universal-video-transcode

Any video or audio file in, one MP4 out that plays on both an iPad and an Android device — at the best quality the target codec allows.

Give it a path. A file converts; a folder converts everything in it.

python scripts/transcode.py "D:/video/clip.mkv"
python scripts/transcode.py "D:/video" --recursive --dry-run    # plan only, no encoding
python scripts/transcode.py "D:/video" --out D:/converted

What it does

It reads the source file (never the filename) and picks one of three recipes:

recipe	when	video	audio
copy	H.264/HEVC with a usable timeline	-c copy — bit-identical	copy if AAC
fix-audio	same, but the audio is not AAC	-c copy	AAC 256k
encode	VP9/AV1, or a timeline that cannot be copied	HEVC (or x264)	copy if AAC, else AAC 256k

A copy is used whenever a copy is possible, because a copy is lossless by definition — the strongest possible answer to "keep the quality the same".

Then it verifies the output and refuses to accept it otherwise: duration, resolution, codec, hvc1 tag, 8-bit pixel format, uniform frame cadence, no flat composition offsets, and a full decode pass.

Why it exists

A file can drop ~23% of its frames in a browser while r_frame_rate == avg_frame_rate reads exactly equal — so every fps-based check calls it healthy. The cause is deterministic and header-readable: the MP4's composition-offset table is unusable, either a single constant ctts entry (PTS == DTS + const) or no ctts box at all (PTS == DTS). The timestamps are then in decode order, and the decoder's reordered output frames carry the wrong times.

This repo gates on that, structurally, without playing anything.

python scripts/selftest.py     # builds both defect shapes and asserts the detector

fixture                            expect   got      verdict
src.mp4                            False    False    ok    healthy MP4, proper ctts
src.mkv                            False    False    ok    Matroska - no ctts by design
flat_noctts.mp4                    True     True     ok    MP4 with NO ctts box
flat_ctts1.mp4                     True     True     ok    MP4 with a single constant ctts entry

Measured on the file that prompted this: 23.0% of frames dropped before, 0.0% after.

The full benefit case — the ten real failures this pipeline hit and the guard each one produced — is in docs/WHY.md.

Compatibility

	iPad / iOS	Android
Container	MP4/MOV only — no Matroska	MP4 and MKV both fine
Video	H.264 (all), HEVC hvc1 (A9+)	H.264 Baseline mandatory; HEVC 5.0+
Audio	AAC — never Opus	AAC and Opus both fine

Apple is the constraint, not Android: iOS has no Matroska demuxer and cannot play Opus, so MP4 + AAC is what forces the conversion. Two traps: HEVC must be tagged hvc1 (ffmpeg defaults to hev1, which Apple refuses), and Android only guarantees HEVC Main profile Level 3 — use --codec x264 when cheap/old Android hardware is in scope.

Details and citations: references/compatibility-matrix.md.

Quality policy

• Never upscale — the source resolution is preserved; there is no scale in the pipeline.
• HEVC at -cq 27: measured SSIM 0.9937 (visually transparent) at ~source size.
• H.264 (when chosen for reach) at -crf 20.
• Audio copied when already AAC; otherwise AAC 256k, where the residual error is the source's own lossy error — 320k buys nothing measurable.
• -fps_mode cfr on every re-encode: without it the encoder silently drops 0.5–1.3% of frames, leaving doubled intervals that play as judder.

Running it unattended

Safe to schedule — it is idempotent and self-locking:

• a lock file refuses a second instance (two concurrent NVENC jobs starve one encoder and drop frames, the exact defect this tool removes)
• an already-playable file is skipped, so a re-run costs one probe
• a failed item leaves the input untouched, so the next run retries it
• aborts below 5 GB free rather than filling the work drive

python scripts/transcode.py "Z:/media/incoming" --recursive \
  --out "Z:/media/converted" >> /var/log/transcode.log 2>&1

Install as a Hermes skill

This repo is the canonical source. Copy it into the skill directory rather than editing the skill copy in place:

cp -r . "$LOCALAPPDATA/hermes/skills/media/universal-video-transcode"

See AGENTS.md for the non-negotiables.

Options

flag	meaning
--dry-run	print the plan, encode nothing
--codec auto|hevc|x264	force the video codec (x264 for maximum reach)
--cq N / --crf N	quality (HEVC default 27 / H.264 default 20)
--audio 256k, --channels 0	audio bitrate; 0 keeps the source layout
--cpu	force libx265/libx264 (no NVENC)
--keep	keep the input file
--overwrite	replace an existing output
--no-decode-check	skip the full decode pass (faster, less safe)
--lock PATH	lock file location

ffmpeg/ffprobe are taken from PATH, or from TRANSCODE_FFMPEG / TRANSCODE_FFPROBE.

License

MIT © 2026 Lee Teong Hoe (Hoelee Enterprise)