SSTV Transmission Modes
General Characteristics
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
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
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
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 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.
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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.
Vertical Interval Signaling Codes
---------------------------------
Compiled by John Langner WB2OSZ
Version of March 1996
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 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.
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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 and I will publish it here. Everyone will
benefit.
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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.
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