MFSK Modes
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.
Protocol
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
Mode |
Symbol Rate |
Typing Speed1 |
Duty Cycle2 |
Modulation |
Bandwidth3 |
ITU Designation4 |
MFSK165 |
15.625 baud |
~ 4.0 cps (40 wpm) |
100% |
16-FSK |
316 Hz |
316HF1B |
MFSK Image |
1000 px/sec |
128x128 B&W 16 sec |
100% |
Analog FSK |
316 Hz |
316HF1C |
MFSK85 |
7.8125 baud |
~ 2.0 cps (19 wpm) |
100% |
32-FSK |
316 Hz |
316HF1B |
Notes:
1.
WPM is based on an average 5 characters per word, plus word space.
Values are approximate because a variable length code is used.
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 http://en.wikipedia.org/wiki/Types_of_radio_emissions
5. Double spaced mode.
6. Default and normal calling mode.