[FFmpeg-devel] [PATCH] avfilter/asrc_sine: Generate full amplitude sine wave.

[Santanu Biswas]

Previously, generating a full amplitude sine wave require to either
use the 'volume' filter in combination with this filter or manually
generate sine wave using 'aevalsrc' (e.g "aevalsrc=sin(2*PI*t*440)").
This method is not bit-exact; therefore it is much coherent to not
scale the amplitude by 1/8 and keep it as it is.

Signed-off-by: Santanu Biswas <shbisws at gmail.com>
---
 doc/filters.texi        | 2 +-
 libavfilter/asrc_sine.c | 8 ++------
 2 files changed, 3 insertions(+), 7 deletions(-)

diff --git a/doc/filters.texi b/doc/filters.texi
index 0e10946cca..a42e27b9b5 100644
--- a/doc/filters.texi
+++ b/doc/filters.texi
@@ -7452,7 +7452,7 @@ Set number of taps for low-pass filter.
 
 @section sine
 
-Generate an audio signal made of a sine wave with amplitude 1/8.
+Generate an audio signal made of a sine wave.
 
 The audio signal is bit-exact.
 
diff --git a/libavfilter/asrc_sine.c b/libavfilter/asrc_sine.c
index c0d8d2265b..6c2f9d40f3 100644
--- a/libavfilter/asrc_sine.c
+++ b/libavfilter/asrc_sine.c
@@ -83,13 +83,11 @@ static const AVOption sine_options[] = {
 AVFILTER_DEFINE_CLASS(sine);
 
 #define LOG_PERIOD 15
-#define AMPLITUDE 4095
-#define AMPLITUDE_SHIFT 3
 
 static void make_sin_table(int16_t *sin)
 {
     unsigned half_pi = 1 << (LOG_PERIOD - 2);
-    unsigned ampls = AMPLITUDE << AMPLITUDE_SHIFT;
+    unsigned ampls = 32767;
     uint64_t unit2 = (uint64_t)(ampls * ampls) << 32;
     unsigned step, i, c, s, k, new_k, n2;
 
@@ -116,9 +114,7 @@ static void make_sin_table(int16_t *sin)
             sin[half_pi - i - step / 2] = (k * c + 0x8000) >> 16;
         }
     }
-    /* Unshift amplitude */
-    for (i = 0; i <= half_pi; i++)
-        sin[i] = (sin[i] + (1 << (AMPLITUDE_SHIFT - 1))) >> AMPLITUDE_SHIFT;
+
     /* Use symmetries to fill the other three quarters */
     for (i = 0; i < half_pi; i++)
         sin[half_pi * 2 - i] = sin[i];
-- 
2.35.1