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content/blog/audioreplace.md

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+---
+title: "Replace Audio in Video"
+date: 2020-04-20T20:32:26-04:00
+draft: false
+tags: []
+---
+
+I recorded a video and wanted to touch up my audio in audacity. Here's how I used `ffmpeg` to extract the audio, and then replace it with a modified version.
+
+## Extract Audio
+
+If you know the format of the audio (mp3, ogg, aac) then it's possible to do a byte copy of the audio track into a file:
+
+```bash
+ffmpeg -i input_video.mkv -vn -acodec copy output.aac
+```
+
+| Argument       | Description                |
+| -------------- | -------------------------- |
+| `-i`           | Input                      |
+| `-vn`          | No Video                   |
+| `-acodec copy` | Copy audio stream directly |
+
+If you don't know the audio codec and have `mediainfo` installed, then run
+
+```bash
+mediainfo --Inform="Audio;%Format%" input_video.mkv
+```
+
+If you gave up, then you can transcode the audio (will take longer than direct copy)
+
+```bash
+ffmpeg -i input_video.mkv -vn output.aac
+```
+
+## Replacing Audio
+
+Once you're done touching up the audio (`touchup.mp3`), you'll want to replace the existing audio with it.
+
+```bash
+ffmpeg -i input_video.mkv \
+       -i touchup.mp3 \
+       -c:v copy \
+       -map 0:v:0 \
+       -map 1:a:0 \
+       output_video.mp4
+```
+
+| Argument               | Description                                                  |
+| ---------------------- | ------------------------------------------------------------ |
+| `-i`                   | Inputs                                                       |
+| `-c:v copy`            | Make this a copy operation                                   |
+| `-c:v copy -map 0:v:0` | Map the video from the first input to the first video output |
+| `-map 1:a:0`           | Map the audio from the second input to the first video output |
+