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.
Specifications
- Audio Tracks
- 1
- Channels
- 2
- Content
- digital silence — all samples zero
- Language
- und
- Codec
- AAC LC 48 kHz
- Faststart
- true
- Duration
- 8s
What is a .mp4 file?
MP4 (MPEG-4 Part 14) is a widely supported multimedia container based on ISOBMFF that holds video, audio, subtitles, and metadata, most commonly H.264 or H.265 video with AAC audio. It supports streaming, chapters, and multiple tracks. It is the dominant format for distributing and playing digital video.
How to use this file
Use an example MP4 to test container demuxing, track and codec detection, seeking and streaming, and transcoding or thumbnail-extraction pipelines.
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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- webmAlpha Channel — VP9 in WebMVideo with a real per-pixel alpha channel: VP9 yuva420p in WebM, the one alpha path browsers decode natively. Composite it over a page background and the transparency is genuine, not a chroma key. Roughly 90% of each frame is fully transparent. Two traps this file exists to expose. First, auto-alt-ref must be disabled at encode time or libvpx silently drops the alpha plane, producing a valid file with no transparency and no error. Second, WebM stores VP9 alpha in BlockAdditional and signals it with AlphaMode=1, so FFmpeg's NATIVE vp9 decoder reports pix_fmt yuv420p and decodes fully opaque — you must force `-c:v libvpx-vp9` to see the alpha at all. Probing this file with default settings and concluding it has no alpha is the expected mistake.
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- mp4Bitstream — Heavy B-Frames (8 Consecutive)Eight consecutive B-frames with a B-pyramid, so decode order and display order diverge sharply and DTS runs well behind PTS. The case that breaks naive timestamp handling and any code that assumes frames arrive in display order. Same picture and same codec as every other clip in this group — only the GOP and frame-type structure differ, so the effect on size and seekability is directly attributable.
- mp4Bitstream — Long GOP (24 Frames, One Per Second)One keyframe per second — the delivery default, and the setting that determines HLS/DASH segment boundaries, since a segment must start on a keyframe. Same picture and same codec as every other clip in this group — only the GOP and frame-type structure differ, so the effect on size and seekability is directly attributable.
- mp4Bitstream — No B-FramesI and P frames only. Required by some low-latency and legacy decoders, and it removes the reordering that makes decode order differ from display order. Same picture and same codec as every other clip in this group — only the GOP and frame-type structure differ, so the effect on size and seekability is directly attributable.
Generated by generation/video_accessibility.py. Free for any use, no attribution required — license.