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\chapter{Slow-scan television} Edit

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{\em Slow-scan television (SSTV)} is a type of communication mode designed for image transfer. Because SSTV is narrowband mode, it can be transmitted via voice channels with standard SSB transceiver on all radio amateur frequency bands. World-wide communication is possible during good conditions on high frequency bands.

\section{The beginnings} Edit

In 1957 a student at the University of Kentucky, Copthorne ``Cop Macdonals, WA2BCW (now VY2CM) found an article about a device developed&nbsp by Bell laboratories for image transmission via telephone lines. The communication system which needed a bandwidth as narrow as voice broadcast can be transfered by common ham radio transmitters.

There was available another image mode the radio fax (facsimile), but there was needed the long time (about 20 minutes) for a high resolution image transfer. Such a long time can't give the feeling of time consistency during a QSO and it also needed an intricate mechanical printer on electrosensitive paper. It was necessary to invent something else.

There was an idea to transfer images coded into audio signal and display them on long persistence displays (CRTs used in radars or slow-scan oscilloscopes).

Then Copthorne started to work on how to transfer images via radio waves with common ham radio transceivers. Within six month he carried out many experiments with amplitude and frequency modulation and it resulted in the design of slow-scan television. During the next six months he created an SSTV image scanner, so practical experiments could be done on the ham bands. The first television image crossed the Atlantic on the 20th December 1959.

\placefigure[][]{Early slow-scan television images.}
\startcombination[2*1]
{\externalfigure[sstv/obr/obrazek1.png][width=0.4\makeupwidth]}{The first image crossed the Atlantic, received by John Plowman G3AST.}
{\externalfigure[sstv/obr/copthorn.png][width=0.4\makeupwidth]}{Copthorne Macdonald's broadcast.}
\stopcombination

During the next ten years Copthorne and a group of amateurs worked on SSTV improvements and they created the basic standard for SSTV and developed a sampling camera.

Their work was completed in 1968 when the FCC (Federal Communications Commission) formally authorized the SSTV operation.

A few months later ham radio magazines brought the first articles about the new communication mode. It led to a huge interest by ham operators and a real SSTV boom.

\section{Image transmission} Edit

The basic idea of SSTV is to transfer a television image with a standard transceiver, however a television broadcast requires a large bandwidth. The reduction of the television signal is achieved by lowering of horizontal (row) and vertical (image) scans, which must be reduced to a minimal frequency. This means that a typical 3MHz signal of black and white television must be reduced to 3\,kHz~-- reduction of bandwidth in around 1000\,:\,1. Nowadays the bandwidth reduction is bigger, because color images needs approx. 6\,MHz. Only static images with lower resolution can be transfered due to the great bandwidth reduction.

During experiments was found that the image was visible about 8 second on a long persistence CRT with P7 phosphor, so after reception of the last scan-line the first is still visible, but int a while the image slowly disappeared. For the best impression it was necessary to view SSTV monitor in a darkened room. Usually several of the same images were transmitted in a row. Each following image slowly redrew the original still visible on phosphor. So it was possible to display image for longer times or record it on a tape for later playback.

It was found that the ideal time of line synchronization pulses for correct detection by electronic circuits is 5\,ms and for image (vertical) synchronization it is 30\,ms. The vertical sync is for automatic start of monitor displaying.

The synchronization frequency for scan-lines and frames was derived from the electric mains frequency. For horizontal sync is used 50\,Hz divided by three~-- 16.6\,Hz and for vertical scan $1/7.2\,\rm{s}=0.1388$\,Hz, it is mains frequency divided by 360 (3$\times$ number of lines 120). The parameters are derived in same way for countries with 60\,Hz mains.

The video signal band was chosen in the range from 1500\,Hz for black up to 2300\,Hz for white. Sync pulses have a frequency of 1200\,Hz and because they are ``blacker than black then they do not affect the image information.

All frequency components of SSTV are insidea low-frequency band and it is possible to transfer them via voice channels.

Other SSTV modes came out from this original standard and in most casesthey differ only in scan speed and the addition of colour transmission.

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