Make webm audio recordings seekable\scrub (e.g. using ts-ebml)

pjeby · December 20, 2021, 7:01am

Use case or problem

Skip ahead to later parts of a recording made using the audio recorder core plugin, without first playing through the whole thing. (You can’t, because the file doesn’t contain cue metadata.)

Proposed solution

The ts-ebml package on npm has a command-line tool to make a Chrome webm recording seekable; its Typescript API could be used directly in the audio recorder to rewrite the webm file when recording is complete. The actual CLI implementation is only a few lines, and should work reasonably well even if the recording has to be read back into memory and written out again:

github.com

legokichi/ts-ebml/blob/09fceb199ee53dfdab2b578ee395378443940a5a/src/cli.ts#L24-L35


      
          const decoder = new Decoder();
          const reader = new Reader();
          reader.logging = false;
          reader.drop_default_duration = false;
          const buf = fs.readFileSync(args[0]);
          const elms = decoder.decode(buf);
          elms.forEach((elm)=>{ reader.read(elm); });
          reader.stop();
          const refinedMetadataBuf = tools.makeMetadataSeekable(reader.metadatas, reader.duration, reader.cues);
          const body = buf.slice(reader.metadataSize);
          const refined = new Buffer(tools.concat([new Buffer(refinedMetadataBuf), body]).buffer);
          process.stdout.write(refined);

Current workarounds

Alternative 1: Use the ts-ebml CLI to copy to another file, then copy back to overwrite the original (since it can’t be done in place in the shell).

Alternative 2: Convert the webm to mp3 with an audio converter or ffpmeg, then manually replace the link and delete the original file.

Related feature requests (optional)

neonplants · January 8, 2022, 8:57pm

Thanks for posting. I don’t understand how is the lack of this is acceptable in a software that seems made to capture and sort ideas.

WhiteNoise · March 24, 2022, 2:04pm

https://bugs.chromium.org/p/chromium/issues/detail?id=642012

Chrome has a long standing bug report/feature request to add metadata to the recorded webm in order to make it seekable. They don’t seem inclined to implement it. Maybe we will implement some form of workaround for this issue (like the one proposed by pjelby) if it works on all platforms.

A workaround will be tested in 0.14.3 for newly recorded files.

system · March 31, 2022, 2:04pm

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