        Note: EZ SSTV supports a limited number
        of SSTV modes.  Pasokon TV has about 30.


                SSTV Transmission Modes
                -----------------------

The original 8 second Black + White mode used tones in the
range of 1500 to 2300 Hz to represent shades of gray.
A 5 millisecond pulse of 1200 Hz separated scan lines and
a 30 millisecond pulse of 1200 Hz signaled the beginning of
a new image.  A complete image was 120 scan lines sent at 
a rate of 15 lines per second.

Since that time dozens of new modes have been developed to 
add higher resolution and/or color.

The Robot color modes were developed back when everyone was
using only B+W equipment.  They send a B+W compatible image at
the beginning of each scan line and color information at the
end of each line.  Thus, the large installed base of B+W only
equipment was able to receive the images.

Since that time all the newer modes have used separate scans
of Red, Green, and Blue components because this is the way 
images are generally stored in files and displayed on computer
monitors.

Characteristics of these modes are summarized in the table 
below.


   Mode         Mode    Color   Time    Scan    
   Family       Name    Type    (sec)   Lines   Notes
   ------       ----    ----    -----   -----   -----
   AVT          24      RBG     24      120     d
                90      RGB     90      240     d
                94      RGB     94      200     d
                188     RGB     188     400     d
                125     BW      125     400     d

   Martin       M1      RGB     114     16+240  b
                M2      RGB     58      16+240  b
                M3      RGB     57      8+120   c
                M4      RGB     29      8+120   c

   Pasokon TV   P3      RGB     203     16+480
   High         P5      RGB     305     16+480
   Resolution   P7      RGB     406     16+480

   Robot        8       BW      8       120     a, e
   B&W          12      BW      12      120     e
                24      BW      24      240     e
                36      BW      36      240     e

   Robot        12      YC      12      120     
   Color        24      YC      24      120
                36      YC      36      240
                72      YC      72      240

   Scottie      S1      RGB     110     16+240  b
                S2      RGB     71      16+240  b
                S3      RGB     55      8+120   c
                S4      RGB     36      8+120   c
                DX      RGB     269     16+240  b

   Wraase SC-1  24      RGB     24      128     
                48      RGB     48      256     
                96      RGB     96      256     

   Wraase SC-2  30      RGB     30      128
                60      RGB     60      256
                120     RGB     120     256
                180     RGB     180     256

There are also some experimental modes implemented in only
one or two systems.  Some are still evolving, some will be
phased out.  Information here might not be accurate.  Whether 
others decide to add them remains to be seen.

   ProSkan      J120    RGB     120     240

   WinPixPro    GVA 125 BW      125     480     
                GVA 125 RGB     125     240
                GVA 250 RGB     250     480

   Acorn        PD 65   ?       65      ?
                PD 160  ?       160     ?
                PD 180  RGB     180     480
                PD 240  RGB     240     480

   "J.A."       ?       ?       ?       480


Color Type:

RGB - Red, Green, and Blue components sent separately.
YC - Sent as Luminance (Y) and Chrominance (R-Y and B-Y).
BW - Black and White.


Notes:

a - Similar to original 8 second black & white standard.
b - Top 16 lines are gray scale.  240 usable lines.
c - Top 8 lines are gray scale.  120 usable lines.
d - AVT modes have a 5 second digital header and no
    horizontal sync.
e - Robot 1200C doesn't have B&W mode but it can send red,
    green, or blue memory separately.  Traditionally, 
    just the green component is sent for a rough approximation
    of a B&W image.


Initial SSTV Mode
-----------------

The first time you run Pasokon TV, the Scottie S1 mode is
selected because this is currently the most popular mode
used in North America.  You can change this default by 
following these steps:

1. Select the desired SSTV mode.
2. Pick "SSTV Mode" from the "Setup" menu.
3. In the dialog box, pick the button labeled "Default".


VIS Codes
---------

The original vertical sync signal was simply 30 milliseconds
of 1200 Hz.  The Robot 1200C used a much longer vertical sync
signal containing digital information.  This is called the
Vertical Interval Signaling (VIS) code and allows a receiving
station to select the proper mode automatically.

All modern SSTV systems use this VIS code but many people 
still announce the mode about to be transmitted because VIS
decoding is not very reliable under noisy conditions.

The codes used are listed below.


                Vertical Interval Signaling Codes
                ---------------------------------

                                        Compiled by John Langner WB2OSZ
                                        Version of March 1996
                

                Note: The most recent version of this
                can be found on the Internet at
                http://www.ultranet.com/~sstv/modes.html

                I've talked with most of the major SSTV
                developers and there is general agreement
                that the on-line doc should be the world
                wide reference to avoid conflicts.


Low                     High Order Bits (MSB = Even Parity)
Order
Bits  | 0/8x    1/9x    2/Ax    3/Bx    4/Cx    5/Dx    6/Ex    7/Fx
      | p000    p001    p010    p011    p100    p101    p110    p111
------+----------------------------------------------------------------
x0    | Robot   SC-1    Martin  Scottie AVT     AVT     Acorn   Pasokon
0000  | 12 sec  24      M4      S4      24 [3]  125     PD 180  TV  [6]
      | Color   Color                           [4a]            
      |
x1    | Robot                           AVT     AVT     Acorn   Pasokon 
0001  | 8 sec                           24      125[4a] PD 240  TV  P3
      | Red [1]                         Narrow  Narrow
      |
x2    | Robot                           AVT     AVT     Acorn   Pasokon
0010  | 8 sec                           24      125[4a] PD 160  TV  P5
      | Green [1]                       QRM     QRM             GVA 125
      |
x3    | Robot                   SC-2    AVT     AVT             Pasokon 
0011  | 8 sec                   30 sec  24      125 [4a]        TV  P7
      | Blue [1]                Color   Nar+QRM Nar+QRM         PD 65
     -+-
x4    | Robot   SC-1    Martin  Scottie AVT     [5]     ProSkan [5]
0100  | 24 sec  48 [2]  M3      S3      90              J120
      | Color   Color
      |
x5    | Robot                           AVT                     
0101  | 12 sec                          90
      | Red [1]                         Narrow
      |
x6    | Robot                           AVT                                        
0110  | 12 sec                          90                 
      | Green [1]                       QRM
      |
x7    | Robot                   SC-2    AVT             GVA BW
0111  | 12 sec                  180 sec 90              125
      | Blue [1]                Color   Nar+QRM
     -+-
x8    | Robot  SC-1     Martin  Scottie AVT
1000  | 36     48 [2]   M2      S2      94
      | Color  Color
      |
x9    | Robot                           AVT                  
1001  | 24 sec                          94
      | Red [1]                         Narrow
      |
xA    | Robot                           AVT                   
1010  | 24 sec                          94
      | Green [1]                       QRM
      |
xB    | Robot                   SC-2    AVT                  
1011  | 24 sec                  60 sec  94
      | Blue [1]                Color   Nar+QRM
     -+-
xC    | Robot  SC-1     Martin  Scottie AVT 188         GVA 250                
1100  | 72     96       M1      S1      Scottie
      | Color  Color                    DX [4b]
      |
x1    | Robot                           AVT                     
0001  | 8 sec                           188
      | Red [1]                         Narrow
      |
x2    | Robot                           AVT                   
0010  | 8 sec                           188
      | Green [1]                       QRM
      |
x3    | Robot                   SC-2    AVT                     
0011  | 8 sec                   120 sec 188
      | Blue [1]                Color   Nar+QRM
     -+-

Notes:
  [1]   The Robot 1200C can send either composite color or just one
        of the Red, Green, or Blue memories.  
        Some SC-1, Martin, and Scottie implementations also allow
        transmission of only one color component.
        This means columns 0 through 4 are completely used.
  [2]   There are two different Wraase SC-1 48 second modes.  One has 
        twice the scan line time but half the number of lines as the 
        other.
  [3]   Narrow uses a shift narrower than the usual 1500 - 2300 Hz
        so a narrower bandpass can be used on the receiver.
        QRM mode is interlaced.  
  [4a]  A couple sources indicate that Scottie DX has the same
        VIS code as AVT 125.  The Robot 1200C - the defacto
        standard - uses code CCh, same as AVT 188.
  [4b]  Scottie DX and AVT 188 both have the same VIS code due to
        lack of communication between developers.  Let's not make
        this mistake again!
  [5]   These table entries do not have standardized uses yet.
        Column 5 - last 12 rows.
        Column 6 - entire column.
        Column 7 - last 12 rows.
  [6]   First 4 rows of column 7 are for new modes such as 640 x 480.
        The first one is reserved for future use.  The others are for
        P3, P5, and P7.

The VIS code is sent as:

        30 mS start of 1200 Hz.
        7 data bits sent LSB first,
                30 mS each, 1100 Hz for 1, 1300 Hz for 0.
        Even parity bit of 30 mS.
        30 mS stop of 1200 Hz.

  

New High Resolution Modes
-------------------------

One of the most popular requests for the next version of Pasokon
TV was higher resolution modes for 640 x 480 images.  There were 
no existing standards so I had to develop a new specification.

The new modes are similar to most existing modes in that they:
- Send images as R-G-B.
- Use 1500 - 2300 Hz for image intensities.
- Use 1200 Hz for horizontal sync.
- Have VIS code at the beginning.
- Have extra gray scale lines at the top.

These new modes differ from existing modes in a couple 
significant ways:
- Well defined and published.  Developers of new SSTV modes
  generally tried to keep details secret.  Other developers
  guessed from their own measurements of signals heard and all 
  the guesses came out a little different.  As a result, 
  different systems aren't exactly compatible with each other 
  especially for the lesser used modes.
- Based on nice round numbers.  The AVT system sends all pixels
  at precisely 2048 per second but all the others use VERY 
  BIZARRE timing.  

Here is the specification with enough detail that others should
be able to implement it.

There are 3 new modes for 640 x 480 image transmission.
They all begin with a different VIS code which does not 
conflict with any other known modes.  (Was true at the time of
publication but it was discovered months later that some 
other experimental modes used the same codes.)

We need a central registry for allocating unique numbers.  
I suggest http://www.ultranet.com/~sstv/modes.html because
it can be accessed electronically from around the world.

They all have 16 lines of gray scale at the top, black on
left and white on right.  Text may also appear in this region.
The transmitting station adds it automatically and most 
receiving systems will display it.  These top 16 lines generally 
won't be saved when the image is written to a file.

After the 16 gray scale lines, we have 480 image lines.
Assuming 640 pixels per line, the timing for each line is:

        [ VIS code (first line) or horizontal sync here ]
        Back porch - 5 time units of black (1500 Hz).
        Red component - 640 pixels of 1 time unit each.
        Gap - 5 time units of black. 
        Green component - 640 pixels of 1 time unit each.
        Gap - 5 time units of black.
        Blue component - 640 pixels of 1 time unit each.
        Front porch - 5 time units of black.
        Horizontal Sync - 25 time units of 1200 Hz.

A total of 1965 time units per line.

The 3 modes differ only in the length of the "time unit" 
mentioned above.

        Mode name                P3      P5      P7
        ---------               ----    ----    ----
        Time units / second     4800    3200    2400
        Total time (sec)         203     305     406
        Total time (min)         3.4     5.1     6.8
        VIS code (hexadecimal,    71      72      F3 
           includes even parity)

As you might have guessed, the names come from the number of
minutes required to transmit a picture.

The porches allow more accurate detection of the sync edge
than if an unknown video level was adjacent to the sync.

Received images often have a slight amount of slant or 
horizontal shift from the ideal.  By placing black gaps of
a few pixels between the color components, edges will have
less noticable (and annoying) black instead of the other
edge of the image wrapped around in the wrong color.

For the highest quality mode, P7, a pixel rate of 2400 Hz was 
picked because it is a nice round number close to the rates 
used by M1, and S1.  It is also a standard serial port 
baud rate so some low cost implementations might want to use
a serial port somehow for timing.  It is also harmonically
related to the sync frequency but that probably doesn't
make any difference.

The P3 mode, has twice as many pixels per second resulting
in half the transmission time and lower image quality.  P5 is
somewhere in the middle.

You will notice that all the numbers listed are multiples of
5 time units.  Implementations choosing to use 512 instead of 
640 pixels  per line can simply multiply everything by 4/5 and 
it all still comes out in nice round numbers.  For example, P7 
would have a pixel rate of 2400 * 4 / 5 = 1920 Hz.

The front and back porches, and the gaps would be 4 units
instead of 5.  Each line is a total of 1572 time units.
The total line time, in milliseconds, comes out the same.

For best image quality, the receiving station will want to use
"free run" or synchronous mode.  Uncalibrated systems can 
follow the horizontal sync pulses for timing.  The presence of
horizontal sync also allows you to receive the rest of an image
even if you missed the beginning.  Modes without horizontal
sync, such as AVT, do not have this property.
