How to keep your original camera negative safe
To ensure the secure transfer of your original camera data, avoid using simple file copy methods (e.g. macOS Finder, Windows Explorer, or basic command line copy). Instead, follow these steps to ensure you don't lose a day's worth of footage because missing or corrupt files weren't noticed before the original recording media was erased. Your minimum standard should be checksum-verified data backups.
- Checksums: A checksum is a small block of data representing a file, used to detect errors that may have been introduced during file transmission. Different types of checksums exist, calculated using various algorithms, but they all serve the same purpose.
- Checksum Verification: A data management tool offering "checksum verify" will calculate and compare the checksum of a file with the checksum of its copy. The verification result (verified or failed), along with additional information, can then be stored in a checksum directory listing called a Media Hash List (MHL).
A Media Hash List (MHL) is an XML file which contains:
The next time the files will be copied, the hash values from the MHL file will be used as the reference for the verification. If any file was modified in any way (i.e. corrupted or edited), the checksums will no longer match and the verification will fail. Checksums therefore should be created as early as possible, i.e. when the original camera data is copied from the recording media and should be kept along with the original files all the way into archival.
Though most agree that checksums are vital, different tools, companies or individuals may utilize any of the verification methods/checksum types. This makes it difficult to track where a corruption or unintended modification occurred within the production’s lifespan.
To overcome this issue, a sub-committee of the American Society of Cinematographers (ASC), together with members from studios, software companies, DITs, and post facilities, developed an extension to the original MHL concept: the ASC MHL (version 2).
In a nutshell, the ASC MHL refines the XML structure of the MHL file and adds a so-called "chain file", which serves as an MHL history. The chain file keeps track of the path and file hash of each ASC MHL file so their integrity, in turn, can be verified. This enables productions to determine exactly where in the chain the problem occurred.
More information on the ASC MHL can be found on the website of the American Society of Cinematographers.
The following is a list of data management tools for ensuring secure handling and integrity verification of your original camera data. These offer the recommended level of security though verification. Please note that this list cannot be exhaustive!