[FFmpeg-devel] [PATCH] opusdsp: add ability to modify deemphasis constant

[Lynne <dev@lynne.ee>]

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xHE-AAC relies on the same postfilter mechanism
that Opus uses to improve clarity (albeit with a steeper
deemphasis filter, using a z=0.64).

The code to apply it is identical, it's still just a
simple IIR low-pass filter. This commit makes it possible
to use alternative constants.

Patch attached.


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From c4ad107076970a555ed838a75c0343dffe0cad36 Mon Sep 17 00:00:00 2001
From: Lynne <dev@lynne.ee>
Date: Mon, 29 Jan 2024 04:31:43 +0100
Subject: [PATCH] opusdsp: add ability to modify deemphasis constant

xHE-AAC relies on the same postfilter mechanism
that Opus uses to improve clarity (albeit with a steeper
deemphasis filter).

The code to apply it is identical, it's still just a
simple IIR low-pass filter. This commit makes it possible
to use alternative constants.
---
 libavcodec/aarch64/opusdsp_init.c |  2 +-
 libavcodec/aarch64/opusdsp_neon.S | 28 +++++-----------------------
 libavcodec/opusdec_celt.c         |  4 +++-
 libavcodec/opusdsp.c              | 28 ++++++++++++++++++++++++++--
 libavcodec/opusdsp.h              |  2 +-
 libavcodec/x86/opusdsp.asm        |  9 +++------
 libavcodec/x86/opusdsp_init.c     |  2 +-
 7 files changed, 40 insertions(+), 35 deletions(-)

diff --git a/libavcodec/aarch64/opusdsp_init.c b/libavcodec/aarch64/opusdsp_init.c
index bb6d71b66b..449d95e8d3 100644
--- a/libavcodec/aarch64/opusdsp_init.c
+++ b/libavcodec/aarch64/opusdsp_init.c
@@ -23,7 +23,7 @@
 #include "libavcodec/opusdsp.h"
 
 void ff_opus_postfilter_neon(float *data, int period, float *gains, int len);
-float ff_opus_deemphasis_neon(float *out, float *in, float coeff, int len);
+float ff_opus_deemphasis_neon(float *out, float *in, float coeff, float *weights, int len);
 
 av_cold void ff_opus_dsp_init_aarch64(OpusDSP *ctx)
 {
diff --git a/libavcodec/aarch64/opusdsp_neon.S b/libavcodec/aarch64/opusdsp_neon.S
index e933151ab4..253825aa61 100644
--- a/libavcodec/aarch64/opusdsp_neon.S
+++ b/libavcodec/aarch64/opusdsp_neon.S
@@ -18,29 +18,11 @@
 
 #include "libavutil/aarch64/asm.S"
 
-           // 0.85..^1    0.85..^2    0.85..^3    0.85..^4
-const tab_st, align=4
-        .word 0x3f599a00, 0x3f38f671, 0x3f1d382a, 0x3f05a32f
-endconst
-const tab_x0, align=4
-        .word 0x0,        0x3f599a00, 0x3f38f671, 0x3f1d382a
-endconst
-const tab_x1, align=4
-        .word 0x0,        0x0,        0x3f599a00, 0x3f38f671
-endconst
-const tab_x2, align=4
-        .word 0x0,        0x0,        0x0,        0x3f599a00
-endconst
-
 function ff_opus_deemphasis_neon, export=1
-        movrel          x4, tab_st
-        ld1             {v4.4s}, [x4]
-        movrel          x4, tab_x0
-        ld1             {v5.4s}, [x4]
-        movrel          x4, tab_x1
-        ld1             {v6.4s}, [x4]
-        movrel          x4, tab_x2
-        ld1             {v7.4s}, [x4]
+        ld1             {v4.4s}, [x2], #16
+        ld1             {v5.4s}, [x2], #16
+        ld1             {v6.4s}, [x2], #16
+        ld1             {v7.4s}, [x2]
 
         fmul            v0.4s, v4.4s, v0.s[0]
 
@@ -63,7 +45,7 @@ function ff_opus_deemphasis_neon, export=1
         st1             {v1.4s, v2.4s}, [x0], #32
         fmul            v0.4s, v4.4s, v2.s[3]
 
-        subs            w2, w2, #8
+        subs            w3, w3, #8
         b.gt            1b
 
         mov             s0, v2.s[3]
diff --git a/libavcodec/opusdec_celt.c b/libavcodec/opusdec_celt.c
index fd8e9929e9..2dd3e12c48 100644
--- a/libavcodec/opusdec_celt.c
+++ b/libavcodec/opusdec_celt.c
@@ -460,7 +460,9 @@ int ff_celt_decode_frame(CeltFrame *f, OpusRangeCoder *rc,
         /* deemphasis */
         block->emph_coeff = f->opusdsp.deemphasis(output[i],
                                                   &block->buf[1024 - frame_size],
-                                                  block->emph_coeff, frame_size);
+                                                  block->emph_coeff,
+                                                  ff_deemph_opus_weights,
+                                                  frame_size);
     }
 
     if (channels == 1)
diff --git a/libavcodec/opusdsp.c b/libavcodec/opusdsp.c
index 0764d712e4..e46079c271 100644
--- a/libavcodec/opusdsp.c
+++ b/libavcodec/opusdsp.c
@@ -18,8 +18,31 @@
 
 #include "config.h"
 #include "libavutil/attributes.h"
+#include "libavutil/mem_internal.h"
 #include "opusdsp.h"
 
+static const DECLARE_ALIGNED(16, float, ff_deemph_opus_weights)[] = {
+    CELT_EMPH_COEFF,
+    CELT_EMPH_COEFF*CELT_EMPH_COEFF,
+    CELT_EMPH_COEFF*CELT_EMPH_COEFF*CELT_EMPH_COEFF,
+    CELT_EMPH_COEFF*CELT_EMPH_COEFF*CELT_EMPH_COEFF*CELT_EMPH_COEFF,
+
+    0,
+    CELT_EMPH_COEFF,
+    CELT_EMPH_COEFF*CELT_EMPH_COEFF,
+    CELT_EMPH_COEFF*CELT_EMPH_COEFF*CELT_EMPH_COEFF,
+
+    0,
+    0,
+    CELT_EMPH_COEFF,
+    CELT_EMPH_COEFF*CELT_EMPH_COEFF,
+
+    0,
+    0,
+    0,
+    CELT_EMPH_COEFF,
+};
+
 static void postfilter_c(float *data, int period, float *gains, int len)
 {
     const float g0 = gains[0];
@@ -43,10 +66,11 @@ static void postfilter_c(float *data, int period, float *gains, int len)
     }
 }
 
-static float deemphasis_c(float *y, float *x, float coeff, int len)
+static float deemphasis_c(float *y, float *x, float coeff, float *weights, int len)
 {
+    float c = weights[0];
     for (int i = 0; i < len; i++)
-        coeff = y[i] = x[i] + coeff*CELT_EMPH_COEFF;
+        coeff = y[i] = x[i] + coeff*c;
 
     return coeff;
 }
diff --git a/libavcodec/opusdsp.h b/libavcodec/opusdsp.h
index c2a301e832..0745348e21 100644
--- a/libavcodec/opusdsp.h
+++ b/libavcodec/opusdsp.h
@@ -23,7 +23,7 @@
 
 typedef struct OpusDSP {
     void (*postfilter)(float *data, int period, float *gains, int len);
-    float (*deemphasis)(float *out, float *in, float coeff, int len);
+    float (*deemphasis)(float *out, float *in, float coeff, float *weights, int len);
 } OpusDSP;
 
 void ff_opus_dsp_init(OpusDSP *ctx);
diff --git a/libavcodec/x86/opusdsp.asm b/libavcodec/x86/opusdsp.asm
index 418cc16330..635f59f83c 100644
--- a/libavcodec/x86/opusdsp.asm
+++ b/libavcodec/x86/opusdsp.asm
@@ -22,16 +22,13 @@
 
 SECTION_RODATA
 
-         ; 0.85..^1    0.85..^2    0.85..^3    0.85..^4
-tab_st: dd 0x3f599a00, 0x3f38f671, 0x3f1d382a, 0x3f05a32f
-
 SECTION .text
 
 INIT_XMM fma3
 %if UNIX64
-cglobal opus_deemphasis, 3, 3, 8, out, in, len
+cglobal opus_deemphasis, 4, 4, 8, out, in, weights, len
 %else
-cglobal opus_deemphasis, 4, 4, 8, out, in, coeff, len
+cglobal opus_deemphasis, 5, 5, 8, out, in, coeff, weights, len
 %endif
 %if ARCH_X86_32
     VBROADCASTSS m0, coeffm
@@ -41,7 +38,7 @@ cglobal opus_deemphasis, 4, 4, 8, out, in, coeff, len
     shufps m0, m0, 0
 %endif
 
-    movaps m4, [tab_st]
+    movaps m4, [weightsq]
     VBROADCASTSS m5, m4
     shufps m6, m4, m4, q1111
     shufps m7, m4, m4, q2222
diff --git a/libavcodec/x86/opusdsp_init.c b/libavcodec/x86/opusdsp_init.c
index 582fbb4f0c..f9ea4eac8b 100644
--- a/libavcodec/x86/opusdsp_init.c
+++ b/libavcodec/x86/opusdsp_init.c
@@ -23,7 +23,7 @@
 #include "libavcodec/opusdsp.h"
 
 void ff_opus_postfilter_fma3(float *data, int period, float *gains, int len);
-float ff_opus_deemphasis_fma3(float *out, float *in, float coeff, int len);
+float ff_opus_deemphasis_fma3(float *out, float *in, float coeff, float *weights, int len);
 
 av_cold void ff_opus_dsp_init_x86(OpusDSP *ctx)
 {
-- 
2.43.0.381.gb435a96ce8


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