[FFmpeg-devel] [PATCH] avcodec/atrac3: fix inconsistent band num calculation (PR #20308)

[Daniil Cherednik via ffmpeg-devel <ffmpeg-devel@ffmpeg.org>]

PR #20308 opened by Daniil Cherednik (dcherednik)
URL: https://code.ffmpeg.org/FFmpeg/FFmpeg/pulls/20308
Patch URL: https://code.ffmpeg.org/FFmpeg/FFmpeg/pulls/20308.patch

'decode_spectrum' reads 5 bits from bitstream to get
number of encoded subbands – so 31 means all 32
subbands are encoded. This value also is used to
determinate the number of used band in the hybrid
filterbank.

'subband_tab' array contains 33 values of MDCT spec
line positions started from 0 line and used to map
subband number in to the range of mdct lines.

Since the subband_num returned by decode_spectrum
actually is number – 1 and subband_tab started from 0
we need to add 1 to make num_bands calculation correct.


From 08cccba8e8bd0e2d0608ec19d195bac3da05a9c9 Mon Sep 17 00:00:00 2001
From: Daniil Cherednik <dan.cherednik@gmail.com>
Date: Wed, 20 Aug 2025 21:29:42 +0200
Subject: [PATCH] avcodec/atrac3: fix inconsistent band num calculation
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

'decode_spectrum' reads 5 bits from bitstream to get
number of encoded subbands – so 31 means all 32
subbands are encoded. This value also is used to
determinate the number of used band in the hybrid
filterbank.

'subband_tab' array contains 33 values of MDCT spec
line positions started from 0 line and used to map
subband number in to the range of mdct lines.

Since the subband_num returned by decode_spectrum
actually is number – 1 and subband_tab started from 0
we need to add 1 to make num_bands calculation correct.
---
 libavcodec/atrac3.c  | 2 +-
 tests/fate/atrac.mak | 4 ++--
 2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/libavcodec/atrac3.c b/libavcodec/atrac3.c
index fe156fa482..d91bdc79ac 100644
--- a/libavcodec/atrac3.c
+++ b/libavcodec/atrac3.c
@@ -616,7 +616,7 @@ static int decode_channel_sound_unit(ATRAC3Context *q, GetBitContext *gb,
 
     /* calculate number of used MLT/QMF bands according to the amount of coded
        spectral lines */
-    num_bands = (subband_tab[num_subbands] - 1) >> 8;
+    num_bands = (subband_tab[num_subbands + 1] - 1) >> 8;
     if (last_tonal >= 0)
         num_bands = FFMAX((last_tonal + 256) >> 8, num_bands);
 
diff --git a/tests/fate/atrac.mak b/tests/fate/atrac.mak
index 098063e298..847eed5b61 100644
--- a/tests/fate/atrac.mak
+++ b/tests/fate/atrac.mak
@@ -11,11 +11,11 @@ FATE_ATRAC1-$(call PCM, AEA, ATRAC1, ARESAMPLE_FILTER) += $(FATE_ATRAC1)
 
 FATE_ATRAC3 += fate-atrac3-1
 fate-atrac3-1: CMD = pcm -i $(TARGET_SAMPLES)/atrac3/mc_sich_at3_066_small.wav
-fate-atrac3-1: REF = $(SAMPLES)/atrac3/mc_sich_at3_066_small.pcm
+fate-atrac3-1: REF = $(SAMPLES)/atrac3/mc_sich_at3_066_small_with_band_fix.pcm
 
 FATE_ATRAC3 += fate-atrac3-2
 fate-atrac3-2: CMD = pcm -i $(TARGET_SAMPLES)/atrac3/mc_sich_at3_105_small.wav
-fate-atrac3-2: REF = $(SAMPLES)/atrac3/mc_sich_at3_105_small.pcm
+fate-atrac3-2: REF = $(SAMPLES)/atrac3/mc_sich_at3_105_small_with_band_fix.pcm
 
 FATE_ATRAC3 += fate-atrac3-3
 fate-atrac3-3: CMD = pcm -i $(TARGET_SAMPLES)/atrac3/mc_sich_at3_132_small.wav
-- 
2.49.1

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