VK2QW SSTV



VK2QW An introduction to SSTV

Slow Scan Television is a picture transmission mode developed and used by the Amateur community for quite some time. The original SSTV required expensive commercial equipment. Today, with Personal Computers, Software and a Interface to your transceiver you can be on the air with SSTV.

While SSTV signals are FAX-like in function they do not possess the scratching quality of the FAX signal. The sound of an SSTV signal is more tonal in its composition. It is considered that each mode cannot be distinguished apart by ear. SSTV unlike domestic television is a method of transmitting still pictures over a very narrow bandwidth, on frequencies normally used for speech by Radio Amateurs.

SSTV was originally invented by Copthorne Macdonald and first used by Radio  Amateurs. The original idea was to find a method of transmitting a television picture over a single speech channel. This meant that a typical (at that time) 3MHz wide television picture had to be reduced to around 3kHz (1000:1 reduction). Slow Scan Television occupies the same amount of bandwidth as a typical SSB signal. The choice of time base for synchronising was the readily available domestic power supply at 50 or 60 Hz (depending on the country of origin). This gave a line speed of 16.6Hz and 120 or 128 lines per frame (against the then UK standard of 405 lines (now 625) per frame), giving a new picture frame every 7.2 or 8 seconds.

The hardware used to receive/transmit SSTV is a sound card or you can build/buy an interface which uses one of your serial ports of your computer. The shareware software can be freely downloaded from the internet. There are also programs available for different operating systems. 

Modulation

An Amateur Radio SSTV transmission is transformed to an audio signal. Each colour has its own tone between 1500Hz (BLACK) and 2300Hz (WHITE). A SSTV picture has 256 lines with each line  transmitted 3 times (the 3 RGB colours). There is a compromise between image quality and time. At one extreme we have low resolution (120 line) black and white images which take only 8 seconds. At the other end we have software providing high resolution with 16 million colours at 640 x 480 resolution which take up to 7 minutes. Most images these days are full colour 320 x 240 resolution. The average time taken to send  a HF SSTV picture varies although 2 minute period  could be considered average using Scottie S1 for example.

In other words, the signal frequency shifts up or down to designate brighter or darker pixels, respectively. Colour is achieved by sending the brightness of each colour component (usually red, green and blue) separately. This signal can be fed into an SSB transmitter, which in part modulates the carrier wave.

There are a number of different modes of transmission, but the most common ones are Martin M1 (popular in Europe) and Scottie S1 (used mostly in the USA). Using one of these, an image transfer takes 114 (M1) or 110 (S1) seconds. Some black and white modes take only 8 seconds to transfer an image.
 

Scan Lines

A transmission consists of horizontal lines, scanned from left to right. The RGB colour components are sent separately one line after another in the order R, G, B. Some Robot modes use a YC color model, which consists of luminance (Y) and chrominance (R-Y and B-Y). The modulating frequency changes between 1500 and 2300 Hz, corresponding to the intensity (brightness) of the color component. The modulation is analogue, so there is not a defined number of pixels in each line; they can be sampled using any rate (though in practice, the image aspect ratio is conventionally 4:3). Lines end in a 1200 Hz horizontal synchronization pulse of 5 milliseconds (after all colour components of the line have been sent).

An SSTV signal begins with a digital mode identifier and then starts scanning between 1500 and 2300 Hz, with regular 1200 Hz clicks.

SSTV VIS Code

With the introduction of Robot 1200C, Robot Research introduced the VIS code, which is used to indicate the speed and mode at the beginning of the transmission. The VIS code, when decoded by the receiving station, will let the receiver automatically set the necessary parameters for proper reception. The VIS code is sent as part of the vertical sync pulse and is 10 bits long lasting 10*30ms. The start and stop bits are represented as a 1200Hz tone with the remaining 8 bits (including 1 even parity bit) left for encoding information. This breaks down as 1 30ms start bit at 1200Hz, 7 data bits, each 30ms, sent Lowest Significant Byte (LSB) first (logical '1' is transmitted as 1100Hz, logical '0' is transmitted as 1300Hz). 1 30ms even parity bit and 1 30ms stop bit as 1200Hz.

Modes

Below is a table of some of the most common SSTV modes and their differences. These modes share many properties, such as synchronization and/or frequencies and grey/colour level correspondence. Their main difference is the image quality, which is proportional to the time taken to transfer the image and in the case of the AVT modes, related to synchronous data transmission methods and noise resistance conferred by the use of interlace.

SSTV Standards for Synchronization

Parameter

Value

 Synch Tone

 1200 Hz

 Black Tone

 1500 Hz

 White Tone

 2300 Hz

 Picture Synch

 30 ms

 Line Synch

 5 ms

SSTV Modes

Mode

Type

Time [s]

Resolution[Pixel x Line]

Notes

Martin

M1

RGB

114

320 x 256

a

 

M2

RGB

58

160 x 256

a

 

M3

RGB

57

320 x 128

b

 

M4

RGB

29

160 x 128

b

 

 

 

 

 

 

Scottie

S1

RGB

110

320 x 256

 

 

S2

RGB

71

320 x 128

 

 

S3

RGB

55

320 x 128

 

 

S4

RGB

36

160 x 128

 

 

DX

RGB

269

320 x 256

*

 

 

 

 

 

 

Robot

8

B/W

8

160 x 120

c

 

12

B/W

12

320 x 120

 

 

24

B/W

24

320 x 240

 

 

36

B/W

36

320 x 240

 

 

12

YUV

12

160 x 120

*

 

24

YUV

24

320 x 120

*

 

36

YUV

36

320 x 240

*

 

72

YUV

72

320 x 240

*

 

 

 

 

 

 

B/W Mode

SC-1 8

B/W

8

128 x 128

 

 

SC-1 16

B/W

16

256 x 128

 

 

SC-1 32

B/W

32

256 x 256

 

 

 

 

 

 

 

Wraase

SC-1 24

RGB

24

128 x 128

b

 

SC-1 48

RGB

48

256 x 128

a

 

SC-1 96

RGB

96

256 x 256

a

 

SC-2 30

RGB

30

256 x 128

 

 

SC-2 60

RGB

60

256 x 256

 

 

SC-2 120

RGB

120

512 x 256

 

 

SC-2 180

RGB

180

512 x 256

 

 

 

 

 

 

 

Scanmate

1

ROB

391

310 x 512

*

 

2

RGB

261

310 x 512

*

 

DX

RGB

269

256 x 256

*

 

 

 

 

 

 

AVT

24

RGB

24

128 x 128

*, d

 

90

RGB

90

320 x 240

*, d

 

94

RGB

94

320 x 200

*, d

 

188

RGB

188

400 x 320

*, d

 

Mode Name

VIS Code

Picture Size

Pixel Time (exact)

Total Picture Time (exact)

PD240

97*

640 x 480

382 microseconds

248.00000 seconds

PD180

96

640 x 480

286 microseconds

187.05152 seconds

PD120

95

640 x 480

190 microseconds

126.10304 seconds

PD160

98*

512 x 384

382 microseconds

160.88320 seconds

PD90

99

320 x 240

532 microseconds

89.98912 seconds

PD50

93*

320 x 240

286 microseconds

49.68448 seconds

PD290

94*

800 x 600

286 microseconds

288.68224 seconds

 

Notes:

 *        Not implemented
 a        Top 16 lines gray scale
 b        Top 8 lines gray scale
 c        Similar to original SSTV
 d        No Horizontal Sync (start of a new line)


SSTV - Experimental modes

Mode Time (sec) Pixel resolution Comments
MP73 73 320x256 Colour differentiation
MP115 115    320x256   Colour differentiation (in use)
MP140 140  320x256 Colour differentiation
MP175 175 320x256 Colour differentiation
MR73 73 320x256 Colour differentiation
MR90 90 320x256 Colour differentiation
MR115 115 320x256 Colour differentiation
 MR140  140 320x256  Colour differentiation
MR175 175  320x256 Colour differentiation
ML180 180  640x496 Colour differentiation
ML240 240 640x496 Colour differentiation
ML280 280 640x496 Colour differentiation
ML320 320 640x496 Colour differentiation


MP mode is based on the colour difference scheme that the PD mode uses.
This mode should be useful for reducing multi-path distortion and jitter.
The scan time for each pixel is longer than the other modes, but the
colour information along the vertical axis is compressed.

MR/ML mode is based on the colour difference scheme that the Robot 24/72 mode uses. This mode is aimed for offering good resolution in good conditions. The colour information along the horizontal axis is compressed.

These modes are not new to SSTV and are a compromise between the transmission time, picture size, and quality. These modes are in the experimental phase and are subject to alteration.
 

Frequencies

SSTV transmissions can be seen on the following frequencies: (List is not conclusive or official)

Band Frequency Sideband
80 meters 3.630 MHz kHz (AU) 3730 (Europe)    LSB
40 meters 7.033 MHz (International call )      LSB
30 meters 10.132 MHz (Region II) 10.144 (Region I) MP73-N mode - 10.132 (Region III)    USB
20 meters 14.230 MHz (International call ) 14.227, (14.236 MHz VK5ZW S&F SSTV repeater)    USB
15 meters 21.340 MHz (International call )    USB
10 meters 28.700 MHz (VK8NSB SSTV analogue store and forward repeater.)    USB
10 meters 28.680 MHz (International call )    USB


Data Snippets

1. 640 x 480 image has 4 times the resolution of a 320 x 240 picture and will require four times as much time to transmit. 

2. Colour images require from 2-3 times the time required for a gray scale image of the same resolution. 

3. The rate for transmission if the pixels in an image line cannot go much above 2000 pixels/second without exceeding the bandwidth permitted for SSTV operation. 

4. Some SSTV modes make take excessive time to complete and time out a repeater. (primarily UHF/VHF).

5. Slow Scan Analogue builds a picture "line by line" whereas the digital mode is decoded "block by block" which requires a much faster computer of at least 1Ghz, as decoding time depends on the speed of the CPU and RAM.
Digital (DSSTV) transmitted pictures do not require slant adjustment which is often necessary with analogue transmissions. SSTV slant adjustment help.

6. The standard mode used by European stations is MARTIN1. USA and JAPAN and Australia mainly use SCOTTIE1 as the standard. All other modes are not often used, only for short experiments! which were developed by Volker Wraase in Kiel, Germany. Martin was developed by Martin H. Emmerson, G3OQD/England. Scottie was developed by E.T.J. Murpy, GM3SBC/Scotland. Robot was developed by Robot Research.
 

SSTV Mode Report

In the SSTV mode, a report is given in the RSV form.

R = RADIO (from 1 to 5, 5 is the best)
S = SIGNAL STRENGTH (1 to 9)
V = VIDEO (from 1 to 5, 5 is the best)

Therefore when a good picture is received with signal strength of 9 a RSV 595 is given.



Approximate guide to frequency bands and ranges
 
Approximate distance Day Night
Less than 300 km 34 MHz 24 MHz
Between 300 km and 750 km 46 MHz 37 MHz
Between 750 km and 1,500 km 59 MHz 610 MHz
Between 1,500 km and 2,500 km 812 MHz 711 MHz
Between 2,500 km and 5,000 km 1018 MHz 815 MHz

  Band (meter) MHz Use*
HF 160 1.8 - 2.0 night
80 3.5 - 4.0 night and local day
40 7.0 - 7.3 night and local day
30 10.1 - 10.15 CW and digital
20 14.0 - 14.350 world wide day and night
17 18.068 - 18.168 world wide day and night
15 21.0 - 21.450 primarily a daytime band
12 24.890 - 24.990 primarily a daytime band
10 28.0 - 29.70 daytime during sunspot highs
VHF 6 50 - 54 local to world-wide
2 144 - 148 local and medium distance
UHF 70 cm 430 - 440 local

* It should be noted that band conditions vary for many reasons and thus all of these bands can at times take on the characteristics of others. See the section on Propagation. This table should be considered a general guideline. (http://www.eham.net)


Store and forward is a communications technique in which data is sent to an intermediate station where it is kept and sent subsequently to the final destination or to another intermediate station.

VK5ZW uses a single frequency of 14.236 MHz USB. An access tone of 1750 Hz is required for activation.

Operating hours are between 19:30 and 12:30 UTC 7 days a week. 


It is recommended that you have both FSKID Encode and Decode enabled on your software.
EncodeE FSKID - This will send a FSK signal with your call sign at the end of the image which can be seen in the call window on the receiving station and emplaced in template Macro commands.

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Website designed and maintained by Trevor VK2QW
Last updated 05/01/13

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