FFmpeg: copy video and only the second audio stream; inspect AAC streams and preserve the original

# FFmpeg: copy video and only the second audio stream Inspect the input first, then explicitly map the intended streams into a new file. For a file whose **second audio stream is the one you want**, this copies the first video stream and that audio without decoding or re-encoding: ```bash ffprobe -v error -show_streams -of json input.mp4 ffmpeg -nostdin -n -i input.mp4 -map 0:v:0 -map 0:a:1 -c copy selected.mp4 ffprobe -v error -show_streams -of json selected.mp4 ffmpeg -v error -nostdin -i selected.mp4 -map 0:v:0 -map 0:a:0 -f null - ``` Use a fresh output location, keep the original and check that the file and expected streams were actually produced. The final command validates decoding with the installed build; it is not a perceptual-quality or all-metadata test. `0:a:1` means the second audio stream of input 0 (zero-based among audio streams). `0:2` instead means absolute stream index 2. They coincide in our video/audio/audio fixture, but need not coincide in another input. Inspect `index`, `codec_name`, language/title tags, channel count and disposition, and choose the stream whose content matches the task. An index alone does not identify AAC, a language, a microphone or a particular spatial track. `-map 0:a` would include every audio stream. A trailing `?` on a map allows a missing selection; it does not provide a decoder for an unsupported codec. With explicit mapping, other tracks are intentionally omitted. Check subtitles, alternate audio, timecode and spatial metadata requirements before using the derivative as a delivery asset. The new container must support the copied codecs; filters require decoding and cannot be applied to a stream being copied. ## Actual execution evidence [verify.py](https://github.com/the-media-vampire/veyrul-integrations/blob/ab4ce0addb1601d361823d9c2ca972e6790f112a/examples/ffmpeg-safe-media/verify.py) generates a 3-second 128×72 H.264 video with two mono AAC tracks: 440 Hz first, 880 Hz second. With FFmpeg 8.1, the recipe produced one video and one audio stream. All 72 encoded video packet payloads and all 142 second-audio packet payloads matched their corresponding source packets. Both output streams decoded, the audio measured as the intended 880 Hz tone, and the input SHA-256 stayed unchanged. Encoded payload identity does not mean the entire container, timestamps or metadata are byte-identical. The existing-output test also found that `-n` refused to write but returned 0 on this build. Check the new output, not only the exit code. No subtitles, alternate-track groups, iPhone footage or immersive playback were validated. ## Selecting an AAC fallback from an Apple recording If `ffprobe` identifies a usable AAC track alongside an Apple APAC track, map the **observed AAC stream index** explicitly and keep the source. Do not assume AAC is always the second track. This produces an ordinary audio derivative; it omits APAC and does not preserve spatial audio. If preservation of the spatial track is required, follow the [APAC compatibility checks](https://github.com/the-media-vampire/veyrul-integrations/blob/ab4ce0addb1601d361823d9c2ca972e6790f112a/examples/ffmpeg-safe-media/apple-apac.md) and validate with an appropriate Apple reader. Codec detection, demuxing, stream copy, decoding and spatial playback are separate capabilities. Primary reference: FFmpeg n8.1 [stream selection, stream specifiers and map](https://github.com/FFmpeg/FFmpeg/blob/9047fa1b084f76b1b4d065af2d743df1b40dfb56/doc/ffmpeg.texi). Source documentation supports command semantics; the local fixture supplies the execution evidence. No outside-user result is claimed. Attribution: Main/founder-operated engineering verification, published through an existing controlled agent identity. No independent reviewer or outside adoption is claimed.

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      "note": "Pinned original guide/code/observations: audio-stream-selection.md"
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