Image_Formats.md

What’s the Most Convenient Optical Image Format for Daily Use?

There are many different optical disc image formats out there, with varying purposes. They support different features and amounts of information stored. Some of these have become commonly used standards that are very widely supported throughout different software, and those will (mostly) be the ones covered by this guide. And while it's always best to store the most information possible for an archival copy, it's often easier to keep the simplest possible copy of an image on hand for daily use due to factors such as software support.

Unfortunately, some discs really don't want to be copied, and need a more advanced image format than most in order for the content to be usable. Not all of these formats are open or documented, but they're important to know about and to use for these tougher-to-copy discs. Keep in mind that this guide is only intended to cover the most basic, widely known and used formats. That means that other formats (such as Aaru's Aaru Format images) aren't covered here. It's also worth noting that while a format may be able to support a copy protection in theory, a copy protected disc also needs to be dumped correctly to that format in order to work. Some copy protections may also need the image to be mounted to a virtual drive in a very specific way in order to work properly.

ISO-9660 Image Format (.ISO)

ISO images are the simplest possible image format there is, and are named after the ISO-9660 filesystem used on most PC CD-ROMs. This has become a misnomer, however, as the image format is also used for discs with other filesystems.

This format is essentially just a copy of the disc's filesystem that is present on Track 1, and doesn't store any additional data. Any specific information about the disc's structure is lost, along with any other tracks. This makes it essentially the worst image format to use in terms of preservation, but the best to use in terms of program compatibility. Due to its simplicity, it's also compatible with a wider variety of media, including CDs, DVDs, and BDs. DVDs and BDs are most commonly dumped to ISO images, due to the fact that they don't use multiple tracks and rarely require additional data that isn't present in the ISO.

CDRWIN Cue Sheet Image Format (.CUE/.BIN)

Cue sheet images, usually referred to as CUE images, are the next simplest commonly used image format. At its core, the format is made up of a .CUE file and a .BIN file. The .CUE file contains metadata about the image, along with essential information about the disc such as the track and session information. The .BIN file itself is, basically, a raw binary file that contains the disc's (more or less) exact contents. The data of the .BIN shouldn't change between different dumps of the same disc (whether from different users, drives, or software), but the .CUE can contain other metadata that's expected to differ. There are also multiple ways that the metadata and other information in a .CUE file can be organized, with sessions being a particularly non-standardized feature that can cause confusion between different programs.

The .CUE file itself is also human-readable, making it a particularly easy format to customize and alter by hand, as the .BIN file will generally be unaltered even between differing CUE standards. It also supports MANY more features compared to ISO, including multiple tracks, bad sectors, and different track types. This makes it much more suited to supporting more complicated CDs, such as games with real audio tracks present or that use copy protections. However, it's very much not suited to DVDs or BDs, which don't contain the additional data that .CUE images are designed to store, and instead can make use of features that CUE wasn't designed to support.

While the CUE format was originally used by the program CDRWIN, its accessibility and strong feature support led many other programs to readily adopt it for a variety of use cases, granting it exceptional software support as well. The relative lack of standardization is a key disadvantage, but it's generally not an issue due to the simplicity of the format allowing users to manually change the .CUE file if needed, as long as all the necessary data is present in the file. And while several features, such as sessions and subchannels, aren't universal elements of the format, it can be customized to add support for them. This comes at the cost of the aforementioned wide software support, so for many CDs which are more difficult to copy, more complex formats are generally used, which were often designed with those particular issues in mind.

CloneCD Image Format (.CCD/.IMG/.SUB)

CCD images follow the same basic structure as CUE images, with the .CCD file being the equivalent of the .CUE, and the .IMG file being the equivalent of the .BIN. In fact, the .BIN and .IMG files should be identical for the same disc, as they're both raw copies of the disc's data. In addition, a CCD image usually contains a .SUB file, which is dedicated to storing the subchannel data of the CD. While the .CUE is essentially human-readable information about the dump's metadata and the disc's structure, the .CCD is a more accurate copy of the disc's structure and doesn't contain metadata in the same vein as CUE. In practice, a CCD image can be thought of a more rigidly defined CUE image that also supports subchannels; the format is one step above CUE in terms of feature support, but it can't be customized and doesn't have nearly the same software support.

As opposed to CUE, which is intended to be accessible for more use cases, CCD is much more rigidly defined due to it being the output of the CloneCD program, which creates very high-quality dumps and supports the majority of optical disc copy protections. This makes CCD one of the best formats for copy-protected games, either for direct emulation in a virtual drive or for burning to a physical disc. However, for the same reasons as CUE, CCD only support CDs.

Media Descriptor Image Format (v1) (.MDS/.MDF)

The MDS format is once again similar to CUE and CCD in its basic structure. The .MDS ("Media Descriptor Sidecar") file is similar to the .CCD file in that it contains information about the disc's structure (along with additional data) which is not presented in a human readable format. The .MDF ("Media Descriptor File") file is also similar to the .BIN and .IMG files, except that it can also contain the subchannel data directly, as there isn't a dedicated .SUB file for this format. While this makes for less file clutter, it means that .MDF files with subchannel data will essentially never dump consistently the same. This is because subchannels are far less reliable to dump. Images dumped like this can't be used for things that require consistent dumps that have the same file hash, such as being used for a DAT.

The main reason to use MDS is the fact that it supports more devious copy protection schemes directly, most notably ones that make use of DPM. These discs are mastered to intentionally make certain sectors take non-standard amounts of time to read, and these read times are checked by the DRM to verify the disc's authenticity. The MDS format circumvents this by saving these read times to the .MDS file, which can then be emulated by a virtual drive. Only certain very specific virtual drives support this feature, including Alcohol 120% (the program this format was originally designed for) and Daemon Tools.

MDS is also the simplest and most accessible format for copy-protected DVDs, as the .MDS file can contain additional data needed for most DVD copy protections, such as DMI. It's also worth noting that for DVDs and BDs (due to the relative simplicity of their structure), the .MDF file should always match the .ISO file created by most other software. This means that if you have access to the necessary data, you can craft a functioning .MDS file yourself and rename a standard .ISO file to create a functioning image.

Thankfully, MDS as a whole is decently well documented, and can be converted easily to and from other formats. The main issue is that very few other formats support DPM, and there are very few alternative virtual drives able to run games that require it. None of them have significantly caught on so far, so they aren't mentioned here.

Media Descriptor Image Format (v2) (.MDS/.MDF/.MDX)

Lastly is the MDSv2, or for ease of reference, MDX format, which was created by Daemon Tools as an extension to MDS, and offers additional features. While it can be contained in .MDS and .MDF files, these files aren't compatible with software other than Daemon Tools, and are in fact obfuscated to prevent other programs from reading them. As an alternative, Daemon Tools introduced the .MDX file type, which is a new addition to MDSv2 consisting of .MDS and .MDF files combined into a single file, further reducing clutter and clearly differentiating these images from MDSv1 to avoid confusion.

This format should only be used when absolutely necessary, specifically for the most stubborn of copy protection schemes. Most notably, TAGES discs typically need to be dumped in this format due to its use of twin sectors. Daemon Tools is currently the best tool to circumvent this DRM, but unfortunately it can only dump twin-sector-protected discs to this obfuscated format, and the method by which it does so remains unknown, so these images are only currently usable with the Daemon Tools software itself. Always supplement an MDX dump with a dump from the same disc to an alternate, documented format.

In Conclusion

In my opinion, it's best to have at least two dumps of a disc on hand: one created in the format that stores the most data possible, and one in the simplest possible format that supports the disc, for the sake of ease of use. For normal purposes, most would consider CCD to be the best format to use for most CDs, with MDS being used for DVDs or particularly stubborn CDs; I personally dump to CCD using DiscImageCreator as well as AARUF using Aaru. But the most important thing is that the dump you create works. Even if you don't care too much about preservation, it's important not to "underdump" a disc to a format that doesn't support it. There's quite a few disc images online that aren't usable because the user dumped it to ISO when it has a copy protection that requires CUE, so make sure you're not one of the people doing that!