MFSK Modes

MFSK-4  Spectrum ,  Sound

MFSK-8 Spectrum ,  Sound

MFSK-11  Spectrum ,  Sound

MFSK-16  Spectrum , Sound

MFSK-22  Spectrum , Sound

MFSK-31  Spectrum , Sound

MFSK-32   Spectrum , Sound

MFSK-64  Spectrum , Sound
tick marks at 100 Hz intervals, waterfall palette is "blue-2"

General Description

MFSK16 and MFSK8 are multi-frequency shift keyed (MFSK) modes with low symbol rate. A single carrier of constant amplitude is stepped (between 16 or 32 tone frequencies respectively) in a constant phase manner. As a result, no unwanted sidebands are generated, and no special amplifier linearity requirements are necessary. The tones selected are set by the transmitted (4 or 5 bit) bit pattern and a gray-code table.

The mode has full-time Forward Error Correction, so it is very robust. Tuning must be very accurate, and the software will not tolerate differences between transmit and receive frequency. The mode was designed for long path HF DX, and due to its great sensitivity is one of the best for long distance QSOs and skeds. MFSK8 has improved sensitivity, but is very difficult to tune, and suffers more from Doppler. It is useful as the band fades out.

MFSK16 and MFSK8 were developed by Murray ZL1BPU and Nino IZ8BLY.


These are unconnected, manually controlled message asynchronous symbol synchronous simplex chat modes, with full-time Forward Error Correction. MFSK tone spacing is in both cases equal to the symbol rate (15.625 Hz and 7.8125 Hz). In order to maintain sync during idle periods, every few seconds a few non-printing characters is sent. The default calling mode is MFSK16.

MFSK16 also has an image transfer mode. This is controlled and triggered from MFSK16, and will transmit B&W or Colour pictures of any size and shape, although smaller is better, as transmission is only 1000 pixels/sec. This image transmission is an analog mode without sync. The image transmission is FSK of the same bandwidth as MFSK16, and again transmitter linearity is unimportant.

Coding and Character Set

A nibble-based varicode with ASCII-256 user interface is used. Lower case characters are sent faster. Modulation is four or five bit symbol synchronous, and in MFSK16 the dibit order is determined automatically. MFSK8 has five bits per symbol and uses an extra trial Viterbi decoder to determine the correct bit order.

The FEC uses binary convolution to generate two dibits per varicode bit. Rate R=1/2, Constraint Length K=7, Interleaver L=10 (40 bits). A matrix interleaver is used. This is a standard NASA design.

Operating Parameters


Symbol Rate

Typing Speed1

Duty Cycle2



ITU Designation4



18 wpm



154 Hz



7.8125 baud

36 wpm



316 Hz



10.767 baud

40 wpm



218 Hz



15.625 baud

58 wpm



316 Hz



21.533 baud

80 wpm



435 Hz


MFSK31 31.250 baud 55 wpm 100% 8-FSK 330 Hz 330HF1B
MFSK32 31.250 baud 120 wpm 100% 16-FSK 630 Hz 630HF1B
MFSK64 62.500 baud 240 wpm 100% 16-FSK 1260 Hz 1260HF1B
MFSK Image 1000 px/sec 128x128 B&W 16 sec 100% Analog FSK 316 Hz 316HF1C

1. WPM is based on an average 5 characters per word, plus word space. Values based on sending 100 "paris " words.
2. Transmitter average power output relative to a constant carrier of the same PEP value.
3. This is the "Necessary Bandwidth" as defined by the ITU.
4. A summary of the ITU Designation system can be found at
5. Double spaced mode.
6. Default and normal calling mode.