Copyright © 2024 Bart Massey (Version 0.1.0)

This Rust library crate wraps Keith Packard's Forward Error Correction C code from AltOS git:git.gag.com/fw/altos. See https://github.com/BartMassey/ao-fec for the underlying code and details.

The C code underlying this library was originally written for AltOS embedded OS, used by Altus Metrum rocket telemetry hardware. For this reason, the error correction here is compatible (tested) with the hardware FEC in the Texas Instruments CC1111 radio chip.

CC1111 compatibility makes things a bit strange. The CC1111 uses a rate 1/2 limit 4 convolutional code. A CRC-16 checksum and one or two 0x00 pad bytes are appended to the payload before encoding. This ensures that the payload is even-length and that correct decoding can be checked.

The soft decoding code accepts one byte per bit and uses a Viterbi-style decoder.

The CC1111 limits block sizes to 128 bytes including header and padding. The underlying C code and this code have been generalized to allow arbitrary block sizes. That said, long blocks are probably not a good idea. If the decoder loses sync, it is likely to corrupt the entire remainder of a block. Further, the small CRC means that the possibility of a false positive on a damaged block is high.

The hard part of this is obviously Keith Packard's work. Thanks much! Thanks also to Keith for helping me debug and test this.

This code is licensed under the "GNU General Public License version 2". See the file csrc/LICENSE.txt in this codebase for licensing terms.