[Libav-user] Creating Panned MP3 Clips

Paul B Mahol onemda at gmail.com
Sun Jan 1 12:02:07 EET 2023

On Sun, Jan 1, 2023 at 7:36 AM Andrew Randrianasulu <randrianasulu at gmail.com>

> 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?
> https://ffmpeg.org/ffmpeg-filters.html#Changing-options-at-runtime-with-a-command
> ====
> Filter pan
>   Remix channels with coefficients (panning).
>     Inputs:
>        #0: default (audio)
>     Outputs:
>        #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.
Nope, your software can't do it.

Use ffmpeg's stereotools filter with asendcmd. Supports runtime changing of

> 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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