SSTV Transmission Modes

General Characteristics

(by John Langner, WB2OSZ)

 

Here are some characteristics of SSTV modes currently in use. You will notice that number of pixels per line is not listed. This is because SSTV uses analog modulation which can be sampled at any rate. Various systems might sample the same scan line with 256, 320, 512, 640, or other number of pixels. They are all compatible but those with more samples per scan line produce better detail.

Based on my own observations, around 80% of images sent in North America are Scottie S1. You will also hear a few S2, Martin M1, Robot 36 and 72. Others are extremely rare.

Someone from Europe told me it is about 95% Martin M1 there.

Robot and AVT are apparently popular in Japan. At the 1994 Dayton Hamvention, Tasco demonstrated their incredible new TSC-100 scan converter. Everyone was really impressed with the product but no one was buying them. The reason? It only had four modes: Robot 36 & 72, AVT 90 & 94. Tasco had failed to do any market research on what the rest of the world was using.

                SSTV Transmission Modes
                -----------------------         
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     240     b                  
             M2      RGB     58      240     b                  
             M3      RGB     57      120     c                  
             M4      RGB     29      120     c                  
             HQ1     YC      90      240                  
             HQ2     YC      112     240 


Pasokon TV   P3      RGB     203     16+480     
High         P5      RGB     305     16+480     
Resolution   P7      RGB     406     16+480       
PD           PD 240  YC      248     480                  
             PD 180  YC      187     480
             PD 160  YC      161     384    
             PD 120  YC      126     480   
             PD 90   YC      90      240


Robot        8       BW      8       120     a, e
             12      BW      12      120     e      
             24      BW      24      240     e       
             36      BW      36      240     e        
             12      YC      12      120           
             24      YC      24      120         
             36      YC      36      240         
             72      YC      72      240       


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


Wraase SC-1  24      RGB     24      120     c         
             48      RGB     48      240     b        
             96      RGB     96      240     b       


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  
"J.A."       ?       ?       ?       480       
MSCAN        TV1     ?       ?       ?                  
             TV2     ?       ?       ?   
 


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 really 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.

VIS Codes

The original 8 second B&W SSTV mode (about 30 years ago) used 5 milliseconds of 1200 Hz for horizontal sync (separating scan lines) and 30 milliseconds of 1200 Hz at the beginning of a frame. The Robot 1200C introduced a new concept called Vertical Interval Signaling (VIS). All recent SSTV systems now use a longer vertical sync with a digital code representing the mode of the following image. This allows the receiving station to select the proper mode automatically.

Thanks to everyone who provided information to keep this table complete and up to date. Several have suggested that this be recognized as the world-wide location for registering this information to avoid conflicts.

		Vertical Interval Signaling Codes
  		---------------------------------
		Compiled by John Langner WB2OSZ  
                Version of April 1997
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      M1      S4      24 [3]  125     PD 180  TV  [6]  
      | Color   Color                           [4a]                  
      |
x1    | Robot           [7]     [8]     AVT     AVT     Acorn   Pasokon   
0001  | 8 sec                           24      125[4a] PD 240  TV  P3      
      | Red [1]                         Narrow  Narrow     
      |  
x2    | Robot           [7]     [8]     AVT     AVT     Acorn   Pasokon  
0010  | 8 sec                           24      125[4a] PD 160  TV  P5  
      | Green [1]                       QRM     QRM             GVA 125 
      |  
x3    | Robot           [7]     SC-2    AVT     AVT     PD 90   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           [7]     [8]     AVT                       
0101  | 12 sec                          90  
      | Red [1]                         Narrow        
      |  
x6    | Robot           [7]     [8]     AVT                                 
0110  | 12 sec                          90                   
      | Green [1]                       QRM   
      |  
x7    | Robot           [7]     SC-2    AVT             GVA BW
0111  | 12 sec                  180 sec 90              125  
      | Blue [1]                Color   Nar+QRM   
     -+-  
x8    | Robot  SC-1     Martin  Scottie AVT             MSCAN  
1000  | 36     48 [2]   M2      S2      94              future   
      | Color  Color  
      |  
x9    | Robot           [7]     [8]     AVT             MSCAN             
1001  | 24 sec                          94              future  
      | Red [1]                         Narrow  
      |  xA
      | Robot           [7]     [8]     AVT             MSCAN             
1010  | 24 sec                          94              future   
      | Green [1]                       QRM  
      |  
xB    | Robot           [7]     SC-2    AVT             MSCAN          
1011  | 24 sec                  60 sec  94              future
      | 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           [7]     [8]     AVT                       
0001  | 8 sec                           188  
      | Red [1]                         Narrow   
      |  
x2    | Robot           [7]     [8]     AVT                     
0010  | 8 sec                           188  
      | Green [1]                       QRM   
      |  
x3    | Robot           [7]     SC-2    AVT     PD 120                                 
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.    

[7]   Unused positions in this column reserved by Martin Emmerson          
for future Martin modes.  I would assume HQ1 and HQ2 are          
somewhere in this range but I don't know.    

[8]   Martin Emmerson wants these too. 

     
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.

    


More Details

This section will take a little longer to prepare. There are numerous descriptions of SSTV modes floating around. I've accumulated many over the years and they are incomplete, inaccurate, and inconsistent. They seem to be based on approximate measurements of signals heard on the air rather than the precise specifications from the original developer.

This explains why we have so many interoperability problems between different SSTV systems.

I'd like to help clean up this mess by collecting and publishing complete and accurate specifications of the SSTV modes but I can't do it without your help. If you have accurate and reliable information on any of the SSTV modes, please send it to me WB2OSZ and I will publish it here. Everyone will benefit.


New High Resolution (640 x 480) "P" Modes

These 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. In some cases, the total line time is not even a multiple of the pixel time.

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.

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 receving 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 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 

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

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.

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. 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.

 


johnl@world.std.com

John's Email is bouncing, has anyone got a up to date Email for John ?

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