FFMPEG blank tiny audio spot when concating

Question

I am trying to concat some mp3 audio files with the following command:

ffmpeg -y -i concat:"1.mp3|2.mp3|3.mp3" -c copy output.mp3

FFMPEG leaves some tiny empty audio spots between the concatenated files, looking output.mp3 with Audacity i have:

Is there some functionality on FFMPEG to avoid these empty audio spots?

Answer 1

The LAME Technical FAQ has some relevant information. Here are some excerpts:

Why does LAME add silence to the beginning each song?

This is because of several factors:

Decoder delay at start of file:

All decoders I have tested introduce a delay of 528 samples. That is, after decoding an mp3 file, the output will have 528 samples of 0's appended to the front. This is because the standard MDCT/filterbank routines used by the ISO have a 528 sample delay.

Furthermore, because of the overlapped nature of MDCT frames, the first half of the first granule (1 granule=576 samples) doesn't have a previous frame to overlap with, resulting in attenuation of the first N samples.

Encoder delay at start of file:

ISO based encoders (BladeEnc, 8hz-mp3, etc) use a MDCT/filterbank routine similar to the one used in decoding, and thus also introduce their own 528 sample delay. A .wav file encoded & decoded will have a 1056 sample delay (1056 samples will be appended to the beginning).

Starting with LAME 3.55, we have a new MDCT/filterbank routine written by Takehiro Tominaga with a 48 sample delay.

Refer to the LAME Technical FAQ for additional related answers and more in-depth information.

Some options that aren't great but might do the job:

• Do you have to use MP3? Can you re-output your files to a different format such as PCM ("wav")? Note that simply re-encoding the existing MP3 files to another format will preserve the delay.

• You can use the afade audio filter to add a fade out/fade in per section, or the atrim audio filter to possibly make the gaps less abrupt. However, filtering requires re-encoding.