VFT (US Military BR-6028)

VFT, Voice Frequency Telegraphy, refers to any of several systems for sending multiple RTTY signals over one voice-bandwidth radio channel, usually by audio multiplexing. Some combine different programs, while others duplicate the same one for error checking. All can be recognized by ear because they are several kHz wide, with a busy, buzzing sound. Depending on VFT mode and ionosphere conditions, the continuous, rotating, selective fading of various sub-channels can sound an awful lot like a rock flanger sweep or a 50s movie space ship.

Here we see a received signal from a US military station using a clever diversity mode called BR-6028. All seven of its evenly spaced, 170-Hz shift, AFSK channels transmit the same information, delayed one more second each successive time, in the order 3614752. There is also a 560-Hz pilot tone, as seen at bottom.

This mode's wide time spreading combines with the frequency diversity to give a redundancy well suited to random noise and ionospheric fading. The receiver puts it all back together, and prints all characters exceeding some set level of confidence. Redundancy and error checking are common in most modern HF modes.