From 23d461fe8714a20ee5e6929f22c61512fdda568e Mon Sep 17 00:00:00 2001
From: John Stebbins <stebbins@jetheaddev.com>
Date: Mon, 13 Jan 2014 17:04:30 +0000
Subject: [PATCH] ac3dec: Allow asymmetric application of DRC when drc_scale >
 1

Amplification of quiet sounds is enhanced.
Inspired by gbooker's A52Decoder.
---
 libavcodec/ac3dec.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/libavcodec/ac3dec.c b/libavcodec/ac3dec.c
index 2cceadc878..fcb52e11d0 100644
--- a/libavcodec/ac3dec.c
+++ b/libavcodec/ac3dec.c
@@ -778,8 +778,13 @@ static int decode_audio_block(AC3DecodeContext *s, int blk)
     i = !s->channel_mode;
     do {
         if (get_bits1(gbc)) {
-            s->dynamic_range[i] = powf(dynamic_range_tab[get_bits(gbc, 8)],
-                                       s->drc_scale);
+            /* Allow asymmetric application of DRC when drc_scale > 1.
+               Amplification of quiet sounds is enhanced */
+            float range = dynamic_range_tab[get_bits(gbc, 8)];
+            if (range > 1.0 || s->drc_scale <= 1.0)
+                s->dynamic_range[i] = powf(range, s->drc_scale);
+            else
+                s->dynamic_range[i] = range;
         } else if (blk == 0) {
             s->dynamic_range[i] = 1.0f;
         }
@@ -1493,7 +1498,7 @@ static av_cold int ac3_decode_end(AVCodecContext *avctx)
 #define OFFSET(x) offsetof(AC3DecodeContext, x)
 #define PAR (AV_OPT_FLAG_DECODING_PARAM | AV_OPT_FLAG_AUDIO_PARAM)
 static const AVOption options[] = {
-    { "drc_scale", "percentage of dynamic range compression to apply", OFFSET(drc_scale), AV_OPT_TYPE_FLOAT, {.dbl = 1.0}, 0.0, 1.0, PAR },
+    { "drc_scale", "percentage of dynamic range compression to apply", OFFSET(drc_scale), AV_OPT_TYPE_FLOAT, {.dbl = 1.0}, 0.0, 6.0, PAR },
     { NULL},
 };