The ISO format is an optical disc image format named as such because it follows the ISO 9660 standard for optical disc formats. It contains everything that would be written to an optical disc, sector-by-sector, including the optical disc file system. This has several advantages and disadvantages.
- Because it contains the original disc's file system and sector information, if you ever burn the ISO back to an optical disc, you're guaranteed it will be a perfect copy (excluding any weird security issues like purposely damaged sectors). While this was a wonderful feature back when people were still burning ISOs back to discs, it's not really a big deal anymore.
- It's always a single file, so you don't need to deal with header files like CUE or MDF.
- As one of the first disc image formats, it has a wide variety of support.
- Because it only stores sector information and not track information, it can only store a single track. Any disc with multiple tracks (like CD audio or CD-ROM with audio tracks) cannot be stored in a single file. This is only a problem for multi-tracked CDs which only make up a minority of discs.
- Because it stores all of a disc's information, it also stores unused information. So, if a 700 MB disc only contains 1 MB of usable files, the ISO will still be 700 MB. However, the empty space will compress especially well.
- Since the format doesn't have its own file system, but copies whatever is on the disc, any program which allows you to extract the format's data must implement a variety of file systems. Because of this, many ISO readers only support some ISO files, but not all.
- ISO doesn't include the control headers and error correction data. This is rarely a problem.
I learned about ISO disc images after it became feasible to download entire CDs rather than have them mailed or bought in stores. I tended to appreciate the simplicity of an ISO which was always a single file, but recognized that they were usually much larger than the other formats. They are still my go-to format for CD-ROM rips because they're so universal.
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