FLDIGI Users Manual  3.21
MFSK

Table of Contents

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.

MFSK-32 and MFSK-64 are high baud rate and wide bandwidth modes designed for use on VHF and UHF. These are very useful for send large documents or files when some transmission errors are can be tolerated.

This is an example of properly tuned MFSK16 signal with a s/n of approximately 9 dB.

mfsk-9db.png
MFSK16 signal

The same signal viewed with the waterfall expanded to the x2 factor.

mfsk-9db-x2.png
MFSK16 signal

MFSK Picture Mode

Fldigi can send and receive images using all MFSK baud rates. When operating with other modem programs you should limit sending pictures to the MFSK-16 baud rate. The program can send and receive MFSK images in both black and white and in 24 bit color. The transmission mode for MFSKpic is similar to FAX.

Reception of an MFSKpic transmission is fully automatic. The MFSKpic transmission has a preamble sent which will be visible on the text screen. The preamble reads as "Pic:WWWxHHH;" or "Pic:WWWxHHHC;" for b/w or color respectively. The WWW and HHH are numbers specifying the width and height of the picture in pixels.

The successful reception of a MFSKpic is highly dependent on s/n conditions. The data is transmitted as an FM modulated signal and is subject to burst and phase noise on the transmission path. It can provide excellent photo transmission on a really good path.

mfskpix-bee.png
Received MFSK Image

This is an example of a photo received on a bench test. The received image is an exact replica of the transmitted image. The color depth is a full 24 bits.

Images should be carefully selected for size before beginning a transmission. To calculate the transmit time for an image use the following formula:

Time(sec) = W * H / 1000 for black and white

Time(sec) = W * H * 3 / 1000 for color

Where the W and H are the dimensions of the photo in pixels. A 200 x 200 image will take 120 seconds to transmit in color and 40 seconds to transmit in b/w. The symbol rate for this mode is 1000 data bytes per second. The color image consists of 3 bytes; red, blue and green for each pixel.

k0og.jpg
Picture received from K0OG

This is an example of a picture received live on 80 meters (thanks K0OG)

Received images are saved in the default folder $HOME/.fldigi/images (Linux) or <defaultpath>/fldigi.files/images (Windows).

Transmitting an Image

Xmt-Pix.jpg
Xmit Picture Dialog box

You can only transmit an image while in the MFSK-16 mode. The image can be prepared for transmission while in the receive mode. Right click in the transmit text box and select "Send Image" from the popup menu. This will open up the transmit image dialog which will be blank to start.

Press the "Load" button and a file selection dialog will allow you to select a suitable image for transmit. The file selection dialog also has a preview capability so you will see what the image looks like.

You may also open a window manager file browser and drag and drop an image to the center part of the Send image dialog.

The "X1" button is a three-way toggle that allows you to transmit an image file in
X1 - normal and compatible with other modem programs
X2 - double speed, and
X4 - quadruple speed. X2 and X4 are fldigi specific image modes.

Xmt-Pix1.jpg
Xmit Picture Dialog Box with Image

The Send image dialog after the image was drag and dropped onto the dialog.

The properties box said this image was 120 x 119 24 bit color. So it should take 42.8 seconds to transmit in full color. You can send a color or a b/w image in either color mode or b/w mode. If you transmit a color image in b/w the program will convert the image before transmitting. If you transmit a b/w image as full color you are in effect transmitting redundant information, but it can be done. I selected the "XmtClr" button for a trial run. Pressing either the "XmtClr" or "XmtGry" will put the program and the transceiver into the transmit mode if it was in the receive mode. The image is cleared and then repainted as the transmission proceeds. You see the same image progression that the receiving station should see. The main display also displays the % completion on the status bar. Hold the mouse over either the XmtClr or the XmtGry button and the tooltip will tell you the transmit time for this image.

You may abort the transmission at any time by pressing the "Abort Xmt" button. That will return you to the text mode for MFSK. You will then have to toggle the T/R button if you want to return to receive.

Xmt-Pix3.jpg
Received MFSK Image

The receiving program decodes the "Pic:110x119C;" as a color picture 110 wide by 119 high. Here is shown being received on a computer running Vista Home Premium.

Xmt-Pix4.jpg
Waterfall of a MFSK Image

This is what the waterfall will look like during the reception of an MFSK-16 image.

The actual spectrum signature will vary with the image bytes being transmitted. The waterfall scale is in the x4 mode and the above photo was being transmitted in 24 bit color for this screenshot. The waterfall clearly shows that the image transmission is within the bandwidth occupied by MFSK-16.

Picture with a slant

If either the send, receive or both ends of the transmission are using an uncalibrated sound card whose sampling rate is not an exact multiple of 8000 Hz the resulting picture at the receive end will appear slanted. The degree of slant is directly related to the accumulation of the frequency error at both ends of the transfer. Stations wishing to send and receive MFSK pic's should calibrate their sound card. The WWV calibration mode is used to measure and set the parts per million (ppm) correction factor for the sound card.

Your sound system may be fully corrected, but the sending station may have an uncorrected sound card. You can usually correct for small errors in the following way. After the full picture is received move the mouse to bottom left or right corner of the slanted images (the corner that clearly visible). Then left click on that corner. The program will correct for the slant. The correction will not be perfect but it may help to make the image more viewable.


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