[Libav-user] Creating Panned MP3 Clips

Andrew Randrianasulu randrianasulu at gmail.com
Sun Jan 1 08:35:38 EET 2023

вс, 1 янв. 2023 г., 09:10 Terry Corbet <tcorbet at ix.netcom.com>:

> I have recently discovered how to use the Audacity Envelope Tool to turn
> a standard stereo MP3 file into a modified one in which throughout the
> entire duration of the clip the apparent source of the sounds will
> traverse from left to right.

may be pan filter can do something by altering volumes of individual
channels but as far as I can see you can't change its parameters at runtime?


Filter pan
  Remix channels with coefficients (panning).
       #0: default (audio)
       #0: default (audio)
pan AVOptions:
   args              <string>     ..F.A......

no T , as you can see (ffmpeg 5.1)

I wonder if our software (cinelerra-gg, video editor, so a bit heavyweight)
can do this via built-in keyframing .. I'll ask on our maillist.

While I could use that workflow to
> manually perform the same transformation on multiple files, for my own
> use as well as to help other family members [who generally have limited
> computer skills] I want to automate that workflow.
> Over the past four days I have played as much catch-up on the many
> topics and toolkits which appear might permit me to engineer a software
> solution to this requirement.  As a newbie, I probably will not
> correctly summarize what I believe to be the possible tools and
> approaches, so please forgive any misuse to the correct terminology.  I
> hope/believe that I might be able to state my concepts/questions in a
> manner which will be most considerate of the time of those who
> participate in this mailing list and most quickly help me move closer to
> a good approach to the challenge.
> 01.  I have managed to download the libraries which are used for the
> maintenance of the ffmpeg, ffprobe and ffplay triumvirate of tools.
> 02.  I have managed to successfully build some sample C programs [taken
> from the doc\examples sub-directory and other miscellaneous snippets
> found by following the wonderful links from your Wiki] using the
> CodeBlocks IDE framework.
> 03.  I have squirreled my way through the parts of the Doxygen
> documentation which seem like they would be most apropos.
> What I did not discover was any functions or examples of what I assumed
> I would be needing to do, which essential would be to process the audio
> frames of the FrontLeft [FL]  and FrontRight [FR] channels of coming out
> of a stream of packets.  That caused me to think that perhaps I would
> find examples of that processing by searching the Audacity sources to
> learn when and how they use the ffmpeg libraries.  And somewhere between
> the Audacity and FFmpeg sites I stumbled upon some sources and some
> documentation concerning what I suppose are two reasonable libraries
> devoted to "resampling" -- soxr and swr.
> It was about at that point that I concluded that my modification of the
> sampled frames probably does not fall within the ambit of what is meant
> by resampling at all and that led to an investigation of what Nyquist
> was all about.  Wow, what a guy Mr. Dannenberg must be.  The 2007
> Nyquist Reference Manual is a jaw-dropping read.
> I think that is enough background/context.  Here's were I would
> appreciate any suggestions:
> A.  Would it be possible to accomplish the steps necessary to achieve
> the desired result just using ffmpeg.exe?  I imagine that, using the
> command line tool and an appropriate shell scripting language, it might
> be necessary to make multiple passes of the original .mp3 file and/or
> the two separate channels.  I am not concerned about that loss of
> throughput; it will always be far faster than any manual procedure.
> B.  Nonetheless, there are some advantages that would accrue from
> accomplishing the work entirely in an application .exe with a little GUI
> glitter to help the user be able to attempt some trial-and-error
> [preview] with slight changes in some of the parameters of the task
> depending upon the nature of the audio content and the manner in which
> the user will eventually play the output on different devices in
> different environments.  Since I will not have the capabilities for
> building an Envelope in the manner that Nyquist [Lisp] accomplishes
> that, can anyone point me to any sample code doing that in C with the
> eight ffmpeg .dll libraries?
> C.  Or -- and I appreciate that it is not fair to ask this of this mail
> group -- but I would appreciate any experience/advice as to whether the
> solution really ought to be accomplished by some scripting and/or macro
> facilities wrapped around Audacity?
> Thank you so much for the fantastic capabilities you have provided with
> the entire FFmpeg effort and for your patience in reading through my
> questions as the bell is about to strike on the New Year.
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