SSTV DIGITAL MODES


International Space Station (ISS) SSTV PD120 mode transmissions
on 145.8MHz nbfm I received/decoded in April 2016 (click to enlarge)



The PD SSTV modes were developed by Paul G4IJE and Don K0HEO
to provide a range of high resolution SSTV modes with reasonable transmission times.

A 640 x 480 picture can be transmitted in 3 minutes with better quality than a 4.5 minute colour FAX transmission.

The original PD mode timings have recently been revised and some new modes added.
All the PD modes use Y, R-Y and B-Y encoding, similar to that used in the original Robot 1200c colour modes.
The chrominance components (R-Y and B-Y) use reduced vertical resolution to minimise transmission time.

Each line starts with a long sync pulse (more on this later) which is then followed by Y0, R-Y, B-Y and Y1 components.
Each component is of identical duration.
Assuming that line numbering starts from 0, Y0 is the luminance information from line 0, the R-Y and B-Y are the AVERAGE of the R-Y and B-Y from lines 0 and 1, and Y1 is the luminance from line 1.
Thus each "line" of SSTV sent is actually encoded from TWO picture lines.
The receive software uses Y0, R-Y and B-Y to decode the first picture line and Y1, R-Y and B-Y to decode the second line.

The PD modes are intended for synchronous (ie "free running") reception.
The line sync pulses serve to mark the start of each line "pair" to allow easy starting if the Vertical Interval Signaling (VIS) is missed.

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.

Modes/Resolutions
Mode Pixel Line (mS) Sync (mS) Porch (mS) Frame(secs) Resolution VIS (decimal) VIS+P (decimal)
PD 90 532 703.04 20 2.08 89.98912 320 x 256 99 99
PD 120 190 508.48 20 2.08 126.10304 640 x 496 95 95
PD 160 382 804.416 20 2.08 160.8832 512 x 400 98 226
PD 180 286 754.24 20 2.08 187.05152 640 x 496 96 96
PD 240 382 1000 20 2.08 248 640 x 496 97 225


The VIS codes are in decimal and the VIS+P codes includes the even parity bit.
The line times are for a complete "double" line, comprising sync + porch + Y0 + R-Y + B-Y + Y1.
Please note that there are no pulses or porches between the four components.

Source: http://www.g0hwc.com/sstv_modes.html

(VIS) Vertical Interval Signaling 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.
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.


Vertical Interval Signaling (VIS) codes by John Langner WB2OSZ April 1997
  High Order Bits (MSB = Even Parity)
Low Order Bits 0/8x 1/9x 2/Ax 3/Bx 4/Cx 5/Dx 6/Ex 7/Fx
x0 0000 Robot 12sec color SC-1 24 color Martin M1 Scottie S4 AVT 24 [3] AVT125 [4a] Acorn PD180 Pasokon TV [6]
x1 0001 Robot 8sec red [1]   [7] [8] AVT 24 narrow AVT125 [4a] narrow Acorn PD180 Pasokon TV P3
x2 0010 Robot 8sec green [1]   [7] [8] AVT 24 QRM AVT125 [4a] QRM Acorn PD160 Pasokon TV P5 GVA 125
x3 0011 Robot 8sec blue [1]   [7] SC-2 30 Sec color AVT 24 Nar + QRM AVT125 [4a] Nar + QRM Acorn PD90 Pasokon TV P7 PD 65
x4 0100 Robot 24 sec Color [1] SC-1 48 [2] Color Martin M3 Scottie S3 AVT 90 [5] Proscan J120 [5]
x5 0101 Robot 12 sec red [1]   [7] [8] AVT 90 narrow      
x6 0110 Robot 12 sec green [1]       AVT 90 QRM      
x7 0111 Robot 12 sec blue [1]   [7] SC-2 180sec color AVT 90 Nar + QRM   GVA BW 125  
x8 1000 Robot 36 color SC-1 28 [2] Color Martin M2 Scottie S2 AVT 94   MSCAN future  
x9 1001 Robot 24 red [1] XA robot   [7] [8] AVT   MSCAN future  
xa 1010 Robot 24 green [1]   [7] [8] AVT 94 QRM   MSCAN future  
xb 1011 Robot 24 blue [1]   [7] SC-2 60 sec color AVT 94 Nar + QRM   MSCAN future  
xc 1100 Robot 72 color SC-1 96 color Martin M1 Scottie S1 AVT 188 Scottie DX [4b]   GVA 250  
x1 0001 Robot 8sec red [1]   [7] [8] AVT 188 narrow      
x2 0010 Robot 8sec green [1]   [7] [8] AVT 188 QRM      
x3 0011 Robot 8sec blue [1]   [7] SC-2 120 Sec color AVT 188 Nar + QRM AVT125 [4a] Nar + QRM PD120  


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 G3OQD for future Martin modes.
I would assume HQ1 and HQ2 are somewhere in this range but I don't know.

[8] Martin Emmerson G3OQD wants these too.

SSTV Transmission Modes
Mode Family Mode Name Color Type Time (sec) Scan Lines notes
AVT 24 RBG 24 120 d
  90 RBG 90 240 d
  94 RBG 94 200 d
  188 RBG 188 400 d
  125 RBG 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 High Resolution PD P3 RGB 203 16+480  
  P5 RGB 305 16+480  
  P7 RGB 406 16+480  
  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
  12 YC 12 120 e
  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

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.