The MFSK16 shares the MFSK interface with DominoEX.
The MFSK16 mode is a standard which uses 16 tones. MFSK16 is a 100% duty cycle mode, and at all times one and only one tone is present. The shortest duration of a tone is 64 milliseconds long -- equivalent to a channel baud rate of 15.625 symbols/second. The tones are separated by 15.625 Hz from one another, with the lowest and highest tones separated by 234 Hz. Including the keying sidebands, an MFSK16 signal is nominally 316 Hz wide.
Each of the 16 possible tones of MFSK16 represents four bits of data. The raw data on the channel is therefore transmitted at 62.5 bits/second. This data is passed through a forward error correction (FEC) logic which reduces the information rate by a factor of two, thus causing the actual throughput of MFSK16 to be 2 bits per 64 milliseconds, or a rate of 31.25 bits/second (the same as PSK31).
The FEC logic consists of a data interleaver and a rate-1/2 convolutional code. The interleaver spreads the data bits over a range of 124 data slots so that any burst error that is caused by fading or interference will not all be concentrated within the constraint length of the convolutional code. The interleaver adds a latency of a little under one second to the data stream but adds tremendously to the robustness of MFSK16.
For cocoaModem's MFSK implementation details, see here.
Please familiarize yourself with the general cocoaModem interface and configuration panels before proceeding with this section.
To use MFSK16, first select the MFSK tab at the top of the main cocoaModem window. The Mode Menu is then used to select between MFSK16 and the DominoEX modes.
Fig 1 shows the cocoaModem MFSK16 interface.
The top part of the interface consists of the familiar waterfall display, together with the MFSK tuning indicator. Below that are the receiver section, the transmitter section and the usual macros and QSO information fields.
Tuning an MFSK signal
The waterfall display in Figure 1 shows a spectograph of a typical MFSK16 signal.
MFSK16 usually requires you to select Upper sideband mode on your transceiver. cocoaModem however, automatically rearranges the tones in the MFSK16 decoder and encoder so that you can choose to operate with either upper sideband or lower sideband.
You have to make sure that the Dial Readout parameters (Figure 2) in the Receiver Config panel agrees with what your transceiver is using. Failure to do this will cause no reception, and your transmission will also not be decoded by anyone else.
Even if you are using USB
transmission, it is well worth entering the AFSK VFO offset
of your transceiver into the VFO field, since the readings
under the waterfall can be used to directly determine your
transmission passband. If you are using voice mode SSB for
transmission, the VFO Offset should be set to zero.
Figure 3 shows in detail waterfall display when the Dial Readout parameters are set to the ones shown above in Figure 2 (one of the Yaesu FT-1000MP's AFSK settings).
As with the other wide band
interfaces in cocoaModem, when LSB is chosen, signals with
lower audio tones will appear on the right side of the
waterfall and signals with higher audio tones will appear
towards the left side of the waterfall.
When the Dial Readout parameters are correctly set, the label that is below the waterfall indicates the frequency (in Hz) that can be applied to your transceiver's VFO dial to get the precise frequencies you are transmitting at. Notice that in the above example, the zero frequency offset is actually near the middle of the waterfall, rather than on the left or right side -- this is typical when the VFO offset is non-zero.
The slider that is below the waterfall display allows you to choose the dynamic range of the waterfall.
You can identify a clean MFSK16 signal from the fact that the signal is about 250 Hz wide on the waterfall and you can sometimes see the 16 separate tones. Whether you have selected to use USB or LSB in the VFO Readout parameters, you "tune" an MFSK16 signal in cocoaModem by clicking on the leftmost tone on the waterfall, or as close to it as you can.
When an MFSK16 transmission begins, it will idle for a short time on this lowest tone -- that should also provide you with a location in the waterfall to click on.
Below the waterfall display section are the MFSK mode selector (you can only select MFSK16 for now) and some tuning aids.
Once you click on the waterfall, you should see a set of yellow columns appear in the tuning indicator. Figure 4 shows what the indicator looks like when receiving and locked to a very clean signal. Initially, the red tick marks may not be centered in between the yellow columns. Once cocoaModem manages to make a guess of which is a base tone, the red tick marks in the tuning indicator will move and position themselves in between the yellow columns, as shown in Figure 4.
Soon after, decoded characters
should appear in the receive text view.
If you had clicked on a DominoEX 16 or DominoEX 8 signal instead of an MFSK16 signal, you will see two additional tones (18 altogether) instead of the 16 tones above.
The "print" will not appear right after you click on the waterfall. Be patient. The demodulated signal has to go through the interleaver which adds a latency of about 1 second. The signal then proceeds to the decoder for the convolutional code, which adds a additional latency of about 1.5 seconds.
The cocoaModem click buffer is present in the MFSK interface -- this mean that if you click on a signal that is already showing in the waterfall for more than 2.5 seconds, the "print" will appear as soon as a third of a second (depending on the speed of your computer).
To further reduce the latency, there is a slider (Figure 5) in the cocoaModem MFSK interface to the right of the AFC slider that allows you to select a lower latency at the expense of fewer errors being corrected in the FEC logic. For almost all but the weakest signal in MFSK16, you should not notice much loss by selecting lower latency, and it will allow you to print a signal with a little over a second less latency.
The automatic tuning described
above happens when the AFC slider (to the right of the
tuning indicator) is set to On. If slider is set
to Off, the red tick marks in the tuning indicator
will not move and it is up to you to manually fine tune the
signal so that the yellow columns line up in between the
red tick marks.
Bear in mind that the red tick marks are just 15.625 Hz from one another. You will need to very slowly tune the VFO of your transceiver. Alternately, you can use cocoaModem's fine tuning mechanism to tune. If cocoaModem's window is active and with your mouse cursor positioned inside the waterfall display, the mouse scroll wheel acts as an the fine tuning mechanism.
Click on the title bar of cocoaModem's window to activate the window. When you fine tune, not click on the mouse since that will cause a different frequency to be selected. Instead, simply move the mouse so the cursor is within the waterfall.
If your mouse does not come with a scroll wheel, you can still fine tune using the arrow keys on your keyboard while the Command key on the keyboard is depressed. The left and right arrow keys provide tuning steps of about 0.5 Hz and the up and down arrow keys provide a coarser tuning step.
Once the yellow columns are locked in between the red tick marks when AFC is On, you can choose to move the slider to the Hold position. This is useful when you are receiving a station that has a stable signal which does not drift with time. cocoaModem's interface will keep the receiver tuned to the right spot even when the signal is no longer transmitting. This way, when the signal resumes transmission, you will already have locked in and can print the signal as soon as possible.
For very weak signals, you can turn on the Weak Signal button below the waterfall. The display will turn into something like what is shown below:
As shown in Figure 1 above, the MFSK interface's receiver section is below the tuning section. Figure 6 below shows in greater detail the MFSK controls that are on the right of the receive text view.
Figure 6 - MFSK Controls
Shown in Figure 6 are two text
fields which show the tone offsets for the MFSK receiver
and the transmitter.
When you click on the waterfall, the audio frequency that corresponds to the location that is clicked is transferred to both the receive and transmit frequency fields. If the AFC is turned on and there is a signal, the receive frequency will lock to and track the MFSK signal's frequency. The receive field will also track any fine tuning that is performed with the mouse scroll wheel or the arrow keys.
However, the transmit frequency will remain tuned to the frequency that you have clicked on.
If you want the current receive frequency to become the transmit frequency, click on the Rx > Tx button.
If you are the station who is initiating a contact (e.g., by calling CQ), click on the waterfall to set a location to transmit. If a station answers, either allow the AFC to automatically lock to the station or use the mouse scroll wheel to tune the station in. Do not touch the Rx > Tx button since the other station has already established the original click location as your transmit frequency.
If you are answering some other station's CQ, click on signal and wait for lock to occur. When the receiver is locked and printing, immediately click on the Rx > Tx button so that your transmit frequency is set to be the same as his transmit frequency.
The receive box also has a Soft Decode check box and a Squelch control.
cocoaModem implements both a soft decision decoder and a hard decision decoder for MFSK. The soft decoder (default state) uses more processor cycles but is also capable of printing a signal that is approximately 1 dB noisier than the hard decision decoder.
The squelch control keys on the signal-to-noise ratio of the received signal, rather than its signal strength. When the squelch is set all the way to the right, cocoaModem will print every symbol, whether they are reliable or not. When the Squelch slider is push all the way to the left, only good signal-to-noise cases allow printing.
The MFSK transmit section behaves similarly to the other interfaces. Once a frequency is selected in the waterfall, clicking on the Transmit button will cause the indicator button to turn red, the PTT will activate and the interface starts transmitting. The Transmit button's caption also turns into Receive.
When Receive is clicked, cocoaModem will wait until all buffered characters have cleared the encoder pipeline. While the buffer is clearing, the transmit indicator light will turn yellow. When all characters have cleared the encoder, the PTT will deactivate, the transmit indicator will extinguish, the button caption again turns to Transmit, and the MFSK receiver will again be active.
Characters that have not yet entered the encoder can be removed from the buffer by clicking on the Flush button.
As with other interfaces in cocoaModem, you can enter the transmit state by using the Command-T keyboard shortcut. You can return to the receive state with the Command-R keyboard shortcut. And you can flush the buffer and return to the receive state by using the Command-X keyboard shortcut.
MFSK is a 100% duty cycle mode and will therefore work fine under VOX control.