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Slow-scan television (SSTV) is a picture transmission method, used mainly byamateur radio operators, to transmit and receive static pictures via radio inmonochrome or color.
A literal term for SSTV isnarrowband television. Analogbroadcast television requires at least 6 MHz wide channels, because it transmits 25 or 30 picture frames per second (seeITU analog broadcast standards), but SSTV usually only takes up to a maximum of 3 kHz ofbandwidth. It is a much slower method of still picture transmission, usually taking from about eight seconds to a couple of minutes, depending on the mode used, to transmit one image frame.
Since SSTV systems operate onvoice frequencies, amateurs use it onshortwave (also known asHF byamateur radio operators),VHF andUHF radio.
The concept of SSTV was introduced by Copthorne Macdonald[1] in 1957–58.[2] He developed the first SSTV system using an electrostatic monitor and avidicon tube. It was deemed sufficient to use 120 lines and about 120 pixels per line to transmit a black-and-white still picture within a 3 kHz telephone channel. First live tests were performed on the 11-meter ham band – which was later given to theCB service in the US. In the 1970s, two forms of paper printout receivers were invented byhams.
SSTV was used to transmit images of the far side of the Moon fromLuna 3.[3]
The first space television system was called Seliger-Tral-D and was used aboardVostok. Vostok was based on an earliervideophone project which used two cameras, with persistent LI-23iconoscope tubes. Its output was 10 frames per second at 100 lines per frame video signal.
A similar concept, also namedSSTV, was used onFaith 7,[4] as well as on the early years of theNASAApollo program.
TheApollo TV cameras used SSTV to transmit images from insideApollo 7,Apollo 8, andApollo 9, as well as theApollo 11Lunar Module television from theMoon. NASA had taken all the original tapes and erased them for use on subsequent missions; however, theApollo 11 Tape Search and Restoration Team formed in 2003 tracked down the highest-quality films among the converted recordings of the first broadcast, pieced together the best parts, then contracted a specialistfilm restoration company to enhance the degraded black-and-white film and convert it intodigital format forarchival records.[5]
Commercial systems started appearing in the United States in 1970, after theFCC had legalized the use of SSTV foradvanced level amateur radio operators in 1968.
SSTV originally required quite a bit of specialized equipment. Usually there was a scanner or camera, a modem to create and receive the characteristicaudio howl, and acathode-ray tube from a surplusradar set. The special cathode-ray tube would have "long persistence"phosphors that would keep a picture visible for about ten seconds.
Themodem would generate audio tones between 1,200 and 2,300 Hz from picture signals, and picture signals from received audio tones. The audio would be attached to a radioreceiver andtransmitter.
A modern system, having gained ground since the early 1990s, uses apersonal computer and specialsoftware in place of much of the custom equipment. Thesound card of a PC, with special processing software, acts as amodem. Thecomputer screen provides the output. A smalldigital camera or digital photos provide the input.
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Aspectrogram of the beginning of an SSTV transmission
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Like the similarradiofax mode, SSTV is ananalog signal. SSTV usesfrequency modulation, in which every different value ofbrightness in the image gets a different audio frequency. In other words, the signal frequency shifts up or down to designate brighter or darker pixels, respectively. Color is achieved by sending the brightness of each color component (usually red, green and blue) separately. This signal can be fed into anSSB transmitter, which in part modulates thecarrier signal.
There are a number of different modes of transmission, but the most common ones areMartin M1 (popular in Europe) andScottie S1 (used mostly in the USA).[7] 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.
A calibration header is sent before the image. It consists of a 300-millisecond leader tone at 1,900 Hz, a 10 ms break at 1,200 Hz, another 300-millisecond leader tone at 1,900 Hz, followed by a digital VIS (vertical interval signaling) code, identifying the transmission mode used. The VIS consists ofbits of 30 milliseconds in length. The code starts with a start bit at 1,200 Hz, followed by 7 data bits (LSB first; 1,100 Hz for 1, 1,300 Hz for 0). An evenparity bit follows, then a stop bit at 1,200 Hz. For example, the bits corresponding the decimal numbers 44 or 32 imply that the mode is Martin M1, whereas the number 60 represents Scottie S1.
A transmission consists of horizontallines, scanned from left to right. The color components are sent separately one line after another. The color encoding and order of transmission can vary between modes. Most modes use anRGB color model; some modes are black-and-white, with only one channel being sent; other modes use a YC color model, which consists ofluminance (Y) andchrominance (R–Y and B–Y). The modulating frequency changes between 1,500 and 2,300 Hz, corresponding to the intensity (brightness) of the color component. The modulation is analog, so even though the horizontal resolution is often defined as 256 or 320 pixels, they can be sampled using any rate. The imageaspect ratio is conventionally 4:3. Lines usually end in a 1,200 Hz horizontal synchronization pulse of 5 milliseconds (after all color components of the line have been sent); in some modes, the synchronization pulse lies in the middle of the line.
Below is a table of some of the most common SSTV modes and their differences.[7] These modes share many properties, such as synchronization and/or frequencies and grey/color 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.
| Family | Developer | Name | Color | Time | Lines |
|---|---|---|---|---|---|
| AVT | Ben Blish-Williams, AA7AS / AEA | 8 | BW or 1 of R, G, or B | 8 s | 128×128 |
| 16w | BW or 1 of R, G, or B | 16 s | 256×128 | ||
| 16h | BW or 1 of R, G, or B | 16 s | 128×256 | ||
| 32 | BW or 1 of R, G, or B | 32 s | 256×256 | ||
| 24 | RGB | 24 s | 128×128 | ||
| 48w | RGB | 48 s | 256×128 | ||
| 48h | RGB | 48 s | 128×256 | ||
| 104 | RGB | 96 s | 256×256 | ||
| Martin | Martin Emmerson - G3OQD | M1 | RGB | 114 s | 240¹ |
| M2 | RGB | 58 s | 240¹ | ||
| Robot | Robot SSTV | 8 | BW or 1 of R, G or B | 8 s | 120 |
| 12 | YUV | 12 s | 128 luma, 32/32 chroma × 120 | ||
| 24 | YUV | 24 s | 128 luma, 64/64 chroma × 120 | ||
| 32 | BW or 1 of R, G or B | 32 s | 256 × 240 | ||
| 36 | YUV | 36 s | 256 luma, 64/64 chroma × 240 | ||
| 72 | YUV | 72 s | 256 luma, 128/128 chroma × 240 | ||
| Scottie | Eddie Murphy - GM3SBC | S1 | RGB | 110 s | 240¹ |
| S2 | RGB | 71 s | 240¹ | ||
| DX | RGB | 269 s | 320 x 256 |
The mode family called AVT (forAmiga Video Transceiver) was originally designed by Ben Blish-Williams (N4EJI, then AA7AS) for a custom modem attached to an Amiga computer, which was eventually marketed by AEA corporation.
The Scottie and Martin modes were originally implemented as ROM enhancements for the Robot Research Corporation SSTV unit. The exact line timings for the Martin M1 mode are given in this reference.[8]
The Robot SSTV modes were designed by Robot Research Corporation for their own SSTV units.
All four sets of SSTV modes are now available in various PC-resident SSTV systems and no longer depend upon the original hardware.
AVT is an abbreviation of "Amiga Video Transceiver", software and hardware modem originally developed by "Black Belt Systems" (USA) around 1990 for theAmiga home computer popular all over the world before theIBM PC family gained sufficient audio quality with the help of specialsound cards. These AVT modes differ radically from the other modes mentioned above, in that they are synchronous, that is, they have no per-line horizontal synchronization pulse but instead use the standard VIS vertical signal to identify the mode, followed by a frame-leading digital pulse train which pre-aligns the frame timing by counting first one way and then the other, allowing the pulse train to be locked in time at any single point out of 32 where it can be resolved or demodulated successfully, after which they send the actual image data, in a fully synchronous and typically interlaced mode.
Interlace, no dependence upon sync, and interline reconstruction gives the AVT modes a better noise resistance than any of the other SSTV modes. Full frame images can be reconstructed with reduced resolution even if as much as 1/2 of the received signal was lost in a solid block of interference or fade because of the interlace feature. For instance, first the odd lines are sent, then the even lines. If a block of odd lines are lost, the even lines remain, and a reasonable reconstruction of the odd lines can be created by a simple vertical interpolation, resulting in a full frame of lines where the even lines are unaffected, the good odd lines are present, and the bad odd lines have been replaced with an interpolation. This is a significant visual improvement over losing a non-recoverable contiguous block of lines in a non-interlaced transmission mode. Interlace is an optional mode variation, however without it, much of the noise resistance is sacrificed, although the synchronous character of the transmission ensures that intermittent signal loss does not cause loss of the entire image. The AVT modes are mainly used in Japan and the United States. There is a full set of them in terms of black and white, color, and scan line counts of 128 and 256. Color bars and greyscale bars may be optionally overlaid top and/or bottom, but the full frame is available for image data unless the operator chooses otherwise. For receiving systems where timing was not aligned with the incoming image's timing, the AVT system provided for post-receive re-timing and alignment.
| Family | Developer | Name | Time [sec] | Resolution | Color | VIS | VIS+P |
|---|---|---|---|---|---|---|---|
| PD[9] | Paul Turner, G4IJE Don Rotier, K0HEO-SK | PD50 | 50.000000 | 320 x 256 | G, R-Y, B-Y | ||
| PD90 | 89.989120 | 320 x 256 | 99 | 99 | |||
| PD120 | 126.103040 | 640 x 496 | 95 | 95 | |||
| PD160 | 160.883200 | 512 x 400 | 98 | 226 | |||
| PD180 | 187.051520 | 640 x 496 | 96 | 96 | |||
| PD240 | 248.000000 | 640 x 496 | 97 | 225 | |||
| PD290 | 289.000000 | 800 x 616 |
Using a receiver capable of demodulatingsingle-sideband modulation, SSTV transmissions can be heard on the following frequencies:
| Band | Frequency | Sideband |
|---|---|---|
| 80 meters | 3.845 MHz (3.73 in Europe) | LSB |
| 43 meters | 6.925 MHz (pirate radio) | USB |
| 40 meters | 7.171 MHz (7.165 in Europe) | LSB |
| 40 meters | 7.181 MHz (New suggested frequency to include General Class licensees) | LSB |
| 40 meters | 7.214 MHz Australian digital SSTV frequency (Easypal and DIGTRX) | LSB |
| 20 meters | 14.23 MHz Frequency 1 analog | USB |
| 20 meters | 14.227 and 14.233 MHz Frequency 2 analog to alleviate crowding on 14.23 | USB |
| 15 meters | 21.34 MHz | USB |
| 11 meters | 27.700 international SSTV calling +/- 30khz | USB |
| 10 meters | 28.68 MHz | USB |
| External videos | |
|---|---|
 Video showing images and the sound generated when sending them as SSTV audio. onYouTube |
In Valve's 2007 video gamePortal, there was an internet update of the program files on 3 March 2010. This update gave a challenge to find hidden radios in each test chamber and bring them to certain spots to receive hidden signals. The hidden signals became part of anARG-style analysis by fans of the game hinting at a sequel of the game – some sounds were ofMorse code strings that implied the restarting of a computer system, while others could be decoded as purposefully low-quality SSTV images. When some of these decoded images were put together in the correct order, it revealed a decodable MD5 hash for abulletin-board system phone number (425)822-5251. It provides multipleASCII art images relating to the game and its potential sequel.[10][11][12] The sequel,Portal 2, was later confirmed. According to a hidden commentary node SSTV image fromPortal 2, the BBS is running from a Linux-based computer and is linked to a 2,400 bit/s modem from 1987. It is hooked up in an unspecified Valve developer's kitchen. They kept spare modems in case one failed, and one did. The BBS only sends about 20 megabytes of data in total.
In the aforementioned sequel,Portal 2, there are four SSTV images. One is broadcast in a Rattman den. When decoded, this image is a very subtle hint towards the game's ending. The image is of a Weighted Companion Cube on the Moon. The other three images are decoded from a commentary node in another Rattman den. These 3 images are slides with bullet points on how the ARG was done, and what the outcome was, such as how long it took the combined internet to solve the puzzle (the average completion time was 71⁄2 hours).[13]
In another video game,Kerbal Space Program, there is a small hill in the southern hemisphere on the planet "Duna", which transmits a color SSTV image in Robot 24 format. It depicts four astronauts standing next to what is either the Lunar Lander from the Apollo missions, or an unfinished pyramid. Above them is the game's logo and three circles.[14] It emits sound if an object is near the hill.[citation needed] As of the latest version of the game (1.12), the hill no longer transmits the signal.[15]
Caparezza, an Italian songwriter, inserted an image on theghost track of his albumPrisoner 709.
TheAphex Twin release2 Remixes by AFX contains a track that displays an SSTV image that has text about the programs used to make the release as well as a picture of Richard sitting on a couch.
SYDNEY: After a three-year search for the lost Apollo 11 tapes and an exhaustive six-year restoration project, digitally remastered footage of the historic Moonwalk is almost ready to be broadcast.
{{cite web}}: CS1 maint: numeric names: authors list (link)
World Ham/11mtr SSTV cams -https://worldsstv.com/
Modem software:
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