Digital preservation and high-performance emulation often seem to be at odds. On one side, there is the drive to maintain absolutely pure, 1:1 bit-perfect copies of original optical media. On the other, there is the practical reality of limited storage space and the need for organized, single-file management. This tension is where the discussion of Redump vs CHD begins. Contrary to popular belief, these two terms do not represent competing technologies. Instead, they represent a professional workflow where one provides the verified source and the other provides the optimized delivery mechanism.

Understanding the relationship between the Redump project and the Compressed Hunks of Data (CHD) format is essential for anyone building a serious digital library. Whether managing a massive PlayStation 2 collection or archiving rare Saturn titles, knowing how to leverage both ensures that games remain functional, verifiable, and efficient.

Defining the Roles of Redump Standards and CHD Technology

To distinguish between the two, it is necessary to move away from viewing them as file extensions. Redump is an organization and a methodology, while CHD is a specific binary container designed for efficiency.

Redump operates as a disc preservation group. Their mission is to create a definitive database of every retail disc ever pressed for consoles and computers. When a user refers to a "Redump set," they are referring to disc images (usually in .bin and .cue format) that have been hashed and verified against the group’s central database. If the hash of a file matches the Redump entry, it is guaranteed to be an identical copy of the physical retail disc, including all sub-channel data and specific track offsets.

CHD, which stands for Compressed Hunks of Data, was originally developed by the MAME (Multiple Arcade Machine Emulator) team. Its purpose was to solve a specific problem: arcade hard drives and CD-ROMs were massive, and keeping them in raw formats was consuming too much disk space. The format was designed to be "lossless," meaning that even though the file size is reduced, every single bit of the original data is retained. When an emulator reads a CHD file, it decompresses the necessary data in real-time, providing the system with exactly what it would expect from a raw disc image.

The synergy is simple: Redump provides the "Master" copy, and CHD provides the "Production" copy.

The Technical Architecture of Redump Preservation

The Redump project is the gold standard because of its rigor. In the world of disc dumping, simple ISO files are often insufficient. Many early game consoles used multi-track discs containing both data and CD-DA (Compact Disc Digital Audio) tracks. A standard ISO cannot represent these multiple tracks, leading to missing music or broken game logic.

Redump solves this by using the .bin/.cue format. The .bin file contains the raw data of the disc, while the .cue file acts as a metadata sheet, telling the system exactly where each track begins and ends. In many cases, a single game might consist of one .cue file and dozens of .bin files. While this is perfect for accuracy, it is a nightmare for file management.

The verification process involves three primary hashes:

  1. CRC32: A quick check for data integrity.
  2. MD5: A more robust message-digest algorithm.
  3. SHA-1: The primary identifier used by the Redump database to ensure no two files are confused.

Maintaining a collection in its raw Redump state ensures that as long as the hashes match the database, the archive is "perfect." However, the lack of compression means a PlayStation 1 library that could fit on a 1TB drive might require 2TB or more when left in raw format.

Decoding CHD as the Modern Standard for Emulation

CHD was built to address the flaws of raw dumps without sacrificing the integrity that Redump enthusiasts demand. The technical brilliance of CHD lies in its "Hunk" based architecture. Instead of compressing the entire file as one giant block—which would require decompressing the whole thing just to read one sector—CHD breaks the data into smaller, manageable chunks (Hunks).

Key technical features of the CHD format include:

  • Lossless Compression: It utilizes Zlib, LZMA, and FLAC (for audio tracks) to shrink data. Because it is lossless, a CHD file can be converted back into its original .bin/.cue state with 100% accuracy.
  • Metadata Storage: Unlike simple ZIP files, CHD stores the track information, sub-channels, and offsets internally. This turns a messy 30-file Redump set into a single, clean .chd file.
  • Hard Drive Support: Beyond optical discs, CHD can also compress raw dumps of IDE and SCSI hard drives used in arcade machines and early PC systems.

As of the current V5 CHD standard, the compression ratios are impressive. PlayStation 1 and Sega Saturn games often see a 40% to 60% reduction in size, especially for games that featured large amounts of red-book audio or empty padding on the disc.

Performance and Compatibility Comparisons

When choosing between keeping files in Redump format or converting them to CHD, performance is a common concern. Modern hardware has reached a point where the overhead of decompressing a CHD file in real-time is negligible.

Storage Efficiency

Redump files (.bin/.cue) take up the exact amount of space as the original physical media. If a disc was 700MB, the file is 700MB. CHD files, however, only store the actual data. If a 700MB disc only contained 200MB of actual game data and 500MB of "null" padding (a common practice to keep data on the outer edges of the disc for faster seek times), the CHD file will shrink down to roughly 200MB.

Loading Times

There is a common misconception that compressed files load slower. In reality, because the CPU can decompress data faster than a mechanical hard drive (or even a standard SATA SSD) can read raw data, loading a CHD file can sometimes be faster than loading a raw .bin file. The bottleneck is shifted from I/O throughput to CPU cycles, which are abundant in modern emulation setups.

Emulator Support

While Redump files are supported by virtually every emulator in existence, CHD support has become the industry standard for modern, high-tier emulators.

  • DuckStation (PS1): Native CHD support.
  • PCSX2 (PS2): Native CHD support, which is a massive space saver for DVD-based games.
  • RetroArch (Beetle PSX, Flycast, Genesis Plus GX): Full support across most disc-based cores.
  • PCSX-ReARMed: Support varies by platform but generally excellent on modern ARM devices.

Real World Experience Converting a Multi Terabyte Library

In our testing, we took a Redump-verified PlayStation 2 library totaling 4.2TB. This collection consisted of over 2,000 titles, many of which were multi-disc sets. The file structure was cluttered, with various ISOs and BIN/CUE files making it difficult to scrape metadata or organize via frontends like LaunchBox or EmulationStation.

Using chdman, the command-line utility provided with MAME, we initiated a batch conversion. The process for a single game looks like this: chdman createcd -i "GameName.cue" -o "GameName.chd"

The results were transformative. The total library size dropped from 4.2TB to approximately 2.6TB. This 38% reduction in space allowed the entire collection to fit on a single, more affordable 4TB drive with room to spare, rather than requiring a more expensive 8TB or dual-drive setup.

From an organizational standpoint, the "Experience" factor cannot be overstated. Instead of seeing:

  • Final Fantasy VII (Disc 1).bin
  • Final Fantasy VII (Disc 1).cue
  • Final Fantasy VII (Disc 2).bin
  • Final Fantasy VII (Disc 2).cue
  • Final Fantasy VII (Disc 3).bin
  • Final Fantasy VII (Disc 3).cue

The library became a streamlined list of single .chd files. Frontends had a 100% success rate in identifying the games, and the "clutter" of the .cue files was eliminated entirely as the metadata was now safely tucked inside the CHD header.

Advanced Verification Workflows with Verifydump

One of the primary reasons purists hesitate to move to CHD is the loss of hash-verifiability. Once a .bin file is compressed into a .chd, its SHA-1 hash changes. You can no longer simply drag and drop the file into a web-based MD5 checker to see if it matches the Redump database.

This is where tools like verifydump become essential. verifydump is a command-line utility designed specifically to bridge the gap between CHD files and Redump DAT files. The tool works by:

  1. Temporarily decompressing the CHD data into memory or a temporary directory.
  2. Reconstructing the .bin/.cue structure based on the internal CHD metadata.
  3. Comparing the reconstructed data against the official Redump DAT file.

In our workflow, running verifydump gave us the peace of mind that our 2.6TB of CHD files were still "perfect" representations of the Redump source. It confirmed that no bit-rot had occurred during conversion and that the chdman utility had correctly mapped the track offsets. For those worried about long-term archival, keeping the Redump DAT files alongside your CHD collection is the professional way to ensure your library remains "standard-compliant" without wasting terabytes of space.

System Specific Considerations for Disc Based Games

Not all disc systems are created equal, and the conversion process can vary depending on the original media's quirks.

PlayStation 1 and 2

These are the easiest to handle. Redump typically provides these as .bin/.cue or .iso. The createcd and createdvd (or createcd for both in newer versions) commands in chdman work flawlessly. Since these systems are the most popular for emulation, CHD support is rock-solid.

Sega Dreamcast

The Dreamcast is a unique case. The original GD-ROMs held 1.2GB of data, more than a standard CD. Early scene dumps used the .cdi format (DiscJuggler), which often downsampled audio or removed data to fit the game onto a standard 700MB CD-R. Redump, however, provides high-quality dumps in .cue format, while the TOSEC project uses .gdi.

For a long time, there was debate about whether chdman could correctly handle .gdi files. Current versions of chdman handle .gdi and Redump .cue files for Dreamcast perfectly. Using CHD for Dreamcast is highly recommended because it eliminates the confusion between .gdi, .bin, and .raw files, merging them into one.

Sega Saturn

Saturn games are notorious for having many small audio tracks. A single game like Rayman might have 50 or more .bin files. CHD is a life-saver here. By using FLAC compression for these audio tracks, CHD can reduce the size of Saturn games significantly while maintaining perfect audio quality.

Multi-Disc Games and M3U Playlists

While CHD turns multi-file games into single-file games, it does not turn multi-disc games (like Metal Gear Solid) into a single file. You will still have a CHD for Disc 1 and a CHD for Disc 2. The best practice here is to use an .m3u playlist file. This is a simple text file that lists the filenames of the CHDs. Most modern emulators will load the .m3u file, allowing you to swap discs via the emulator menu without ever leaving the game.

Summary of the CHD vs Redump Ecosystem

The debate shouldn't be about which format is better, but about how to use them together. Redump is the source of truth—the verification that your data is correct. CHD is the optimized delivery vehicle—the way you actually play and store that data.

For the casual player, downloading or converting to CHD is the most efficient path. It saves space, simplifies organization, and is supported by all major emulators. For the archivist, the path is to verify your files against the Redump database first, then convert to CHD for daily use, while perhaps keeping a backup of the original raw files or at least the DAT files and the verifydump tool.

As storage demands grow and libraries expand into the tens of thousands of games, the efficiency of CHD combined with the accuracy of Redump represents the pinnacle of the retro gaming experience.

Conclusion

Transitioning a game library from the raw Redump format to CHD is one of the most beneficial upgrades a digital collector can perform. It bridges the gap between the rigid requirements of digital preservation and the practical needs of modern emulation hardware. By utilizing lossless compression, single-file containers, and robust verification tools like verifydump, users can enjoy a library that is significantly smaller and more organized without ever sacrificing the 1:1 accuracy that makes the Redump project so valuable.

As we look toward the future of emulation, the adoption of CHD across platforms like the Steam Deck, high-end PCs, and even mobile devices proves that it is more than just a MAME utility—it is the definitive format for the disc-based era of gaming.

FAQ

What is a .chd file used for?

A .chd file is a compressed disk image format used primarily in emulation. It stores data from CDs, DVDs, or hard drives in a single, smaller file. It is widely used because it saves storage space while being "lossless," meaning the original data is perfectly preserved.

How do I open a .chd file?

To play a .chd file, you need a compatible emulator such as RetroArch (with appropriate cores like SwanStation or Flycast), PCSX2, or DuckStation. If you want to "open" it to see the files inside or convert it back, you must use the chdman command-line tool.

Can I convert a .chd file back to .iso or .bin/.cue?

Yes. Because CHD uses lossless compression, you can use the chdman extractcd command to turn a .chd file back into a .bin/.cue or .iso format. The resulting files will be identical to the original ones before they were compressed.

Is CHD better than Redump?

They are different things. Redump is a quality standard and a database project that ensures disc images are 1:1 copies of the original. CHD is a file format used to compress those images. Most people use Redump-verified files as the source and convert them to CHD for better storage and organization.

Does converting to CHD lose any game data?

No. The compression used in the CHD format is entirely lossless. This means that no audio quality is lowered, no textures are compressed, and no game code is changed. It is mathematically identical to the original source.

Why are my Dreamcast CHD files not working?

This is often due to using an outdated version of chdman or an emulator that doesn't support the V5 CHD header. Ensure you are using the latest version of MAME's tools for conversion and a modern emulator like Flycast or Redream.