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-[[Image:SSTV reception.png|thumb|right|SSTV transmissions often include station [[call sign]]s, [[RST code|RST]] reception reports, and [[radio amateur]] jargon.]][[Image:SSTV sunset audio -ogg-.jpg|thumb|right|A Spectral Analysis of the SSTV audio file later in this page, created with Cool Edit 2.0]]
-'''Slow-scan television''' (SSTV) is a picture transmission method used mainly by [[amateur radio]] operators, to transmit and receive static pictures via radio in [[monochrome]] or color.
-A technical term for SSTV is [[narrowband television]]. [[Broadcasting|Broadcast]] television requires huge 5, 6 or 8 MHz wide channels, because it transmits 25 or 30 pictures per second (in the [[NTSC]], [[PAL]] or [[SECAM]] systems), but SSTV usually takes up to only 3&nbsp;kHz of [[Bandwidth (signal processing)|bandwidth]]. It is a much slower method of still picture transmission, usually lasting from about eight seconds to a couple of minutes.
-Since SSTV systems operate on [[voice frequency|voice frequencies]], amateurs use it on [[shortwave]] (also known as [[HF]] by [[amateur radio]] operators), [[very high frequency|VHF]] and [[ultra high frequency|UHF]] radio.
-==History==
-===Concept===
-The concept of SSTV was introduced by Copthorn Macdonald in 1957&ndash;1958<ref name="Miller">{{cite web | url = http://www.darc.de/distrikte/g/T_ATV/sstv-history.htm | title = "SSTV history." | author = Miller, Don | accessdate = May 9 | accessyear = 2006 }}</ref>. He developed the first SSTV system using an electrostatic monitor and a [[video camera tube#Vidicon|vidicon tube]]. Those days it seemed sufficient to use 120 lines and about 120 pixels per line to transmit a black-and-white still picture within a 3 kHz phone channel. First live tests were performed on the 11 Meter ham band - which was later given to the CB service in the US.
-=== Early usage in space exploration ===
-[[Image:S63-07856.jpg|thumb|Astronaut Gordon Cooper, SSTV broadcast from Faith 7]]
-SSTV was used to transmit images of the far side of the Moon from [[Luna 3]] <ref>[http://astrosurf.com/nunes/explor/explor_luna3.htm Luna 3<!-- Bot generated title -->]</ref>.
-The first space television system was called [[Seliger-Tral-D]] and was used aboard [[Vostok]]. Vostok was based on an earlier [[videophone]] project, it used two cameras, with persistent LI-23 [[iconoscope]] tubes. Its output was 10 frames per second at 100 lines per frame video signal.
-* The Seliger system was tested during the 1960 launches of the [[Vostok]] capsule, including [[Sputnik 5]], containing the space dogs [[Belka]] and [[Strelka]], whose images are often mistaken for the dog [[Laika]] and the 1961 flight of [[Yuri Gagarin]], the first man in space on [[Vostok 1]].
-* [[Vostok 2]] and thereafter used an improved 400-line television system referred to as [[Topaz]].
-* A second generation system ([[Krechet]], incorporating docking views, overlay of docking data, etc) was introduced after 1975.
-A similar concept, also named '''SSTV''', was used on [[Mercury-Atlas 9|Faith 7]] as well as on the early years of the [[NASA]] [[Project Apollo|Apollo]] program. The Faith 7 camera transmitted one frame every two seconds.
-SSTV was used to transmit images from inside [[Apollo 7]], [[Apollo 8]], and [[Apollo 9]], as well as the [[Apollo 11]] [[Apollo Lunar Module|Lunar Module]] television from the [[Moon]], see [[Apollo TV camera]].
-* The SSTV system used in [[NASA]]'s early Apollo missions transferred ten frames per second with a resolution of 320 frame lines using less bandwidth than a normal TV transmission.
-* The early SSTV systems used by NASA differ significantly from the SSTV systems currently in use by amateur radio enthusiasts today.
-===Evolution===
-Commercial systems started appearing in the United States in 1970, after the [[Federal Communications Commission|FCC]] had legalized the use of SSTV for [[Amateur radio licensing in the United States|advanced 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 characteristic [[sound reproduction|audio]] howl, and a [[cathode ray tube]] from a surplus [[radar]] set. The special cathode ray tube would have "long persistence" [[phosphor]]s that would keep a picture visible for about ten seconds.
-The [[modem]] would generate audio tones between 1200 and 2300 Hz from picture signals, and picture signals from received audio tones. The audio would be attached to a radio [[receiver (radio)|receiver]] and [[transmitter]].
-==Current systems==
-A modern system, having gained ground since the early 1990s, uses a [[personal computer]] and special [[software]] in place of much of the custom equipment. The [[sound card]] of a PC, with special processing software, acts as a modem. The [[computer screen]] provides the output. A small [[digital camera]] or digital photos provide the input.
-[[Image:SSTV signal.jpg|thumb|220px|right|An SSTV signal begins with a digital mode identifier and then starts scanning between 1500 and 2300 Hz, with regular 1200 Hz clicks.]]
-===Modulation===
-SSTV uses analogue [[frequency modulation]], in which every different value of [[brightness]] 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 an [[single-sideband modulation|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)<ref name="Langner">{{cite web | url = http://web.archive.org/web/20030216064120/http://users.rcn.com/sstv/modes.html | title = "SSTV Transmission Modes." | author = Langner, John | accessdate = May 8 | accessyear = 2006 }}</ref>. 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.
-===VIS code===
-[[Image:VIS code.svg|350px|right]]A [[digital]] ''VIS'' (vertical interval signaling) code can be sent before the image, identifying the transmission mode used. It consists of [[bit]]s of 30 milliseconds in length. The code starts with a start bit at 1200 Hz, followed by 7 data bits ([[least significant bit|LSB]] first; 1100 Hz for 1, 1300 Hz for 0). An even [[parity bit]] follows, then a stop bit at 1200 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.
-===Scanlines===
-A transmission consists of horizontal [[line (video)|lines]], scanned from left to right. The [[RGB color model|RGB]] color 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 (image)|aspect ratio]] is conventionally 4:3). Lines end in a 1200 Hz horizontal synchronization pulse of 5 milliseconds (after all color components of the line have been sent).
-===Modes===
-Below is a table of some of the most common SSTV modes and their differences<ref name="Langner"/>. 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.
-{| class="wikitable"
-! Family !! Developer !! Name !! Color !! Time !! Lines
-|-
-| rowspan=8 | AVT
-| rowspan=8 | Ben Blish / 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
-|-
-| rowspan=2 | Martin
-| rowspan=2 | Martin Emmerson
-| M1 || RGB || 114 s || 240¹
-|-
-| M2 || RGB || 58 s || 240¹
-|-
-| rowspan=6 | Robot
-| rowspan=6 | Robot SSTV
-| 8 || BW or 1 of R, G or B || 8 s || 120
-|-
-| 12 || YC || 12 s || 128 luma, 32/32 chroma × 120
-|-
-| 24 || YC || 24 s || 128 luma, 64/64 chroma × 120
-|-
-| 32 || BW or 1 of R, G or B || 32 s || 256 × 240
-|-
-| 36 || YC || 36 s || 256 luma, 64/64 chroma × 240
-|-
-| 72 || YC || 72 s || 256 luma, 128/128 chroma × 240
-|-
-| rowspan=2 | Scottie
-| rowspan=2 | Eddie Murphy
-| S1 || RGB || 110 s || 240¹
-|-
-| S2 || RGB || 71 s || 240¹
-|}
-<div style="font-size: 80%">¹ Martin and Scottie modes actually send 256 scanlines, but the first 16 are usually grayscale.</div>
-The mode family called AVT (for ''Amiga Video Transceiver'') was originally designed by Ben Blish (N4EJI, then AA7AS) for a custom modem attached to an Amiga computer, which was eventually marketed by AEA corporation.
-The Scotty and Martin modes were originally implemented as ROM enhancements for the Robot corporation SSTV unit.
-The Robot SSTV modes were designed by Robot corporation for their own SSTV unit.
-All four sets of SSTV modes are now available in various PC-resident SSTV systems and no longer depend upon the original hardware.
-==== AVT ====
-AVT is an abbreviation of "Amiga Video Transceiver", software and hardware modem originally developed by "Black Belt Systems" (USA) around 1990 for the [[Amiga]] home computer popular all over the world before the [[IBM PC compatible|IBM PC]] family gained sufficient audio quality with the help of special [[sound card]]s.
-These AVT modes differ radically from the other modes mentioned above, in that 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. Older computers sometimes needed to do this in order to make up for an inability to precisely match the synchronous timing of the frame over long periods.
-The AVT modes are mainly used in [[Japan]] and the [[USA]]. 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.
-===Frequencies===
-Using a receiver capable of demodulating [[single-sideband modulation]], SSTV transmissions can be heard on the following frequencies:
-{| class="wikitable"
-! Band !! Frequency !! Sideband
-|-
-| [[80 meters]] || 3845 kHz (3730 in Europe) || LSB
-|-
-| [[40 meters]] || 7170 kHz (7043 in Europe) || LSB
-|-
-| [[20 meters]] || 14230 kHz || USB
-|-
-| [[15 meters]] || 21340 kHz || USB
-|-
-| [[10 meters]] || 28680 kHz || USB
-|}
-==Media==
-{{Listen|filename=SSTV sunset audio.ogg|title=A sample SSTV transmission|description=An image of a sunset sent as Martin M1.|format=[[Ogg]]}}
-== Interference ==
-SSTV is very susceptible to interference, especially when transmitted over USB or LSB.
-==See also==
-*[[Amateur television]]
-*[[Hellschreiber]]
-*[[Narrow-bandwidth television]]
-*[[Radiofax]]
-*[[Radioteletype]]
-*[[Shortwave]]
-*[[SSTV repeater]]
-==References==
-<div class="references-small">
-*Glidden, Ramon (September 1997). [http://www.arrl.org/tis/info/pdf/99753.pdf "Getting Started With Slow Scan Television."] ''QST''. Accessed on [[April 28]], 2005.
-*[http://cancerweb.ncl.ac.uk/cgi-bin/omd?slow+scan "Slow scan definition."] ''On-line Medical Dictionary''. Accessed on [[April 28]], 2005.
-*Turner, Jeremy (December 2003). [http://www.openspace.ca/outerspace/TavFalcoInterview2003.html "07: Interview With Tav Falco About Early Telematic Art at Televista in Memphis, New Center for Art Activities in New York and Open Space Gallery in Victoria, Canada."] ''Outer Space: The Past, Present and Future of Telematic Art''. Accessed on [[April 28]], 2005.
-*Sarkissian, John. [http://www.parkes.atnf.csiro.au/apollo11/tv_from_moon.html Television from the Moon]. The Parkes Observatory's Support of the Apollo 11 Mission. Latest Update: 21 October 2005.
-</div>
-===Notes===
-{{reflist}}
-==External links==
-* [http://ae6jn.dyndns.org/wwsstv.html Gallery] of SSTV [[webcam]]s around the world
-'''Modem software:'''
-* [http://mmhamsoft.amateur-radio.ca/mmsstv/ MMSSTV] for [[Microsoft Windows]]
-* [http://users.telenet.be/on4qz/ QSSTV] for [[Linux]]
-{{Video formats}}
-[[Category:amateur radio]]
-[[Category:radio modulation modes]]
-[[Category:television technology]]
-[[cs:Slow-scan television]]
-[[de:Slow Scan Television]]
-[[fr:Télévision à balayage lent]]
-[[nl:Slow Scan Televisie]]

Revision as of 01:37, 2 July 2008