[FFmpeg-devel] [RFC PATCH 1/3] aacdec: always skip the first 2048 samples if there's no side data

[Lynne]

For some reason, this was never set, which meant all **raw** AAC in ADTS
streams, except faac, had extra samples at the start.

Despite this being a standard MDCT-based codec with a frame size of 1024,
hence a delay of 1024 samples at the start, all major encoders, excluding
faac and FFmpeg, use 2048 samples of padding.

The FFmpeg encoder will be modified to also output 2048 samples of padding
at the start, to make it in line with other encoders.

Yes, this leaves FATE pretty sad. Will fix it with the real version of the 
patch.



>From 079235e1f1a9caeadfd2b8d78b3fe2273d86018a Mon Sep 17 00:00:00 2001
From: Lynne <d...@lynne.ee>
Date: Fri, 11 Aug 2023 17:50:54 +0200
Subject: [PATCH 1/3] aacdec: always skip the first 2048 samples if there's no
 side data

For some reason, this was never set, which meant all **raw** AAC in ADTS
streams, except faac, had extra samples at the start.

Despite this being a standard MDCT-based codec with a frame size of 1024,
hence a delay of 1024 samples at the start, all major encoders, excluding
faac and FFmpeg, use 2048 samples of padding.

The FFmpeg encoder will be modified to also output 2048 samples of padding
at the start, to make it in line with other encoders.
---
 libavcodec/aacdec_template.c | 17 +++++++++++++----
 1 file changed, 13 insertions(+), 4 deletions(-)

diff --git a/libavcodec/aacdec_template.c b/libavcodec/aacdec_template.c
index f8039e490b..0e4a274fea 100644
--- a/libavcodec/aacdec_template.c
+++ b/libavcodec/aacdec_template.c
@@ -1273,6 +1273,9 @@ static av_cold int aac_decode_init(AVCodecContext *avctx)
     if (ret < 0)
         return ret;
 
+    /* Usually overridden by side data */
+    avctx->internal->skip_samples = 2048;
+
     return 0;
 }
 
@@ -2417,14 +2420,16 @@ static int decode_dynamic_range(DynamicRangeControl *che_drc,
     return n;
 }
 
-static int decode_fill(AACContext *ac, GetBitContext *gb, int len) {
+static int decode_fill(AACContext *ac, GetBitContext *gb, int len)
+{
     uint8_t buf[256];
-    int i, major, minor;
+    int i, major, minor, micro;
 
     if (len < 13+7*8)
         goto unknown;
 
-    get_bits(gb, 13); len -= 13;
+    get_bits(gb, 13);
+    len -= 13;
 
     for(i=0; i+1<sizeof(buf) && len>=8; i++, len-=8)
         buf[i] = get_bits(gb, 8);
@@ -2434,7 +2439,11 @@ static int decode_fill(AACContext *ac, GetBitContext *gb, int len) {
         av_log(ac->avctx, AV_LOG_DEBUG, "FILL:%s\n", buf);
 
     if (sscanf(buf, "libfaac %d.%d", &major, &minor) == 2){
-        ac->avctx->internal->skip_samples = 1024;
+        ac->avctx->internal->skip_samples -= 1024;
+    }
+
+    if ((sscanf(buf, "avc %d.%d.%d", &major, &minor, &micro) == 3)) {
+        ac->avctx->internal->skip_samples -= 1024;
     }
 
 unknown:
-- 
2.40.1


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