Charles Brain is a British amateur radio operator from West Sussex, call sign G4GUO. His claim to fame is the development, in partnership with Andy Talbot, G4JNT, of a digital voice mode for use on High Frequency radio. More information about his protocol can be found at his Digital Voice on HF page.

In a nutshell, the protocol involves the use of the AMBE codec, operating at 2400 bit/s, with FEC added to produce a 3600 bit/s data stream. This data stream is then transmitted on 36 carriers, spaced 62.5 Hz apart, at 2 bits/symbol, 50 symbols/sec using QPSK. This gives the protocol a total RF bandwidth of approximately 2250 Hz (compared to 2700–3000 Hz for an analog single sideband transmission). This has the effect of increasing sound quality and removing static from HF communications.

For maximum flexibility, the G4GUO modem implements the protocol on audible frequencies, which can then be put into a single sideband transceiver, effectively operating as a transverter in this context (similar in prinicpal to the use of PSK31). This way, the modem may be coupled with any single sideband transceiver, enabling a ham radio operator (or, indeed, any type of radio operator) to use it on any band where regulations permit.