Also known as: Codec 2, Codec2
Codec 2 is an open-source, royalty-free low-bitrate speech vocoder created by David Rowe.1 It provides intelligible voice from roughly 700 bps to 3200 bps and is the patent-free alternative to AMBE-family codecs — the reason a fully open digital-voice stack is possible at all.
How it works
Codec 2 is a harmonic sinusoidal coder built on linear-predictive-coding. For each ~20–40 ms frame it estimates a pitch (fundamental) frequency, the energy, a voicing decision, and the spectral envelope — the latter captured as a small set of LPC coefficients that are converted to line spectral pairs and vector-quantized. The decoder places sinusoids at harmonics of the pitch, scales each by the sampled envelope, and applies the voicing to decide how much is periodic versus noise-like. Conceptually it sits close to the MBE family, but its parameter set and quantization were designed independently and, crucially, are unencumbered by patents.
Variants
Codec 2 ships as a set of fixed-rate modes named by their bit rate — 3200, 2400, 1600, 1400, 1300, 1200, 700C — that trade audio quality against bandwidth. The 3200 and 2400 modes sound the most natural; the 700-bps modes are remarkably intelligible for their size and are used where the channel is extremely tight. M17 uses the 3200-bps mode for its 9600-baud RF stream. There is also a companion neural-net vocoder (LPCNet) that Rowe’s FreeDV work pairs with Codec 2 parameters for higher quality at very low rates.
In practice
The open licence (LGPL) is the whole point: unlike AMBE+2, Codec 2 carries no per-unit royalty and no DVSI hardware dongle, so anyone can embed it in firmware or software freely. That is why the amateur-radio M17 project and the FreeDV HF digital-voice mode both adopted it, and why it is the natural choice for hobbyist and open-hardware transceivers. The cost is that its ecosystem is smaller than the entrenched AMBE world, so it is not found in commercial public-safety radio.
Relevance to SDR
Codec 2 lets fully open decoders render digital voice without proprietary vocoder licensing. GopherTrunk supports M17, whose voice payload is Codec 2, so the same pure-Go philosophy that avoids AMBE dongles applies here with no licensing asterisk at all — the codec itself is free software. For FreeDV and other Codec 2 modes it is the reference for what an unencumbered land-mobile-style vocoder can achieve.