bm_lo = _mm256_sub_epi16(c128, v16_lo); + __m256i bm_lo = _mm256_sub_epi16(c127, v16_lo); __m256i bs_hi = _mm256_sub_epi16(v16_hi, c127); - __m256i bm_hi = _mm256_sub_epi16(c128, v16_hi); + __m256i bm_hi = _mm256_sub_epi16(c127, v16_hi); __m256i out_lo = _mm256_blendv_epi8(bs_lo, bm_lo, mask_sel); __m256i out_hi = _mm256_blendv_epi8(bs_hi, bm_hi, mask_sel); _mm256_storeu_si256((__m256i*)(dst + i), out_lo); @@ -296,7 +307,6 @@ static void DSD_FME_TARGET_ATTR("ssse3") widen_rotate90_u8_to_s16_bias127_ssse3( const __m128i mask_sel = _mm_setr_epi16( 0x0000,0x0000,0xFFFF,0x0000,0xFFFF,0xFFFF,0x0000,0xFFFF); const __m128i c127 = _mm_set1_epi16(127); - const __m128i c128 = _mm_set1_epi16(128); const __m128i zero = _mm_setzero_si128(); for (; i + 16 <= len; i += 16) { __m128i v8 = _mm_loadu_si128((const __m128i*)(src + i)); @@ -304,9 +314,9 @@ static void DSD_FME_TARGET_ATTR("ssse3") widen_rotate90_u8_to_s16_bias127_ssse3( __m128i v16_lo = _mm_unpacklo_epi8(sh, zero); __m128i v16_hi = _mm_unpackhi_epi8(sh, zero); __m128i bs_lo = _mm_sub_epi16(v16_lo, c127); - __m128i bm_lo = _mm_sub_epi16(c128, v16_lo); + __m128i bm_lo = _mm_sub_epi16(c127, v16_lo); __m128i bs_hi = _mm_sub_epi16(v16_hi, c127); - __m128i bm_hi = _mm_sub_epi16(c128, v16_hi); + __m128i bm_hi = _mm_sub_epi16(c127, v16_hi); __m128i out_lo = _mm_or_si128(_mm_and_si128(bm_lo, mask_sel), _mm_andnot_si128(mask_sel, bs_lo)); __m128i out_hi = _mm_or_si128(_mm_and_si128(bm_hi, mask_sel), _mm_andnot_si128(mask_sel, bs_hi)); _mm_storeu_si128((__m128i*)(dst + i), out_lo); @@ -341,7 +351,6 @@ static void widen_rotate90_u8_to_s16_bias127_neon(const unsigned char *src, int1 #if defined(__aarch64__) const uint8x16_t tbl_idx = {0,1,3,2,4,5,7,6, 8,9,11,10,12,13,15,14}; const int16x8_t c127 = vdupq_n_s16(127); - const int16x8_t c128 = vdupq_n_s16(128); const uint16_t mpat[8] = {0x0000,0x0000,0xFFFF,0x0000,0xFFFF,0xFFFF,0x0000,0xFFFF}; const uint16x8_t msel = vld1q_u16(mpat); uint32_t i = 0; @@ -353,9 +362,9 @@ static void widen_rotate90_u8_to_s16_bias127_neon(const unsigned char *src, int1 int16x8_t v16_lo = vreinterpretq_s16_u16(vmovl_u8(sh_lo8)); int16x8_t v16_hi = vreinterpretq_s16_u16(vmovl_u8(sh_hi8)); int16x8_t bs_lo = vsubq_s16(v16_lo, c127); - int16x8_t bm_lo = vsubq_s16(c128, v16_lo); + int16x8_t bm_lo = vsubq_s16(c127, v16_lo); int16x8_t bs_hi = vsubq_s16(v16_hi, c127); - int16x8_t bm_hi = vsubq_s16(c128, v16_hi); + int16x8_t bm_hi = vsubq_s16(c127, v16_hi); int16x8_t out_lo = vbslq_s16(msel, bm_lo, bs_lo); int16x8_t out_hi = vbslq_s16(msel, bm_hi, bs_hi); vst1q_s16(dst + i, out_lo); @@ -2037,7 +2046,8 @@ static void rtlsdr_callback(unsigned char *buf, uint32_t len, void *ctx) widen_rotate90_u8_to_s16_bias127(buf, dst, len); } else if (!s->offset_tuning && !combine_rotate_enabled) { rotate_90(buf, len); - widen_u8_to_s16_bias127(buf, dst, len); + /* Use 128 subtraction to avoid +1 bias after byte-wise negation */ + widen_u8_to_s16_bias128_scalar(buf, dst, len); } else { widen_u8_to_s16_bias127(buf, dst, len); }