MFSK-8 and MFSK-16 are multi-frequency shift keying digital transmission modes. MFSK-8 uses a set of 8 tones with the equal distance of 7.81 Hz, MFSK-16 a set of 16 tones with the equal distance of 15.625 Hz. The baud rate of MFSK-8 is 7.81 Bd and that of MFSK-16 is 15.625 Bd.

Both modes are used to transmit text file (ASCII file). The ASCII character is first converted to Varicode, which encodes frequently used characters to short length bit combinations and less frequently used characters to longer length bit combinations. For example, 'e' is mapped to '1000' and 'Z' is mapped to '110111000'.

The Varicode bit stream is then coded with a FEC (Forward Error Correction) algorithm. To combat frequency fading problems during the transmission, the FEC bit stream is interleaved before it is assigned to tones (source coding).

In both decoders only the center frequency needs to be tuned.

After the center frequency is set, the decoder first synchronizes, most importantly, to the polarity of signal. This takes for MFSK-8 between 5 and 40 seconds, for MFSK-16 between 2 and 7 seconds – this mainly depends on the baud rate of the mode. During the sync phase no text will be output on the screen.

After the decoder is synchronized, it enters the Traffic state, and a text file will be output on the screen. The synchronization is maintained as long as the MFSK signal is stable. When the noise is too strong or the signal disappears, the decoder begins to resynchronize – the status changes from Traffic to Sync.

Tuning a MFSK-8 and MFSK-16 Signal

The only tuning parameter in MFSK-8 and MFSK-16 signal is the center frequency. Set Peak Hold to Max Hold using the Options menu. After all tones are displayed, determine the minimum and maximum frequency of the tone set using the cursors. With the center cursor the center frequency may be set more accurately. Especially for MFSK-8 the center frequency needs to be set precisely, because the tone distance is only 7.81 Hz.

After the decoder has been synchronized, it enters the Traffic phase. The polarity status field on top indicating "N" (for Normal) or "I" (for Inverse) shows whether the MFSK signal is USB (Upper Side Band) or LSB (Lower Side Band). The transmitted text file is output in the window.