dl-fldigi/fldigi_doxygen/user_src_docs/MFSK.txt

/**
\page  mfsk_page MFSK

\tableofcontents

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.
<br>

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.
<br>

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

\image html mfsk-9db.png " MFSK16 signal"
\image latex mfsk-9db.png " MFSK16 signal" width=6.0in
<br>

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

\image html mfsk-9db-x2.png " MFSK16 signal"
\image latex mfsk-9db-x2.png " MFSK16 signal" width=1.5in
<br>


\section mfsk_picture_mode 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.
<br>

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.
<br>

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.
<br>

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

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

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

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.
<br>

\image html k0og.png "Picture received from K0OG"
\image latex k0og.png "Picture received from K0OG" width=2.0in

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

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

\section mfsk_tx_image Transmitting an Image

\image html Xmt-Pix.png "Xmit Picture Dialog box"
\image latex Xmt-Pix.png "Xmit Picture Dialog box" width=3.0in
<br>

You can only transmit an image while in the MFSK-16/32/64/128 modes.  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.
<br>

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.
<br>

You may also open a window manager file browser and drag and drop an image
to the center part of the <b><i>Send image</i></b> dialog.
<br>

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

\image html Xmt-Pix1.png "Xmit Picture Dialog Box with Image"
\image latex Xmt-Pix1.png "Xmit Picture Dialog Box with Image" width=3.0in
<br>

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

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.
<br>

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.
<br>

There is an alternate way of sending an image, but one which only transmits a color
image with no provision for slant correction.  This may be more convenient
for sharing images when you do not need the additional utility of the image dialog.
<br>

Open a file folder to the location of your image files that are suitable for MFSK 
transmission.  These images should be constrained to be 240 x 200 or less for 
reasonable transmit times.  Optionally prepare fldigi for the MFSK image 
transmission by entering some suitable text to precede the transmission.  
You may be sending the image, blossoms.jpg, for example.  
<br>

<center>
\image html blossoms.png "image in files folder"
\image latex blossoms.png "image in files folder" width=3.0in
</center>

One might enter:

\image html blossoms-1.png ""
\image latex blossoms-1.png "" width=4.0in

Then drag and drop the the image file from the files folder to the transmit 
text panel in fldigi.
<br>

The TX panel text will then change to

\image html blossoms-2.png ""
\image latex blossoms-2.png "" width=4.0in

The ^! is the character sequence used by the fldigi TX parser to indicate 
that there is a pending execution command.  In this case it is to transmit 
the blossoms.jpg file.
<br>

Add some suitable ending text and the sequence ^r to the next line and 
then press the T/R button (if not already transmitting).  At the end of the 
image the TX parser will find the ^r and return the program to the receive 
mode.

The Rx panel will echo the transmitted data:
<br>
\image html blossoms-3.png ""
\image latex blossoms-3.png "" width=4.0in

The image progress dialog will close at the end of the image transfer.

\section mfsk_rx_image Receiving an Image

The receiving program decodes the "Pic:240x160C;" as a color picture
240 wide by 160 high.
<center>
\image html Xmt-Pix2.png "Received MFSK Image"
\image latex Xmt-Pix2.png "Received MFSK Image" width=3.0in
</center>

This is what the waterfall will look like during the reception of an
MFSK-64 image.
<br>
<center>
\image html Xmt-Pix3.png "MFSK Image - Waterfall"
\image latex Xmt-Pix3.png "MFSK Image - Waterfall" width=4.5in
</center>

The actual spectrum signature will vary with the image bytes being
transmitted.  The image 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-64.

Received images are automatically saved to the images folder and annotated with
a date-time stamp.

<center>
\image html MFSK-rx-images.png "MFSK Rx Images"
\image latex MFSK-rx-images.png "MFSK Rx Images" width=4.5in
</center>

<br>

\section mfsk_picture_slant 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 \ref digiscope_display_wwv_mode "WWV calibration mode" is used to
measure and set the parts per million (ppm) correction factor for the
sound card.
<br>

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.  Right click to undo the slant correction.  Slant corrections are
not saved to the Rx image file.

<br>
\ref mfsk_page "Return to Top of Page"
<br>
\ref main_page "Return to Main Page"


*/