audio/mpeg wrong bitrate is negative

[puzzle9]

Is there an existing issue for this?

• I have searched the existing issues

music-metadata version

v11.12.3

Current Behavior

bitrate is negative

{
    "format": {
        "tagTypes": [
            "ID3v2.4"
        ],
        "trackInfo": [],
        "lossless": false,
        "hasAudio": true,
        "hasVideo": false,
        "container": "ADTS/MPEG-2",
        "codec": "AAC",
        "sampleRate": -1,
        "bitrate": -27.557291666666668,
        "codecProfile": "AAC LTP",
        "numberOfChannels": 8
    },
    "track": {
        "no": null,
        "of": null
    },
    "disk": {
        "no": null,
        "of": null
    },
    "movementIndex": {
        "no": null,
        "of": null
    },
    "title": "我全都相信",
    "artists": [
        "吴克群"
    ],
    "artist": "吴克群",
    "album": "寄生"
}

Expected Behavior

吴克群 - 我全都相信.zip.001.zip
吴克群 - 我全都相信.zip.002.zip

Volume compression requires changing the name to. zip.001. zip.002

Attached audio sample?

• I have provided sufficient information to reproduce the issue

[MohammadYusif]

I traced the negative bitrate to the ADTS sampling-frequency-index handling in lib/mpeg/MpegParser.ts: when the index is the escape value 15 (sampling frequency signalled explicitly, not derivable from the fixed header), MPEG4.SamplingFrequencies[15] returns the sentinel -1, which is then treated as a valid rate. That -1 propagates into sampleRate and the ADTS bitrate calculation (8 * bytesPerFrame * (sampleRate / 1024)), yielding the negative bitrate/sampleRate: -1 seen here. Planning to fix it by mapping index 15 to null like the other reserved indices (13, 14), so the existing null-guards report an unknown rate instead of a bogus negative one — happy to discuss if you'd prefer a different direction before I open a PR.

[Borewit]

Sound like a good improvement @MohammadYusif .

Can you add a unit test represting this issue with a small .aac file?
Note that the sample file has an incorrect extension.

[MohammadYusif]

I’m on it