FFmpeg - Crossfading inputs with a duration < 1s creates an empty output - audio

Question

I am trying to crossfade a silent input with a music to delay the moment when the music starts to play.
I built the command using fluent-ffmpeg so I could choose the duration of the silent input through my program. The duration of the crossfade is calculated according to the duration of the 2 inputs, and equals 0 if one of them is too short.
Below is an example of the resulting command:
ffmpeg -f lavfi -i anullsrc=r=44100 -i music.mp3 -y -filter_complex [0]atrim=duration=0.28[atrim_0];[atrim_0][1]acrossfade=d=0:c1=tri:c2=tri[final] -map [final] output.mp3
However, this command creates an empty output file when the duration of the silent input is inferior to 1 second, regardless of which music input is next. Using the same command with a trim duration > 1 second creates a valid output with the silence and the music.
I have tried to look through the FFmpeg debug report but couldn't really see what was wrong.
Below is an excerpt of the debug log report:
Input file #0 (anullsrc=r=44100):
Input stream #0:0 (audio): 14 packets read (28672 bytes); 14 frames decoded (14336 samples);
Total: 14 packets (28672 bytes) demuxed
Input file #1 (music.mp3):
Input stream #1:0 (audio): 504 packets read (210651 bytes); 504 frames decoded (578372 samples);
Total: 504 packets (210651 bytes) demuxed
Output file #0 (output.mp3):
Output stream #0:0 (audio): 0 frames encoded (0 samples); 0 packets muxed (0 bytes);
Total: 0 packets (0 bytes) muxed
Any idea what could cause this?
PS: I am using FFmpeg 4.4, and the same command with FFmpeg 4.2 lead to a segmentation fault. Don't know if this can be of any help

Answer 1

acrossfade can accept crossfade duration through two exclusive options: nb_samples (default: 44100) and duration (default: 0). When the latter isn't set, the former is used. So, in your command, acrossfade uses a crossfade duration of 44100 samples or 1 second. The filter needs both inputs to be at least as long as the crossfade duration.
However, in your case, it seems you just want to do two things: fade in the audio and maybe delay it. Just use afade for that.
ffmpeg -i music.mp3 -y -af afade=d=1:curve=tri,adelay=0.28s:all=1 output.mp3
This will fade-in the music over one second and delay the start by 0.28s.