gstdec backend adds delay at the start when decoding a .mp3

[MZehren]

Hello,

When using the script decode.py to decode an mp3 into wav, the .wav file generated has 33ms of silence prepend at the start. Thus the original .mp3 and the decoded .wav are really not the same.

The output of the script is:

Input file: 2 channels at 44100 Hz; 452.7 seconds.
Backend: gstdec

A ffprobe of the original .mp3 returns:

$ ffprobe Black\ Blood\ -\ Aiea\ Mwana\ (T.Kolai\ Special\ Edit).mp3
ffprobe version 3.4.6-0ubuntu0.18.04.1 Copyright (c) 2007-2019 the FFmpeg developers
built with gcc 7 (Ubuntu 7t.3.0-16ubuntu3)
configuration: --prefix=/usr --extra-version=0ubuntu0.18.04.1 --toolchain=hardened --libdir=/usr/lib/x86_64-linux-gnu --incdir=/usr/include/x86_64-linux-gnu --enable-gpl --disable-stripping --enable-avresample --enable-avisynth --enable-gnutls --enable-ladspa --enable-libass --enable-libbluray --enable-libbs2b --enable-libcaca --enable-libcdio --enable-libflite --enable-libfontconfig --enable-libfreetype --enable-libfribidi --enable-libgme --enable-libgsm --enable-libmp3lame --enable-libmysofa --enable-libopenjpeg --enable-libopenmpt --enable-libopus --enable-libpulse --enable-librubberband --enable-librsvg --enable-libshine --enable-libsnappy --enable-libsoxr --enable-libspeex --enable-libssh --enable-libtheora --enable-libtwolame --enable-libvorbis --enable-libvpx --enable-libwavpack --enable-libwebp --enable-libx265 --enable-libxml2 --enable-libxvid --enable-libzmq --enable-libzvbi --enable-omx --enable-openal --enable-opengl --enable-sdl2 --enable-libdc1394 --enable-libdrm --enable-libiec61883 --enable-chromaprint --enable-frei0r --enable-libopencv --enable-libx264 --enable-shared
libavutil 55. 78.100 / 55. 78.100
libavcodec 57.107.100 / 57.107.100
libavformat 57. 83.100 / 57. 83.100
libavdevice 57. 10.100 / 57. 10.100
libavfilter 6.107.100 / 6.107.100
libavresample 3. 7. 0 / 3. 7. 0
libswscale 4. 8.100 / 4. 8.100
libswresample 2. 9.100 / 2. 9.100
libpostproc 54. 7.100 / 54. 7.100
Input #0, mp3, from 'Black Blood - Aiea Mwana (T.Kolai Special Edit).mp3':
Metadata:
NITR : NTKB?&?
title : Aiea Mwana
artist : Black Blood
comment : Afro-latino; xHD
encoder : Lavf57.83.100
TKEY : 11m
TBPM : 122
Duration: 00:07:32.68, start: 0.025057, bitrate: 128 kb/s
Stream #0:0: Audio: mp3, 44100 Hz, stereo, s16p, 128 kb/s
Metadata:
encoder : Lavc57.10

I don't have the same issue when using Madmom library to read files which relies on the ffmpeg backend I believe.
Where does this silence come from?

[sampsyo]

Huh, that's interesting! It would be worth digging into more deeply, but at the moment, I don't have an intuition for why GStreamer might be doing this. It might require some deeper GStreamer knowledge than I have to help explain it…

[lazka]

gstreamer doesn't support gapless decoding for mp3, which means there can be (very short) extra silence at the beginning and end of streams compared to other tools.