[FFmpeg-devel] [PR] aacsbr_template: fix SBR USAC coupling (PR #21548)

[Lynne via ffmpeg-devel <ffmpeg-devel@ffmpeg.org>]

PR #21548 opened by Lynne
URL: https://code.ffmpeg.org/FFmpeg/FFmpeg/pulls/21548
Patch URL: https://code.ffmpeg.org/FFmpeg/FFmpeg/pulls/21548.patch

This issue hid under the radar since the codebooks between coupling
modes very often result in identical bit counts regardless of the encoded
data, leading to no frame-level bitstream desyncs except in rare cases.

AAC Mps212 data is parsed immediately after the SBR data, where a loss
of sync in SBR will result in Mps212 being wildly different.


>From 82b768cfae1a1f69cb5ae927496083b935ce3837 Mon Sep 17 00:00:00 2001
From: Lynne <dev@lynne.ee>
Date: Thu, 22 Jan 2026 19:58:31 +0100
Subject: [PATCH] aacsbr_template: fix SBR USAC coupling

This issue hid under the radar since the codebooks between coupling
modes very often result in identical bit counts regardless of the encoded
data, leading to no frame-level bitstream desyncs except in rare cases.

AAC Mps212 data is parsed immediately after the SBR data, where a loss
of sync in SBR will result in Mps212 being wildly different.
---
 libavcodec/aacsbr_template.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/libavcodec/aacsbr_template.c b/libavcodec/aacsbr_template.c
index 3c39da509d..dd9340ff2c 100644
--- a/libavcodec/aacsbr_template.c
+++ b/libavcodec/aacsbr_template.c
@@ -1287,6 +1287,8 @@ int ff_aac_sbr_decode_usac_data(AACDecContext *ac, ChannelElement *che,
         if ((sbr->data[0].bs_add_harmonic_flag = get_bits1(gb)))
             get_bits1_vector(gb, sbr->data[0].bs_add_harmonic, sbr->n[1]);
     } else if (get_bits1(gb)) { /* bs_coupling == 1 */
+        sbr->coupling = 1;
+
         /* if (harmonicSBR) ... */
 
         if (read_sbr_grid(ac, sbr, gb, &sbr->data[0]))
@@ -1317,7 +1319,10 @@ int ff_aac_sbr_decode_usac_data(AACDecContext *ac, ChannelElement *che,
         if ((sbr->data[1].bs_add_harmonic_flag = get_bits1(gb)))
             get_bits1_vector(gb, sbr->data[1].bs_add_harmonic, sbr->n[1]);
     } else { /* bs_coupling == 0 */
+        sbr->coupling = 0;
+
         /* if (harmonicSBR) ... */
+
         if (read_sbr_grid(ac, sbr, gb, &sbr->data[0]))
             return -1;
         if (read_sbr_grid(ac, sbr, gb, &sbr->data[1]))
-- 
2.52.0

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