AAC, M4A, and the container question

AAC refers strictly to the audio codec — the compression algorithm. It usually rides inside one of two containers: a bare .aac file (rare, no metadata wrapper) or an MP4 container with an .m4a extension (common, supports tags, chapter markers, and album art). The transcription pipeline handles both — drop a .aac or a .m4a, the engine decodes the audio and emits Markdown identically.

Where AAC files come from

iTunes Store purchases from Apple are AAC (the iTunes Plus tier at 256 kbps; older iTunes purchases were 128 kbps DRM-locked AAC, now mostly upgraded to DRM-free). YouTube audio streams are AAC at multiple quality tiers; tools that download YouTube audio (yt-dlp, etc.) typically output AAC or repackage to MP3. Apple Podcasts and Apple Music serve AAC. Most TVs, streaming devices, and broadcast workflows use AAC for audio because of its quality-per-bit advantage over MP3.

Practical use cases

The most common path: someone rips a YouTube interview, lecture, or podcast to AAC for offline reading and wants a transcript. The MDisBetter web tool takes the AAC, transcribes it with speaker labels, and gives you Markdown in a few minutes. For batch ripping (e.g., archiving a YouTube channel's spoken-word content), the OSS path is yt-dlp + faster-whisper running locally; the web tool is the no-setup path for one-off files.