How to create good sample files

Sometimes, I ask for samples of photo or video files to adjust something in Klennet Carver. I need these samples for some debugging or fine-tuning of the recovery. I typically set out some guidelines about the samples, and I decided to put them all down here.

There are two distinct classes of samples:

  1. Sample recovered files, and
  2. sample known-good files.

The difference is that known-good files are exactly as produced by the camera, without any post-processing. Recovered files may be damaged or have their headers reconstructed as part of the recovery. Therefore, recovered files (even if playable) may not match the originals exactly.

Sample recovered files

There is little choice in sourcing these. Generally, pick the smallest file which exhibits the problem, and that's it.

Sample known-good files

First, try to find the existing file matching the requirements below. If you do not have one, use a spare card to make a new file. Do not use the card from which you need to recover data.

For image files

  • Use the same resolution and same camera settings you used to shoot the images you are recovering.
  • Make sure the images are representative of what is on the damaged card.
    • If there were just general photos, then just about anything would do;
    • if you took the images of interest at night or in low light, take sample images in similar conditions;
    • if the images of interest were something special, like photos of documents (black text on white), shoot similar material for samples.

For video files

  • Use the same resolution, compression, and whatever other camera settings you used to record the videos you are recovering.
  • Record the video with sound.
  • Record some motion.
  • The sample length should be 20 to 30 seconds.

One good source of samples is a video of a PC monitor playing a random YouTube video full screen with sound playing on speakers. A 30-second record of this usually makes a nice sample.

One more thing is that no post-processing is allowed. Copy the file directly from the card and upload it to some cloud storage, like Google Drive or Microsoft OneDrive. Uploading to YouTube does not work because YouTube converts the files to its own formats. Links to videos on YouTube are not a sample, regardless of what the original was.

Filed under: File carving.

Created Friday, February 15, 2019