sandreas
commented
1 year ago Phew, thank you for this huge and detailed investigation.
I (personally) do not have ANY use case for this - fading in does indeed modify the audio in a way I never would like to have it. Furthermore I don't think this is really an issue... more like a detailed guide to achieve something.
The --ffmpeg-param thing was a quick and dirty approach to provide some extended feature, but it was a really, REALLY bad idea. It causes more issues than it solves in my opinion.
What I should have done instead was to provide a small plugin api to modify commands before they are getting executed. Example:
// my-plugin.php
m4btool_register_command_plugin(function(array $command, CommandContext $context) {
if(in_array("ffmpeg", $command, true)) {
return $command;
}
// modify command as you wish
// ....
// then return it
return $command
});And then running
m4b-tool merge --command-plugin="my-plugin.php" ....What do you think? Would this be better for your use case?
devnoname120
I spent quite a bit of time and attempts to figure out how to add a fade in/out effect between MP3s merged into an M4B. I share my solution here for future visitors. Note that this solution could easily be natively integrated in
m4b-toolbut my schedule is very busy and unfortunately I don't have the bandwidth to do a pull request.My requirements:
ffmpeg -f concat -f copyrun bym4b-toolis required to preserve them.My solution:
Note: For the conversion step I directly use a FFmpeg command (
ffmpeg -i {} -f lavfi -i […]) instead ofm4b-toolfor two reasons: 1)m4b-toolsilently ignores the--ffmpeg-paramfor the Fraunhofer FDK AAC (libfdk_aac) codec (!) becausem4b-tooldirectly runsffmpeginstead of using theFfmpeg.phpexecutable abstraction.--ffmpeg-paramis properly applied when using the native FFmpeg AAC Encoder (aac) codec. I use the Fraunhofer FDK AAC codec as it has a better encoding quality for a given bitrate compared to the native aac encoder. 2) The--ffmpeg-paramoption ofm4b-toolindiscriminately applies to both the conversion step (when using the native FFmpeg AAC Encoder) and the merge step (no matter what). This is due to the fact that they both use theFfmpeg.phpexecutable abstraction.-f concat -c copyused bym4b-toolfor the merge aren't compatible with FFmpeg filters. Removing these options would both force a re-encoding (which degrades the sound quality) and drop the individual metadata of each converted file (they are preserved thanks to the-f concat -c copyoptions).Explanations: The interesting parts are the following options in the first line. They add a fade-in + fade-out effect losslessly without an extra re-encoding step thanks to a filtergraph:
Detailed break down for the curious:
ffmpegis provided with two stream inputs:-i {}.-f lavfi) that just inputs silent audio (-i anullsrc). Check Step 3 to see why we need it.[0]afade=t=in:d=1:curve=tri[a]adds a fade-in effect at the start of the decoded file.[0]is used as the input of theafadefilter command. It corresponds to the first input passed to FFmpeg. Note that we can't use this filter for the fade-out at the end of the input stream as we would need to provide an absolute time offset in the stream, which we can't calculate within the filter pipeline. (Filter streams are non-rewindable and theafadefilter command doesn't support relative time offsets to the end of the stream).t=infor a fade-in effect. Since no start timestis specified, the effect applies at the beginning of the file.d=1means that the fade-in effect has a total duration of 1 second.curve=trito select a triangular linear fade-in transition function.[a]to direct the output of this step to a named streama.[1]atrim=0:0.7[t]cuts theanullsrcvirtual silent stream to last 0.7 seconds. It needs to have the same duration as the one specified by thedparameter ofacrossfadein the next filter.[1]is the input of theatrimfilter command. This corresponds to the second input passed to FFmpeg, hereanullsrc.0:0.7is the trim window. Here the trim will only keep the 0.7 seconds of the silentanullsrcstream.[a][t]acrossfade=d=0.7:o=1:c1=tri:c2=nofadeadds a cross fade effect at the end of the decoded stream[a]+ start of the second stream[t]. I use a trick (detailed below) to make it only add a fade-out effect at the end of[a]without changing its duration.[a]is used as the first input ofacrossfade. It corresponds to the output of the first step i.e. the decoded file stream with a fade-in effect at the start.[t]is used as the second input ofacrossfade. It corresponds to the output of the trim in the second step i.e. a silent stream with a duration of 0.7 seconds.d=0.7is the duration of the fade-out effect. It's important for it to be equal to theatrimlength of Step 2.atrimstep then the effect won't be applied at all (the second input stream needs to have a duration that is at least as long as the crossfade effect).atrimstep, then a silence is added to the end of the output stream. We don't want that to increase the duration of the stream and add a silent section at the end, but instead only add a fade-out effect.o=1means that the two streams should overlap during the cross-fade (fade out the first stream and fade in the second stream at the same time). This is the main trick of thisfilter_complexpipeline.[a]fades out while[t]fades in at the same time.[a]fade-out is done, the[t]silent stream fade-in is also over (because we trim it to 0.7s which is also the cross-fade duration).[t]is silent so the fade-in of[t]doesn't affect the output stream (a no-op).c1=trito select a triangular linear fade-out transition for the first stream.c2=nofadeto select an identity curve for the fade-in transition of the second stream. The choice of this curve shouldn't matter at the[t]stream is silent anyway.acrossfadeis the output of the whole filter pipeline.