What is ASC-MHL? Checksum verification for camera offloads, explained

What is MHL?

MHL stands for Media Hash List. It's a small file that travels with your footage and lists, for every clip, the checksum that was calculated when the files were offloaded. Think of it as a receipt that says "here is exactly what I copied, and here is the fingerprint of each file at that moment." Anyone down the line (editorial, post, the lab) can re-check the media against that list and know instantly whether anything changed.

What is ASC-MHL specifically?

Different tools used to write slightly different MHL files, which caused compatibility headaches. The American Society of Cinematographers standardized it into ASC-MHL: one open, documented format that every tool can read and write. An ASC-MHL file is XML with a .mhl extension, and it can record not just checksums but also file sizes, timestamps and the history of each copy generation. Because it's an open standard, an ASC-MHL file made by one app can be verified by another.

What is a checksum, in plain terms?

A checksum (or hash) is a short string of characters calculated from a file's exact bytes using a hash algorithm. The key property: change even one bit of the file and the checksum changes completely. So if the checksum of the original card file and the checksum of your copy are identical, the two files are byte-for-byte the same.

ASC-MHL supports several algorithms, including xxHash (xxHash64, XXH3, XXH128), MD5, SHA1, SHA256 and C4. On set, xxHash is a favorite because it's extremely fast, which matters when you're verifying terabytes, while still reliably catching copy errors.

Why "just dragging the files" is risky

When you drag a card's contents to a drive in Finder, macOS copies the files, but it never reads them back to confirm they're intact. Card readers, cables, hubs and drives can silently drop or mangle data, and a copy can finish "successfully" while a clip is quietly corrupted or truncated. You often won't find out until you try to use the footage in the edit, by which point the card has usually been formatted and reused.

Verification closes that gap: after copying, the tool reads the destination files back, recomputes their checksums, and compares them to the source. A mismatch is caught while the card is still in your hand, when you can simply copy again. That's the entire point of an MHL workflow.

Where ASC-MHL fits in a professional workflow

On film and TV sets, offloading isn't "copy and hope." It's a verified, documented step usually run by a DIT or data wrangler. Verified offloads with MHL files are expected (and for many high-end deliveries, such as Netflix-spec productions, effectively required) so that every department can trust the media and trace it back to the card. The MHL is the paper trail that says the footage is genuine and unaltered.

How to get ASC-MHL verification for free

You don't need a paid application to verify your offloads. Copsy is a free Mac app that runs ASC-MHL verification automatically: plug in a card, pick a destination (and optionally Google Drive or Dropbox in parallel), and Copsy copies, organizes by date and source, and verifies every file with ASC-MHL checksums — then pings you on Telegram when it's done and confirmed.

Download Copsy for Mac → · macOS 13+ · no signup

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