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3. Modern SSTV systems features -- part 1/2

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\chapter{Modern SSTV systems features} Edit


A milestone between old and modern SSTV image transmission is without doubts an usage of semiconductor memory chips. The first converters amongst fast and slow-scan television (SSTV converters) was created thanks to permanent image storage in memory. Due to it the image transmission could be more improved, because a major constraining fact the usage of long persistence CRTs was eliminated. Due to it some new formats with longer transmission time were developed. They brought more quality to black and white or color image transmission.

There was a trend in design of new formats to create several modes on each system. First the modes with faster transmission and lower resolution and such the modes for transfer better quality images but in longer time. There should be possible to change between them according on actual band condition.

Early phases of development was influenced by two companies~-- the American {\em Robot Research Inc.} and the German {\em Wraase Electronic\/} leaded by radio amateur Volker Wraase, DL2RZ. Each of them introduced a SSTV converter which use their own transmission system. These systems are different in used colour coding, scan line format and synchronization way. Their converters provide several modes. The {\em mode} denotes a format of image transmission, its resolution and transfer speed.

As often happens, the professional device should not fully comply to hamradio uses. So the new systems with more modes were implemented into converter's firmware. And their were reimplemented into other devices for ensuring of compatibility. Sometimes the new genuine system was designed for overcomes some imperfection of originals.

The number of those systems has grown unbelievably. Recently they were some new added for fully utilization of modern computers potential. Modern personal computers with the necessary equipment are full successors of SSTV converters. The advantage of computers is especially bigger memory and better image resolution.

If we count the amountr of all SSTV modes we find approximately the number 70! So SSTV image is possible to transfer by seventy different modes, which are mutually different in transmission time, resolution, colour coding, etc. In the vast majority they are absolutely unique and incompatible\ldots

You can get little scarred from previous lines, but I can reassure you, because only few modes are usually used.

The European amateurs were widely used SSTV mode called~{\em Martin M1}, but in recent time there are modes {\em Martin M2} and {\em Scottie S2} to be heard. The other used mode is special {\em Scottie DX}, it is characterized by very high image quality. The mode {\em Robot 36 Color} undertaken in the space communication.

Fortunately, all modern converters and computer software are able to operate with these popular modes, so the problem that two stations can not establish the QSO may not occur.

There will be described a digital vertical synchronization for automatic mode selection, because every mode uses digital header with its identification. Thanks to it any SSTV device can automatically switch the correct mode and starts the reception. Computer software also supports the mode detection by measuring the time elapsing between two successive sync. impulses of image line.

The details will be described in the next chapters.

\section{Signal modulation} Edit

\subsection[bandwidth]{Bandwidth} Edit

Some different communication channels, whether wired or wireless, have several characteristics. They define their behaviour in transfer of effective signal. These include for example the {\em attenuation }, the attenuation defines, how much the communication channel attenuate a transferred signal. Another important characteristic is the {\em bias}, the various distortions that occurs due to imperfections of communication path.

There are several negative influences, which affect signal transfer on any communication path. Their effect cannot be negligible. The intensity of this effect depends also on frequency of signal. Generally, it is always possible to identify a range of frequencies that a particular transmission path can transfer well and outside this frequency range the transmission is too poor.

The signal bandwidth does not depend only on the frequency range used for modulation, in our case 1,500\,Hz to 2,300\,Hz, but also on the signal spectrum.

The {\em Fourier analysis} is used to determine the spectrum bandwidth. The analysis can express any waveform in the form of the sum of a large number of sine waves~-- harmonic compoments.

Limited bandwidth has the effect that the harmonic components lying inside this band will be transferred more or less without blemish and another harmonic components pass with a huge distortion or not at all (more in~\in{chapter}[sampling], \at{page}[sampling]).

{\em Bandwidth} can be seen as characteristic of the transmission path given by the range of the signal spectrum.

The basic rule for the required bandwidth is called {\em Nuquist rate}. The definition is that optimal bandwidth equals a half of modulation speed. It is true that the necessary bandwidth increases with the amount of transferred information per time unit.

\subsection{Modulation techniques of analog SSTV} Edit

The SSTV broadcast to be carried out using single-sideband (SSB) amplitude modulation with common hamradio transceiver. Frequencies above 2,500\,Hz are strongly suppressed, so the frequency of white colour, the maximal level of SSTV signal, was chosen 2,300\,Hz.

SSTV signal are transmitted by frequency modulation of audio signal. For avoid any phase shift and drift (both has negative impact on picture quality) is the spectrum of video signal modulated on auxiliary carrier frequency 1900\,Hz~-- {\em subcarrier}. This modulation method is called {\em Sub-carrier frequency modulation (SFCM)}.

Frequency of video signal varies from black by gray shades to white. Bandwidth needed for SSTV transmission varies in the range 1.0 to 3.2\,kHz and depends on the SSTV mode, transmission speed and also on image content, see \in{fig.}[imgresol].

Cheap modems (based on Hamcomm) does not use perfect continuous harmonics signals, byt also creates the quantizated signal. Step changes between quantization levels require wider bandwidth, so some image details can get lost.

\placefigure[][fft]{The bandwidth for two images broadcasted in Martin M1 mode.}

Emission classification codes SSTV mode as {\em J3F}, meaning is:


  • \item {\em J}~-- Carrie modulation: Single-sideband with suppressed carrier.
  • \item {\em 3}~-- Nature of modulating signal: One channel containing analogue information.
  • \item {\em F}~-- Detail of signal: television signals.


In case of SSTV transmission by frequency modulated (FM) channel is emission classified as {\em F3F} and {\em A3F} for amplitude modulation (AM) with both side bands.

\section[imgresol]{Image resolution} Edit

\placefigure[][pic:araresol]{Image quality depends on resolution.}{

{\em Resolution} is a feature that tell what amount of details is possible to store in image, see \in[fig.]{pic:araresol}. The resolution has two parameters: horizontal resolution, the number of image columns~$\times$ number of image lines, the vertical resolution.

In television technology there is more important parameter the vertical resolution (number of lines) and it is given by SSTV mode selection. To get the horizontal resolution is more complicated.

As it's described in previous text, the image is broadcast throught SSB channel on short waves and the maximal bandwidth is limited.

The SSTV is {\em analog} mode and cannot transfer images without loss. The image is not exactly same on reception side as on trasmin side. Even it the transmission channel, respectively reception was without any interference or noise, then due to transmission speed and limited bandwidth is image distorted. The distortion is greater the faster the information is sent. Therefore it is very difficult to say what is the horizontal resolution of SSTV image.

The most of the modes carries images with 240 lines and the image is displayed in 4:3 aspect ratio on a screen. So we can say that the number of columns is $240\times4/3=320$. This valuethen corresponds to theoretical resolution, but not {\em real} image resolution.

The test chart (\in{fig.}[pic:resolution]) is used for qualify of image horizontal resolution. With the resolution pattern contains alternating stripes of black and white in some rasters from very rough to the fines. There is the comparison of this image with normal photography in \in{fig.}[pic:fft].

\placefigure[][pic:resolution]{Horizontal resolution comparison for several SSTV modes.}
	{\externalfigure[sstv/obr/resol/original.png][width=.4\makeupwidth]}{}	{Test pattern original}{}
	{\externalfigure[sstv/obr/resol/m2.png][width=.4\makeupwidth]}{} {Martin M2\hfill ($\sim$220\,$\mu$s)}{} 
	{\externalfigure[sstv/obr/resol/r36.png][width=.4\makeupwidth]}{} {Robot	36 Color\hfill ($\sim$280\,$\mu$s)}{}
	{\externalfigure[sstv/obr/resol/m1.png][width=.4\makeupwidth]}{} {Martin	M1\hfill ($\sim$450\,$\mu$s)}{}
	{\externalfigure[sstv/obr/resol/mp115.png][width=.4\makeupwidth]}{} {MP115\hfill ($\sim$680\,$\mu$s)}{}
	{\externalfigure[sstv/obr/resol/dx.png][width=.4\makeupwidth]}{} {Scottie DX\hfill ($\sim$980\,$\mu$s)}{}

All SSTV modes in \in{figure}[pic:resolution] have the number of columns 320, but as we can see that not all can transfer the image in such quality. See note in brackets, it says approximate time needed for transfer of one pixel. While the Martin M2 we can hardly distinguish second fines grid, the M1 mode with double transmission time it can transfer without problem, but the finest raster is biased, compare it on real picture in~\in{fig.}[pic:m1vsm2]. Another two modes with longer times of transmission can transfer the finest details. Unfortunately, it's pricey compensated by reduced speed of transmission.

\placefigure[][pic:m1vsm2]{The comparison of two modes in real conditions of 14\,MHz band.}
	{\externalfigure[sstv/obr/rozliseni_m1.png][width=.5\makeupwidth]}{Martin M1}
  	{\externalfigure[sstv/obr/rozliseni_m2.png][width=.5\makeupwidth]}{Martin M2}

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