Hi everyone,
We are increasingly encountering issues with decoding AAC audio during playback. In particular, we see the following messages in the logs of CasparCG 2.3.2 (4de6d18f8 Dev):
[2026-04-01 09:04:16.239] [error] [ffmpeg] [aac @ 0x7f9886836b00] Reserved bit set.
[2026-04-01 09:04:16.239] [error]
[2026-04-01 09:04:16.280] [error] [ffmpeg] [aac @ 0x7f9886836b00] Reserved bit set.
[2026-04-01 09:04:16.280] [error]
[2026-04-01 09:04:16.285] [error] [ffmpeg] [aac @ 0x7f9886836b00] Reserved bit set.
[2026-04-01 09:04:16.285] [error]
[2026-04-01 09:04:16.319] [error] [ffmpeg] [aac @ 0x7f9886836b00] Reserved bit set.
[2026-04-01 09:04:16.319] [error]
[2026-04-01 09:04:16.324] [error] [ffmpeg] [aac @ 0x7f9886836b00] Reserved bit set.
[2026-04-01 09:04:16.324] [error]
[2026-04-01 09:04:16.359] [error] [ffmpeg] [aac @ 0x7f9886836b00] Reserved bit set.
[2026-04-01 09:04:16.359] [error]
[2026-04-01 09:04:16.363] [error] [ffmpeg] [aac @ 0x7f9886836b00] Reserved bit set.
[2026-04-01 09:04:16.363] [error]
[2026-04-01 09:04:16.400] [error] [ffmpeg] [aac @ 0x7f9886836b00] Reserved bit set.
These messages are logged continuously, creating additional load on the disk and consuming free space. At the same time, the output NDI stream may start stuttering or even freezing.
Here is an example of the audio stream information (MediaInfo):
Audio
ID : 2
Format : AAC LTP
Format/Info : Advanced Audio Codec
Codec ID : mp4a-40-4
Duration : 49 min 15 s
Bit rate mode : Variable
Bit rate : 219 kb/s
Maximum bit rate : 231 kb/s
Channel(s) : 2 channels
Channel layout : L R
Sampling rate : 48.0 kHz
Frame rate : 46.875 FPS (1024 SPF)
Compression mode : Lossy
Stream size : 77.0 MiB (4%)
Encoded date : 2024-10-10 05:48:01 UTC
Tagged date : 2024-10-10 05:48:01 UTC
From what I found, the “Reserved bit set” error means that a reserved bit in the AAC bitstream header is set to 1, while according to the specification it should be 0. FFmpeg treats this as a sign of a malformed or corrupted stream
However, the same file that triggers these errors in CasparCG plays perfectly fine in other players such as FFmpeg, VLC, and KMPlayer
Has anyone encountered a similar issue or has ideas on how to debug or resolve this problem?
Any insights would be greatly appreciated