Editing Slow-scan television (section)

==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.
{{overlay
| image        = SSTV signal.jpg
| width        = 300
| height       = 197
| float        = right
| columns      = 
| grid         = 
| legend1title = {{small|A [[spectrogram]] of the beginning of an SSTV transmission}}
| legend1start = 1
| overlay1     = Calibration header
| overlay1left = 30
| overlay1top  = 70
| overlay2     = VIS code
| overlay2left = 50
| overlay2top  = 160
| overlay3     = RGB scanlines
| overlay3left = 125
| overlay3top  = 80
| overlay4     = Sync pulses
| overlay4left = 140
| overlay4top  = 150
}}

===Modulation===
Like the similar [[radiofax]] mode, SSTV is an [[analog signal]]. SSTV uses [[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 signal]].

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://users.rcn.com/sstv/modes.html  | title = SSTV Transmission Modes. | author = Langner, John | access-date = May 8, 2006 |archive-url = https://web.archive.org/web/20030216064120/http://users.rcn.com/sstv/modes.html |archive-date = February 16, 2003}}</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.

===Header===
A calibration header is sent before the image. It consists of a 300-millisecond leader tone at 1,900&nbsp;Hz, a 10&nbsp;ms break at 1,200&nbsp;Hz, another 300-millisecond leader tone at 1,900&nbsp;Hz, followed by a digital VIS (vertical interval signaling) code, identifying the transmission mode used. The VIS consists of [[bit]]s of 30 milliseconds in length. The code starts with a start bit at 1,200&nbsp;Hz, followed by 7 data bits ([[least significant bit|LSB]] first; 1,100&nbsp;Hz for 1, 1,300&nbsp;Hz for 0). An even [[parity bit]] follows, then a stop bit at 1,200&nbsp;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===
[[File:Moderni SSTV.jpg|thumb|left|Slow-scan test card]]
A transmission consists of horizontal [[line (video)|lines]], 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 an [[RGB color model]]; some modes are black-and-white, with only one channel being sent; other modes use a YC color model, which consists of [[luminance]] (Y) and [[chrominance]] (R–Y and B–Y). The modulating frequency changes between 1,500 and 2,300&nbsp;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 image [[aspect ratio (image)|aspect ratio]] is conventionally 4:3. Lines usually end in a 1,200&nbsp;Hz horizontal synchronization pulse of 5&nbsp;milliseconds (after all color components of the line have been sent); in some modes, the synchronization pulse lies in the middle of the line.

===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-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
|-
| rowspan=2 | Martin 
| rowspan=2 | Martin Emmerson - G3OQD
| 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 || [[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
|-
| rowspan="3" | Scottie
| rowspan="3" | Eddie Murphy - GM3SBC
| S1 || [[RGB]] || 110 s || 240¹
|-
| S2 || [[RGB]] || 71 s || 240¹
|-
|DX
|[[RGB]]
|269 s
|320 x 256
|}
<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-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.<ref name="QEX_Cordesses">{{cite web | url = http://lionel.cordesses.free.fr/gpages/sstv.html | title = "Some Thoughts on "Real-Time" SSTV Processing." | author = Cordesses, L. and R (F2DC) | work = QEX | year = 2003 | access-date = September 2, 2008 }}</ref>

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====
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 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.

==== Other modes<!-- VIS & VIS+P of PD50 & PD290 haven't been found -->====
{| class="wikitable"
! Family !! Developer !! Name !! Time [sec]!! Resolution !! Color !! VIS !! VIS+P
|-
| rowspan="7" |PD<ref>{{cite web|title=The development of the PD modes |url=https://www.classicsstv.com/pdmodes.php |last=Turner |first=Paul |access-date=2021-06-05}}</ref>
| rowspan="7" |Paul Turner, G4IJE<br>Don Rotier, K0HEO-[[Amateur radio operator#Silent Key|SK]]
|PD50
|50.000000
|320 x 256
| rowspan="7" |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
|
|
|}

===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-meter band|80 meters]] || 3.845&nbsp;MHz (3.73 in Europe) || LSB
|-
| [[Pirate radio in North America|43 meters]] || 6.925&nbsp;MHz (pirate radio) || USB
|-
| [[40-meter band|40 meters]] || 7.171&nbsp;MHz (7.165 in Europe) || LSB
|-
| [[40-meter band|40 meters]] || 7.181&nbsp;MHz (New suggested frequency to include General Class licensees) || LSB
|-
| [[40-meter band|40 meters]] || 7.214&nbsp;MHz Australian digital SSTV frequency (Easypal and DIGTRX) || LSB
|-
| [[20-meter band|20 meters]] || 14.23&nbsp;MHz Frequency 1 analog || USB
|-
| [[20-meter band|20 meters]] || 14.227 and 14.233&nbsp;MHz Frequency 2 analog to alleviate crowding on 14.23 || USB
|-
| [[15-meter band|15 meters]] || 21.34&nbsp;MHz || USB
|-
| [[11-meter band|11 meters]] || 27.700 international SSTV calling  +/- 30khz || USB
|-
| [[10-meter band|10 meters]] || 28.68&nbsp;MHz || USB

|}