dl-fldigi/fldigi_doxygen/user_src_docs/FSQ.txt

/**
\page  fsq_page FSQ

\tableofcontents

FSQ, Fast Simple QSO, is an Incremental-Frequency-Keyed mode using an offset
differential modulation scheme similar to DominoEX, and Thor.  It is a unique 
mode for fldigi users in that it is a line by line transmission rather than 
character by character.  FSQ uses 33 tones spaced by 3 times the 3 baud symbol
rate or 8.8 Hz.  The offset rotation of the IFKP sequence offers improved 
performance under NVIS conditions, because the rotation significantly reduces 
the risk of adjacent symbols causing inter-symbol interference.

fldigi can operate on the following FSQ modes without special setup by the
operator:
<br>

<center>

Mode    | Symbol Rate | WPM | Bandwidth
:------:|:-----------:|:---:|:----------:
FSQ 2   | 1.95 baud   | ~20 |   290 Hz
FSQ 3*  | 2.93 baud   | ~30 |   290 Hz
FSQ 4.5 | 3.91 baud   | ~40 |   290 Hz
FSQ 6   | 5.86 baud   | ~60 |   290 Hz

</center>
* - FSQ-3 is the base rate.

At the time of this release there is only one other amateur modem application
that supports FSQ, FSQCALL, written by Con Wassilieff, ZL2AFP.  FSQ is an invention
of Murray Greenman, ZL1BPU.  Both Murray and Con were very supportive during
the development and testing of fldigi's FSQ implementation.

fldigi-fsq supports the undirected, directed and image modes of FSQCALL.

This FSQ help content borrows shamelessly from Murray Greenman's (ZL1BPU)
"FSQ Explained" document.

<br>
<center>
\image html fsq-main-dialog.png "fsq main dialog"
\image latex fsq-main-dialog.png "fsq main dialog" width=4.0in
</center>
<br>
The main dialog fsq user interface provides an Rx-text panel, a Tx-text panel, 
a "Heard" list, fsq specific control buttons, and a signal to noise indicator.

The Rx-text panel displays all decoded text in the UnDirected Mode.  The Tx-text
panel is for entering all transmitted text, including directives.  These two panels
can be resized vertically by dragging the dividing bar which separates them.

<center>
<table>
  <tr>
    <td>
      \image html fsq-undirected-mode.png "Undirected Mode"
      \image latex fsq-undirected-mode.png "Undirected Mode" width=1.5in
    </td>
  </tr>
  <tr>
    <td>
      \image html fsq-directed-mode.png "Directed Mode"
      \image latex fsq-directed-mode.png "Directed Mode" width=1.5in
    </td>
  </tr>
</table>
</center>

The Rx-text panel displays only recognized text in the Directed Mode.  The text
parser must be satisfied that the sending stations callsign/crc8 agree and that
the directed station callsign is your own or "allcall".  FSQ is case sensitive,
and your sending case must match the senders case, i.e. w1hkj and W1HKJ are not
the same for the purposes of FSQ Directed Mode.

<ul>
  <li>FSQ-ON<br>
      when on the xmt and rcv are in FSQ directed mode<br>
      when off the xmt and rcv are in FSQ undirected mode</li>
  <li>ACTIVE<br>
      when on the decoder will accept all incoming directed traffic<br>
      when off the decocer will only respond to the wake-up * trigger</li>
  <li>MON<br>
      when on the monitor panel is visible</li>
  <li>QTH<br>
      appends the myQTH text to the tx panel</li>
  <li>QTC<br>
      appends the myQTC text to the tx panel</li>
  <li>CQ<br>
       sends the FSQ cq call 'cqcqcq'</li>
</ul>

<center>
\image html fsq-heard-list.png "Heard List Panel"
\image latex fsq-heard-list.png "Heard List Panel" width=1.5in
</center>

The "Heard" list displays the callsign, s/n, and time of reception of all correctly
decoded transmissions.

NOTE: fldigi function keys are inoperative in the FSQ mode!

<br>
\section coding_and_modulation Modulation Scheme

FSQ is designed to use 32 tone differences between 33 equally spaced tones; so it is 
possible to allocate 29 different individual differences directly to the most 
frequently used characters. These characters are all lower case, a-z, plus the 
most common punctuation symbols: space, full stop (period), and new line (CR/LF). 
The three ‘spare’ differences are used to define further code tables.

The total alphabet includes upper case and a reasonable range of symbols, 
yielding a total alphabet of 104 ASCII characters. The extra characters are 
allocated to three additional code tables, which are sent as two sequential 
tone differences. These are characterized by an initial difference, describing 
the character, and a continuation difference, defining the code table.

When the receiver sees a tone difference in the range 0 – 29, followed by another 
in the range 0 – 29, it recognises a single-tone character (lower case etc). 
If the tone difference is followed by a difference in the range 30 – 31, it uses 
the second difference to decide which code table to select the character from, 
and the first difference to choose the character from that table. The encoding 
process is the reverse of this.

<center>
\image html fsq-varicode.png "FSQ Varicode Table"
\image latex fsq-varicode.png "FSQ Varicode Table" width=5.0in
</center>

IFK codes the data (the initial and continuation codes) as differences between 
two tones, rather than as an absolute tone. This is the MFSK equivalent of 
differential BPSK, as used in PSK31, etc. MFSK modes such as Piccolo, Coquelet 
and MFSK16 use direct MFSK modulation, assigning the code directly to a tone 
number. These modes may suffer from more inter-symbol interference than desirable, 
cannot tolerate frequency drift, and can be difficult to tune.

FSQ uses very narrow tone spacing. IFK+ reduces inter-symbol interference by 
ensuring that the chance of the same or an adjacent tone being used for 
sequential symbols would be very remote.

Since the tones always change due to the differential action and the tone 
rotation of IFK+, there is no opportunity for the tones to remain the same for 
two consecutive symbols, which allows the sync-less process to operate correctly. 
For example, without IFK+, repeated space characters could be seen as a single 
character.

Finally, since the coding is differential, any drift and frequency offset is 
cancelled out during decoding. IFK+ can handle frequency drift of about a third 
of the tone spacing per symbol, or in FSQ about 18 Hz drift per second at 6 baud. 
Frequency error tolerance is a matter of decoder design, but ± 50 Hz is tolerated 
easily by FSQ.

IFK+, an acronym for Offset Incremental Frequency Keying, was suggested by 
Murray Greenman ZL1BPU (2009), and first used in DominoEX. With each symbol 
transmitted, as well as adding the difference from the alphabet-coding table 
to the previous tone number, an addition rotation offset is added. In the case 
of FSQ, this value is ONE.

Only one tone is transmitted at a time.

In order for the first difference (first character) to be correctly decoded, a 
dummy tone needs to be transmitted first. This could be the lowest tone, but in 
fact any permitted tone will suffice since only the difference to the next tone 
is important. The software simply sends a space character to achieve this.

\section transmit_functions Transmit Functions

The user types text into the Tx panel, which can be edited before transmission.
Transmission starts when an ENTER character is pressed.  It will also start as a 
result of one of the automatic processes, such as ‘Sounding’ or automated replies.

The characters in the Tx panel are then encoded, and transmitted at 
the currently set symbol rate. As they are transmitted, the characters are 
transferred to the receive pane as a record of what has been transmitted.

<b>BOT Sequence</b>

At the start of transmission in directed and non-Directed Mode, the BOT sequence, 
a SPACE followed by a LINEFEED, is sent. The space serves as a dummy symbol to 
allow the next tone difference to be measured. The LF character ensures that 
reception starts on a new line.

In Directed Mode, the LF also serves as one delimiter for the callsign search 
algorithm. The other delimiter is the colon that follows the callsign. This also 
locates the two callsign checksum characters.

<b>EOT Sequence</b>

At the end of transmission in UnDirected Mode, the transmitter is simply turned 
off once the last useful character is sent. In Directed Mode, however, a special 
character sequence is sent just before the transmission stops, to signal EOT.

The BS (backspace) character is not required in FSQ, so the corresponding character 
in the table has been reallocated as an EOT marker.

The purpose of the EOT sequence is to cause the Squelch to close quickly in Directed 
Mode. Print will continue through fades without fldigi closing and stopping print 
partway through a sentence. If the sentence transmission were to finish without 
the EOT, the squelch would close slowly, and junk would print unnecessarily. The 
EOT ‘shorts out’ the Squelch time constant, closing the Squelch instantly.

The transmission consists of 33 tones spaced 8.7890625 Hz, resulting in a spread 
of tones of 290.0390625 Hz. Using the ITU-R SM.1138 assessment method, the signal 
easily meets 300 Hz useful bandwidth, at all signalling speeds, and its ITU 
Emission Designator is therefore 300HF1B. 

In Directed mode, a conventional network model CSMA protocol decides when the 
station may have access to the radio channel. This protocol defines three classes 
of message (chat, responses and soundings), and applies priorities to them. 
Priority is provided by means of delays after Squelch has closed. In addition, 
to limit clashes between stations wanting access for the same priority message, 
random delays are also added.

<center>

Priority |  Delay  |  Message Type
:-------:|:-------:|:-------------------------------:
  1      |  Short  | Chat sentences
  2      |    Med  | Automated responses to commands
  3      |   Long  | Sounding

</center>

<b>Sounding</b>

In Directed Mode, the user may elect to send regular ‘soundings’. These are very 
short messages with no trigger command and no message body. The message consists 
only of the callsign, colon and checksum of the FSQCall preamble.

These sentences do not print at any station, but appear in the Heard List and are 
logged in the Heard Log. This allows all stations on the channel to know what other 
stations are there with minimum use of channel bandwidth.

Sounding is sent with the lowest message priority. Sounding is essentially 
random, although the time between Sounds is selectable at 1, 10 and 30 minute
intervals.  The start time is set by user action, and essentially random.

\section fsq_protocol FSQ Protocol

FSQ is designed as a ‘chat’ mode, not a long-over QSO mode. It is capable of 
sending multiple sentences in each transmission, but it is easiest and quickest 
when just one sentence is sent.

Each transmission starts with the fldigi station callsign (converted to lowercase).

For example, if the user (say zl1xyz) types:

  Hello John – how are you this evening<ENTER>

The software actually transmits:

  zl1xyz:Hello John – how are you this evening

Nothing could be simpler. Every station within range will print this message.

<b>Performance</b>

There is no error correction involved, so users rely on the inherent robustness 
of the mode, and if there are errors, either due to ‘typos’ or reception errors, 
the users will quickly recognise these. If an error causes confusion, they can 
always ask for a repeat.

Operating 20W on the 40m band during the middle of the day, an automated station 
300 km away was used to return transmitted sentences. From 250 words transmitted, 
at 6 baud, 223 were returned correct (89.2% copy), while at 3 baud, copy was 100%. 
For practical purposes, 80% copy is considered adequate for a comfortable QSO.

Tests using an ionospheric simulator bear these measurements out. Reception is 
essentially 100% for white-noise tests with the signal above –15dB S/N.

\section directed_mode Directed Mode

FSQ Directed Mode operates as a simple extension of the FSQ protocol. The 
preamble is changed a little, a direction (address and command) is applied, and 
finally a short EOT signal is added to the end of each message. These differences 
are simple to apply, as the user only needs to learn a few simple commands and 
remember to add a direction and command to each sentence. The rest is automatic.

Further, directed sentences in FSQ mode can be read easily in undirected FSQ mode, 
and undirected text is visible in the monitor panel which can be viewed in 
Directed Mode.  In Directed Mode, only text directed to your callsign will print 
in the Rx panel, and only commands directed to your station will be responded to.

<center>
\image html fsq-mon-dialog.png "FSQ Monitor Panel"
\image latex fsq-mon-dialog.png "FSQ Monitor Panel" width=3.5in
</center>

The Monitor Panel is only shown during Directed Mode operation.  It is hidden
when Undirected Mode is in use.  All incoming traffic is printed on the Monitor 
Panel.  This includes both Directed and Undirected traffic.  The decoding is 
affected by the fldigi squelch setting.

You may see what appears to be a perfectly good Directed data stream on the Monitor 
Panel which never appears on the Rx Panel.  When this occurs, the data stream has 
failed to parse correctly:
<ol>
<li>the BOT is missing or corrupt, 
<li>the CRC8 check has failed,
<li>the EOT is missing or corrupt,
<li>or it is not directed to your callsign or allcall.
</ol>

The lower half of the monitor panel displays text that is queued for transmission, 
for example the response to a ? or a $ trigger.  Automated responses are not sent 
immediately, but after a small random delay AND when the squelch is not open.

<b>FSQCall Preamble</b>

The FSQCall preamble consists of the sending station’s callsign, a colon, and a 
two character hexadecimal checksum. For example, the simplest transmission, a 
Sounding message, consists of only this. For example:

  zl1bpu:b6

The checksum is a standard CCITT CRC8 calculated on the callsign and the colon. 
The maximum callsign length is 16 characters, and the CRC is capable of detecting 
up to four errors in this distance reliably. Pseudo-callsigns (such as place 
names or personal names) can be used, which is very convenient for Public Service 
and Emergency applications.

<b>Directions and Triggers</b>

All FSQ Directed Mode messages, apart from the Sounding, carry at least one 
direction and a trigger. The direction is the callsign or callsigns of the 
recipient station or stations, in other words, where the message is directed. 
The trigger is one of a short list of one-letter commands sent to the directed 
station, which define what it should do with the message. There may or may not 
be a message body following the directions and triggers.

Here are some simple examples:
<center>
\image html fsq-directed-mode-examples.png "Directed Mode Examples"
\image latex fsq-directed-mode-examples.png "Directed Mode Examples" width=3.0in
</center>

In the first example, ZL1BPU is simply chatting to ZL1ABC. He starts typing by 
giving the direction, the other station’s callsign. The trigger used is a space, 
which is understood by the receiving station as an invitation to print the message 
that follows.

The second example illustrates how you can send the same sentence to two stations. 
There is also an ‘all stations’ direction (allcall) that can be used to send 
messages and some commands to all stations within range.

The third is an example of a simple automated query. It is the FSQ way of asking
for a signal report.  The queried station will respond with your s/n report as
measured by the FSQ program that is being used.

<b>Error Checking</b>

FSQ Directed Mode sentences are checked at the receiver in two ways. First, the 
sender’s callsign is checked against the checksum received: if this is incorrect, 
the sentence is rejected. If it is correct, the rest of the sentence is checked 
to find the recipient’s callsign. If this is not received correctly, the sentence 
is again rejected. If the callsign is received correctly, only the remainder of 
the message following the recipient’s callsign is printed or acted upon.

When (and if) the incoming message is displayed, the preamble is shown without 
the checksum, so it looks like the preamble in the UnDirected Mode. However, the 
callsign displayed is in this case the callsign verified by the checksum.

Checking the validity of the originator’s callsign ensures that the message is 
correctly attributed, is correctly logged, and most significantly, responses can 
be sent to the correct station.

<b>Logging</b>

In Directed Mode, FSQ maintains a log of all stations heard, in chronological 
order. With each entry, verified callsign, the date/time, and measured SNR are 
recorded in CSV (comma separated value) format. This is suitable for examination 
as a spreadsheet.  The heard log file is named "fsq_heard_log.txt" and is located
in the fldigi.files/temp folder.

A complete audit log of transactions is also maintained in Directed Mode. This 
includes regular time stamps, all raw received text, and all transmitted sentences.
The audit log is named "fsq_audit_log.txt" and is located in the fldigi.files/temp
folder.

<b>Directed Callsigns</b>
<ol>
<li>Each station responds to three ‘callsigns’.
<li>The operator callsign (fldigi converts operator call to lowercase).
<li>The ‘callsign’ allcall, to which the software will act upon certain messages, 
including chat, file and image transfer, and alerts. This mechanism is widely 
used in nets to send chat to everyone in range.
<li>The ‘callsign’ cqcqcq is recognised as a legitimate chat ‘callsign’.
</ol>
<br>
<b>Use of Lower Case</b>

FSQ is case sensitive, so messages directed (for example) to ‘ZL1EE’ will not 
be recognised by the station with its callsign set to ‘zl1ee’. Lower case is not 
mandatory for operator callsigns, but is encouraged for these reasons:
<ol>
<li>Lower case is quicker and easier to type.
<li>Lower case is faster to send (most characters are sent in one symbol).
<li>The error rate is half that of upper case characters.
</ol>
<br>


\section fsq_triggers Triggers

All the command triggers (currently) consist of one reserved character. These 
are taken from a pool of characters that are not permitted in callsigns.  As far 
as possible, the characters have a meaning easily associated with the character.

This list shows all the reserved characters, and their current or planned use. 
Blank entries are reserved, with no currently planned application.

<center>
CHAR   | ASCII | USE
:-----:|:-----:|:-------------------------------------------:
Space  |   32  | Directed chat
!      |   33  | Repeat command
\"      |   34  | --
#      |   35  | Message or file transfer
$      |   36  | Request Heard List
%      |   37  | Receive image
&      |   38  | Report QTC message
‘      |   39  | --
(      |   40  | --
)      |   41  | --
&lowast;      |   42  | Restore FSQ to ACTIVE from SLEEP
+      |   43  | Message retreive
,      |   44  | --
-      |   45  | Message delete (1)
.      |   46  | --
:      |   58  | Reserved, not used.
;      |   59  | Relay command
<      |   60  | Reduce sending speed
=      |   61  | Reserved for ECC file transfer
\>      |   62  | Increase sending speed
?      |   63  | Report reception
@      |   64  | Report QTH (or position) message
[      |   91  | Used in file transfers
\      |   92  | --
]      |   93  | Used in file transfers
^      |   94  | Report FSQ program version
_      |   95  | Report the squelch setting in dB (1)
`      |   96  | --
{      |  123  | --
\|      |  124  | Send Alert
}      |  125  | --
~      |  126  | Delayed Repeat command
(1) not supported
</center>

\section fsq_commands Commands

Command sentences consist of a direction followed immediately by a command. 
Multiple directions can be used in the same sentence under limited circumstances. 
Two directions in the same sentence, which are understood by the same station, 
will cause confusion, and only the second will be responded to. For example, one 
should not send ‘zl1xyz@ allcall message’, as the first part will be ignored by 
station zl1xyz.

<b>Command Syntax</b>

The syntax for currently defined commands is listed below.

<ul>
<li><b>callsign</b>  <i>text</i> Addressee station starts to print text. No trigger character used except space following callsign. You MUST use a space or the trigger will not be recognised.
<li><b>callsign?</b> <i>text</i> (Is station hearing me?) Addressee station starts to print text. When squelch closes, station responds: origin_callsign snr=xxdB
<li><b>callsign\*</b> Switches station to ACTIVE if FSQSEL is in SLEEP. Addressee station starts to print. When squelch closes, station responds: callsign:CRC-VAL Active
<li><b>callsign!</b> <i>message</i> (Repeat my message) Addressee station starts to print. When squelch closes, station responds: callsign:CRC-VAL message. This is a simple relay mechanism. message may contain further trigger commands.
<li><b>callsign~</b> <i>message</i> (Repeat my message later) Addressee station starts to print. When squelch closes, after a delay of about 15 seconds, station responds: callsign:CRC-VAL message This delayed relay is useful when stations mentioned in message can hear and respond to the original message.
<li><b>callsign;</b> <i> [relay_to]{trigger}{message}</i> Relay my message to the 'relay_to' station.  See Relay section for details.
<li><b>callsign!sendto</b><i> message</i> Relay message to the 'sendto' station.  Addressee station starts to print.  When squelch closes, station relays the message.
<li><b>callsign#[nnn]</b> (Send file to station) Addressee station starts to print. Text following [nnn] will be saved in or appended to text file nnn.txt in the same folder as the log/audit files. When squelch closes, station responds callsign:CRC-VAL Ack if the message stored OK. There is no  response if the message was not received. [nnn] can be alpha, numeric or mixed.
<li><b>callsign#[filename.txt]</b> (Send file to station) Addressee station starts to print. File is opened from menu, and filename appended to message before file body text, File will be saved in or appended to text file filename.txt in the same folder as the log/audit files. When squelch closes, station responds callsign:CRC-VAL if the message stored OK. There is no  response if the message was not received. [nnn] can be alpha, numeric or mixed.
<li><b>callsign\@</b> (Request station position or location information) Addressee station starts to print. When squelch closes, station responds with pre-recorded sentence, typically containing QTH information. This could be GPS position, locator, or physical address. This is the message stored and retreived by the QTH button.
<li><b>callsign&</b> (Request station message) Addressee station starts to print. When squelch closes, station responds with pre-recorded sentence, typically containing station information or an 'Out to Lunch' message. This is the message stored by the QTC button.
<li><b>callsign^</b> (Request Software Version) Addressee station starts to print. When squelch closes, station  responds with current FSQCALL software version.
<li><b>callsign%</b> (Send image) Addressed station receives the command to record 
an analog graphic image. The Rx Image Dialog will open and the image painted as it
is being received. Images are in color or grey scale and always 
displayed in native size.. Images can be directed to allcall.
<li><b>callsign|</b><i>message</i> (Send alert) Addressee station starts to print. Alert pop-up box containing message is placed on the screen. When the operator closes this dialog, a response transmission is made: origin_callsign Alert ack
<li><b>cqcqcq</b> text (Call for general chat) Following text prints.
<li><b>allcall</b> text (All stations print) Following text prints at all stations.
<li><b>callsign\></b> or <b>callsign\<</b> (Speed change)
\> increases speed, \< decreases speed at target station. 
Response is origin_callsign: 4.5 baud (or whatever the new speed is). 
If the speed is already at the maximum or minimum speed, the response is the same, 
and there is no speed change.
<li><b>allcall#[nnn]</b> (Send file to all stations) All stations within range start to print. 
Text following [nnn] will be saved in or appended to text file nnn.txt. When 
squelch closes, station responds callsign:CRC-VAL Ack if the message stored OK. There 
is no response if the message was not received. allcall#[filename.txt] works the 
same as callsign#[filename.txt].
</ul>
<br>

\section relaying_message Relaying messages

This is a relay mechanism which preserves the origin callsign as the message propagates 
so that any response can be relayed back to that station. message can contain further 
trigger commands. 

Originating station: relay_callsign; dest_callsign message

\image html fsq-relay-examples.png "Relay Examples"
\image latex fsq-relay-examples.png "Relay Examples" width=4.5in

\section fsq_keyboard_keys Special keys

Pressing the F1 key will transfer the "last call" selected from the heard list to the
Tx panel.  "last call" defaults to allcall on program startup.

Pressing the F2 key will transfer the last command to the Tx panel.  The last command
is an empty string on program startup.

\section fsq_heard Heard List

The FSQ heard list is automatically added to, modified, and aged in accordance
with the FSQ user configuration parameters.  Each time a valid callsign/crc8 pair
is discovered it is either added to the heard list, or it's entry is updated with
the last GMT and signal-to-noise ratio.

A single click on a heard list entry will make it the "last call" string.

A double click on a heard list entry will also add the call to the Tx panel.  If multiple
heard list entries are double clicked each one is added to the Tx panel separated by
a space.

<center>
\image html fsq-heard-list.png "Heard List"
\image latex fsq-heard-list.png "Heard List" width=1.5in
</center>

Heard list entries have an associated pop-up menu, available by right-clicking on
the desired entry:

<center>
<table>
<tr>
  <td>
    \image html fsq-allcall-popup.png "All Call Menu"
    \image latex fsq-allcall-popup.png "All Call Menu" width=2.0in
  </td>
  <td>
    \image html fsq-directed-popup.png "Directed Menu"
    \image latex fsq-directed-popup.png "Directed Menu" width=2.0in
  </td>
</tr>
</table>
</center>

\section fsq_image_mode Image Mode

fldigi-fsq has the ability to transmit and receive small digital images selected
from those stored on the disk.  The image transmission is similar to but not
compatible with the MFSKpic modes.  FSQ images are always transmitted in a fixed
width to height ratio.  The W x H format is not limited to a single ratio, nor is
it limited to just color, or just gray scale.

The start of the picture is signalled digitally, and the receiver simply captures 
data continuously for W lines of H pixels, as requested, resulting in a WxH pixel 
image, then reception stops.

The images are approximately in 4:3 or 3:4 aspect ratio. Images must be sent and 
received in Directed Mode, and can be sent to just one recipient, to several 
recipients, or everyone on the channel, just as with other messages.

Available image aspect ratios / formats include:

<ul>
<li>160x120 color (1)
<li>320x240 color (1)
<li>640x480 grey (1)
<li>640x480 color (1)
<li>240x300 color
<li>240x300 grey
<li>120x150 color
<li>120x150 grey
</ul>
(1) compatible with the program FSQCALL

<b>Narrow-Band Images</b>

FSQ Image Mode was designed for NVIS propagation on the lower HF bands. 
Unlike SSTV, the pictures are sent relatively slowly, in a narrow bandwidth 
(1/4 of SSTV), and as a result the picture signal to noise ratio is generally 
better than SSTV for the same transmission power. As a guideline, FSQ signals 
(before the picture is started) need to be about +10dB SNR for noise-free picture 
reception.

FSQ pictures also offer significantly better reception than SSTV under NVIS 
conditions. Because there is no picture sync, there is no risk of the picture 
tearing due to timing changes, or breaking up during selective fades. All that 
happens is that the image may noise up momentarily, but will stay correctly 
aligned. The mode is also significantly less affected by ghosting.

<b>Image Format</b>

The image mode sampling rate is 12,000 samples/sec, the same as the text mode. 
Each pixel consists of 10 samples, and there are P pixels/line. Therefore the 
line period is 10 x P/12000 seconds, or the line rate 12000/(10 x P) lines/sec. 
In colour, each line is sent three times, in BGR order.

As an example, if there were 320 pixels per line, and 240 lines, the line rate 
would be 3.75 lines/sec, and 240 x 3 (for RGB) lines would take 192 seconds, or 
3.2 minutes for a colour picture containing 76,800 pixels.

There is no horizontal (line) sync sent with the picture, and no vertical sync 
at the start of the picture. As a result, it is possible for reception to start 
slightly earlier or later than the standard delay from the time of the command, 
depending on the relative speed of the computers used at either end. This has 
two effects: first, the picture may be recorded shifted left or right, and the 
pixel colour order may be wrong. This is easily fixed after reception using the 
Phase control.

If the sound cards in the transmitting and receiving computers differ markedly 
in sample rate, the resulting picture can also be skewed. A small timing 
difference is enough to cause a noticeable slant in the picture. This is easily 
fixed after reception using the Slant control. High quality sound cards should 
be better than 5ppm, and result in barely noticeable slant, but some cheaper 
computer built-in sound modules may be rather worse.  If you plan to send FSQ
images you should calibrate your sound card to WWV.

<b>Receive Image</b>

<center>
\image html fsq-image-rx.png "Image Reception"
\image latex fsq-image-rx.png "Image Reception" width=4.5in
</center>

Reception is completely automatic. Provided the software is in Directed Mode, and 
the sending station uses the correct callsign and trigger command, the decoder 
will identify the picture start, and record the picture. In doing so, it 
automatically opens a separate "FSQ Rx Image" dialog.

Demodulation uses a phase detection algorithm based on quadrature demodulation 
using the current and previous samples at 12000 samples/sec. All the samples are 
converted to pixels and stored, but only the average over every 10 samples is 
displayed.

The raw image is stored in a temporary buffer of expanded size, which allows 
fine adjustment of the slant and phase to be made before the image is sampled 
and displayed.

If the transmitting station goes off the air or reception is lost during 
recording of a picture, the recording will continue until the necessary number 
of samples has been made, then reception stops.

\image html fsq-rx-image-raw.png "Received - no correction"
\image latex fsq-rx-image-raw.png "Received - no correction" width = 4.0in

The Rx Image dialog controls from left to right are:
- Reset: clears all rx data
- Phase adjustment: moves image left and right
- Slant adjustment: slews image diagonally
- Save: saves the image to the fldigi image folder with a date-time stamped file name
- Close: closes the image dialog

\image html fsq-rx-image-corrected.png "Received - phase corrected"
\image latex fsq-rx-image-corrected.png "Received - phase corrected" width = 4.0in

Image phase and slant should not be adjusted before the picture reception has 
finished, as then only part of the image will be corrected.

<b>Transmit Image</b>

Access to the transmit image functions is only available via the heard list pop-up
menus.  Right click on the recipient call sign and select the "Send Image to ..." item
from the pop-up menu.

<center>
\image html fsq-image-xmt.png "Image Transmission"
\image latex fsq-image-xmt.png "Image Transmission" width=4.5in
</center>

A file open dialog is displayed when the "Load" button is pressed on the TX Image 
dialog.  This allows the user to select any image in a number of different standard 
formats. The image is rendered to the selected image size using a resampling process.  
fldigi will try to force fit the image to the selected aspect ratio.  You should
try to work with images that are in a 4:3 or a 3:4 aspect ratio.  The selected
image does not need to be the same size.  Fldigi will enlarge or reduce the image 
using a resampling technique to fit the selected image size.  The example photo 
of Star Gazer lilies is a 3.2 MB file, 3008 x 2000 pixels.  It was taken in my lily
patch in June 2014.

Once the image has been loaded, the user starts the transmission by pressing the
"Xmt" button on the transmit image dialog.  fldigi formats the correct command
sequence and starts the transmission.

The recipient callsign for image transmission can be a single callsign, or allcall.  
Sending to a single recipient or to allcall is accomplished as described above.  To 
send to multiple addresses you must add their callsigns to the Tx panel, separated 
by spaces and with a trailing space.  Then press the Xmt button on the Tx Image dialog.

fldigi returns to the receive mode at the conclusion of the image transfer.

\section fsq_rsid Disable RxID

You will probably want to disable the RxID detector during FSQ operations.  If RxID
is enabled and an RsID signal is detected the program will switch to the mode 
specified by the RxID.  That will completely alter the user interface.

\ref fsq_page "Return to Top of Page"
<br>
\ref main_page "Return to Main Page"
*/