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Repair a corrupted Sony FX3 video

When a long FX3 take stops opening, it's rarely lost footage — it's a cine clip the camera never finalized. On a Cinema Line body built to keep rolling, the frames are all still there; what's missing is the moov index that points to them, and on the FX3 the file that's missing it is usually a big one. It never has to leave your machine to get that index back.

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Your data is the big block — usually intact. What breaks is the small index. Repair rebuilds it.

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The Sony FX3 (ILME-FX3) is a dedicated Cinema Line body — not a stills camera with a movie mode, but a full-frame cine tool with no mechanical shutter, an active cooling fan, and a form factor built around long continuous takes. That cooling is the whole point: where a hybrid like the a7 IV can throttle and thermally shut down on a long 4K run, the FX3 is rated to keep rolling, so its clips tend to be long and its files large. Shooters run it in S-Cinetone for a finished look straight out of camera, or in S-Log3/S-Gamut3 to grade later — footage that's expensive to lose. It records standard MP4 in three XAVC flavours (XAVC S H.264 Long-GOP, XAVC HS H.265/HEVC, and XAVC S-I All-Intra), and each only becomes playable when the camera stamps its moov index as the take ends cleanly. The FX3's failure mode follows straight from its strengths: a very long take cut short by a dead battery or a power loss, or a CFexpress Type A card yanked while it was still flushing, leaves a large, truncated .MP4 full of real XAVC frames with no moov to map them — and the longer you'd been rolling, the more footage there is to strand. Drop the clip above and the tool checks for exactly that, in your browser, and rebuilds the container when the footage is recoverable.

How the FX3 writes XAVC — and where a long take breaks

The FX3 is a video-first camera in a way its a7-series cousins aren't: it has no mechanical shutter, no viewfinder hump, and an active cooling fan built into the body. That fan is why it can hold a 4K take far longer than a hybrid like the a7 IV, which is exactly what makes FX3 clips run long and the resulting files run big — and a big, still-recording file is precisely what's at risk when power or storage drops out.

Whatever mode you pick, the footage lands in an MP4 container; the three XAVC formats differ only in the codec inside. XAVC S is H.264 (AVC) Long-GOP, the most broadly compatible. XAVC HS is H.265/HEVC, smaller for the same detail but it needs an HEVC-capable player. XAVC S-I is All-Intra H.264, where every frame is a full keyframe at very high bitrate — the format graders reach for when they want maximum headroom, and the one that survives truncation best because each frame stands on its own. In every mode the picture is streamed to the card as you roll, but the moov atom — the table that tells a player where each frame lives and how to decode it — is written only when the take is finalized, at the moment you press stop.

Because the fan takes overheating off the table, the FX3's unfinalized clips almost always trace back to power or storage, and its long-take habit makes each incident costlier:

  • Battery or power loss deep into a long take. High-bitrate 4K empties an NP-FZ100 quickly, and on external power a knocked cable does the same; the longer you've been rolling, the more footage sits in a file that never got its finalizing write.
  • A CFexpress Type A card pulled while it was still flushing. The FX3 buffers heavily at high bitrate — eject during that flush, or open the door while the access lamp is lit, and the tail of the file, index included, is never committed.
  • A card write error (Sony's C:13:01). A memory-card fault that surfaces under heavy writes; it can truncate the take in progress rather than wipe it.
  • A Relay Rec hand-off between the dual slots. When one card fills, the FX3 rolls onto the second mid-take; lose power or a card during that hand-off and the clip being written is left unfinalized.

CFexpress Type A, S-Log3 footage, and the reference clip

The FX3 records to CFexpress Type A and SD UHS-II in its dual slots, and the format decides what the card must sustain: XAVC S-I and the highest-bitrate 4K modes really want CFexpress Type A, while lighter XAVC S can live on a fast V60/V90 SD. Long-form shooting is where this bites — a marginal card that coasts through a short clip can stall deep into a 40-minute take, and a stalled write is one more way an FX3 clip ends up truncated and unfinalized.

There's a grading angle too, because so much FX3 work is shot flat. If you recorded S-Log3/S-Gamut3 to grade later, that log data lives in the same XAVC stream as everything else — a rebuilt container hands your colourist back the exact flat frames the camera wrote, without baking in a look or clipping the range. What the rebuild needs to be faithful is the codec configuration, and that's where a reference helps.

Rebuilding a missing moov means scanning the surviving stream and reconstructing the sample tables the camera would have written. That job is far more faithful with a healthy clip from the same FX3 shot in the same format and settings — same XAVC flavour (S, HS or S-I), same resolution, frame rate and bit depth. That reference carries the exact codec configuration (SPS/PPS for H.264, or VPS/SPS/PPS for HEVC) the broken file is missing. Grab a normal clip from the same card, keep it handy, and if you switched between XAVC S and XAVC HS make sure the reference matches the codec of the broken clip, not the other one. You'll only be asked for it if the file needs it.

What this can and can't fix

Can fix

  • The last long take when a dead battery or a power loss cut it short (unfinalized MP4, missing moov)
  • A large clip truncated when a CFexpress Type A card was pulled mid-flush or threw a C:13:01 write error
  • XAVC S (H.264), XAVC HS (H.265/HEVC) and XAVC S-I files — including S-Log3/S-Gamut3 footage — that copied off the card but won't play
  • Container damage: broken or missing index, wrong sample offsets, bad timestamps

Can't fix

  • Footage past the point the recording was cut — a battery or power loss truncates the file, and frames never written to the card can't be invented back
  • Footage physically lost to a failing or worn CFexpress Type A / SD card (recover the card first, then repair)
  • A broken XAVC HS (HEVC) clip when you have no healthy XAVC HS sample to supply its codec configuration
  • Sony's encrypted/DRM-protected or otherwise locked exports

If a repair fails, we tell you why (missing data versus broken structure), and you are never charged for a failed repair.

FAQ

It's a Cinema Line body with a fan — so why did my long take break?

The FX3 rarely dies of heat; the fan is built for exactly the long takes it's known for. So when a clip breaks it's almost always power or storage: a battery or external-power loss deep into a take, a CFexpress Type A card pulled mid-flush, or a C:13:01 write fault. Any of those cuts the recording before the moov index is written — and because FX3 takes run long, the orphaned file tends to be a large one.

I shot S-Log3 to grade later. Will the repair change my footage?

No. S-Log3/S-Gamut3 is simply how the frames were encoded into the XAVC stream; rebuilding the container re-maps those exact frames without touching their colour or dynamic range. You get back the same flat log footage to grade — the repair fixes the index, not the picture.

The take was cut short — does XAVC S-I recover more than Long-GOP?

If the clip was truncated, the codec matters. XAVC S-I keeps every frame whole, so an All-Intra clip can be salvaged frame by frame right up to the cut. XAVC S and XAVC HS are Long-GOP, so a truncated clip recovers cleanly up to its last complete group of pictures. Either way, footage from after the cut was never written and can't be recovered.

Do I need Sony Catalyst Browse or my NLE to fix it?

No. Catalyst Browse and editors like Resolve or Premiere read a finished container — they can play back gyro-stabilised, graded FX3 footage, but they can't rebuild a moov that was never written. This repair reconstructs the container first, in any modern browser, with nothing to install and the file never leaving your device — then you take the fixed clip into your grade.

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