[FFmpeg-user] Question about using Webm/Matroska container in audio transconding

[Ivan]

Thanks,
just curious why it then works with ogg container and not in webm. What's  reason that header must be updated afterwards?
I. 

On 11 December 2018 15:31:44 CET, Carl Eugen Hoyos <ceffmpeg at gmail.com> wrote:
>2018-12-11 15:13 GMT+01:00, Ivan Zderadicka
><ivan.zderadicka at gmail.com>:
>
>> ffmpeg -i kapitola.mp3 -y -map-metadata 0 -map a -acodec libopus -vbr
>on
>> -b:a 24k -compression_level 6 -cutoff 12000 -f webm pipe:1 >
>test1.webm
>
>(Complete, uncut console output missing.)
>
>If you use pipe output, it is impossible to update the file header
>information with the actual file duration.
>
>Carl Eugen
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