#9544: adding empty channels while downsampling with dithering leads to noise on new channels -------------------------------------+------------------------------------- Reporter: Harald | Type: defect Linden | Status: new | Priority: normal Component: | Version: undetermined | unspecified Keywords: | Blocked By: Blocking: | Reproduced by developer: 0 Analyzed by developer: 0 | -------------------------------------+------------------------------------- Summary of the bug: 1. Starting with a mono, 24bit, 48kHz audio file. 2. Adding 3 new channels leads to 3 additional, silent channels. 3. Downsampling the output to 16 bit with dithering leads to 1 channel with signal and 3 silent channels. 4. Doing step 2 and 3 at once needs to accumulating noise on the silent channels. How to reproduce: {{{ ./ffmpeg --version ffmpeg version N-104730-gbdc920574d Copyright (c) 2000-2021 the FFmpeg developers built with gcc 11 (Debian 11.2.0-12) ffmpeg -f lavfi -i "sine=frequency=1000:duration=1200" -c:a pcm_s24le sine_mono_24bit.wav ffmpeg -i sine_mono_24bit.wav -ac 4 sine_4c_24bit.wav ffmpeg -i sine_4c_24bit.wav -af aresample=osf=s16:dither_method=rectangular ./sine_4c_16bit_dither_noremap.wav ffmpeg -i sine_mono_24bit.wav -af aresample=osf=s16:dither_method=rectangular -ac 4 ./sine_4c_16bit_dither_remap.wav }}} sine_4c_16bit_dither_noremap.wav will have one channel with signal and three silent ones. sine_4c_16bit_dither_remap.wav should be the same, but will have accumulating noise on the silent channels. I discovered this while reencoding a combined 24-bit/5.1/2.0 source to 16-bit/5.1/5.1. The result had accumulating noise on the second audio stream. -- Ticket URL: <https://trac.ffmpeg.org/ticket/9544> FFmpeg <https://ffmpeg.org> FFmpeg issue tracker