Multi-Track Audio — Present but Digitally Silent (MP4)
An audio track that exists, declares stereo, and contains nothing but zero samples. The twin of the no-audio-track file in this group, and the reason that file exists. This is the fixture that catches the false negative in every 'is the audio missing?' check built on a container probe: `ffprobe` reports a healthy AAC stereo track, the duration is right, the bitrate is plausible, and the viewer hears nothing. Detecting it requires decoding and measuring, not inspecting. The language is deliberately `und`, which is what encoders emit when nobody set one — another thing worth being able to reproduce.
Browser-playable test video · 8 seconds
Specifications
- Audio Tracks
- 1
- Channels
- 2
- Content
- digital silence — all samples zero
- Language
- und
- Codec
- AAC LC 48 kHz
- Faststart
- true
- Duration
- 8s
Testing contract
Expected to pass- Scenario
- Open the file, confirm an audio track is present, then measure its sample values.
- Expected result
- 1 AAC LC track at 48 kHz, 2 channels, language und, whose samples are all zero - digital silence. This is the distinction from the no-audio member of this group: a track exists and must be reported, but every sample is zero, so 'has audio' and 'has audible audio' are different questions and this fixture separates them.
What is a .mp4 file?
MP4 (MPEG-4 Part 14) is the dominant container for digital video, holding video, audio, subtitle, and metadata tracks in a tree of typed boxes. `ftyp` declares the brand, `moov` carries the sample tables that make seeking possible, and `mdat` holds the media; when an encoder writes `moov` last, playback cannot begin until the file has fully downloaded, which a faststart remux fixes. It derives from Apple's QuickTime format, generalized by ISO into the ISOBMFF base that MOV, 3GP, and HEIF share.
How to use this file
Use an example MP4 to test box parsing and track detection, seeking, range-request streaming, and transcode or thumbnail pipelines: verifying that a file with a trailing `moov` box is still handled, and that codec support is checked per track rather than inferred from the extension.
How to use this file for testing
“Multi-Track Audio — Present but Digitally Silent (MP4)” is a deterministic Novus Examples fixture for Media accessibility, Video QA, Video codecs, Conversion testing. Containers carrying several subtitle and audio tracks at once, with real language tags and the default, forced, hearing-impaired and visual-impaired flags actually set, for testing track pickers, accessibility menus, and the flag handling that only shows up when more than one track exists.
Documented properties for this file: 2 ch · 8s · und. Compare results against paired or grouped companions on this page when present (clean↔damaged, searchable↔scanned, or format twins) so scores stay reproducible across runs.
Download the file once, keep the path stable in CI or local scripts, and treat the spec table as the contract: dimensions, seeds, field lists, and roles are intentional. Corrupt or invalid samples are labelled as such, expect parsers to fail loudly rather than silently accept them.
Media fixtures are short and synthetic by design. Prefer waveform or transcript ground truth in the same group when measuring ASR, trim, upscale, or sync tools; do not assume broadcast-quality masters.
Code examples
<video controls preload="metadata" width="640" src="mp4-silent-audio-track.mp4"></video>Related files
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Generated by generation/video_accessibility.py. Free for any use, no attribution required, license.