What is the SFF file format?
An .sff file is a Structured Fax File - a binary raster image format used as the native output of ISDN fax adapters implementing the CAPI 2.0 (Common ISDN Application Programming Interface) standard. The format was widely used in Germany and Europe throughout the 1990s and 2000s, especially with AVM Fritz!Box hardware and Fritz!Fax software.
All SFF files begin with the four-byte ASCII signature Sfff at offset 0. After the file header comes a sequence of page records: each page has its own header describing width, height, and resolution, followed by bi-level raster scan lines compressed using CCITT Group 3 (Modified Huffman or Modified READ) or Group 4 (MMR) coding - the same algorithms used in standard .tif fax profiles.
SFF stores only black-and-white (1 bpp) images at standard fax resolutions: 1728 pixels wide at 203 dpi horizontally, with 98 or 196 lines-per-inch vertical resolution. Color and grayscale content are not supported.
The format is now largely obsolete. Internet fax systems migrated to TIFF-F (RFC 3949) and subsequently to PDF. XnView, IrfanView, and the LEADTOOLS SDK remain among the few tools that can read .sff files today.
Security & safety
RISK: LOWBinary image format; no executable content. No known security risks for SFF files.
Format details
in a nutshellPrograms that open SFF files
Windows5 apps
macOS1 app
Linux1 app
Technical details
deep spec| Magic bytes | Sfff (4-byte ASCII signature, hex 53 66 66 66, at offset 0) |
| Byte order | Little-endian |
| Color depth | 1 bpp - bi-level black-and-white only; color and grayscale not supported |
| Compression | CCITT Group 3 (Modified Huffman MH or 2D Modified READ MR) or Group 4 (MMR) per page |
| Standard fax widths | 1728 px (A4 at 203 dpi) or 2432 px (B4) |
| Vertical resolution | 98 lpi (standard) or 196 lpi (fine) |
| Typical size | 20 KB - 2 MB (depends on page count and content density) |
| Container | SFF proprietary wrapper - distinct from TIFF despite similar compression schemes |
| Multi-page support | Yes; page records stored sequentially, each with its own page header |
| MIME type | image/x-sff |
| Origin standard | CAPI 2.0 (Common ISDN Application Programming Interface) |
| Primary source | AVM Fritz!Box and Fritz!Fax ISDN fax hardware/software |
| Successor formats | TIFF-F (RFC 3949) for internet fax; PDF for modern document exchange |
| Released | circa 1993 (tied to CAPI 2.0 ISDN standard) |
| Latest version | SFF 1.0 (single version; format stable since CAPI 2.0 era) |
| Open standard | Yes · royalty-free |
| Specification | fileformats.archiveteam.org |
SFF conversions
Community Q&A
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