\chapter{Computer operations} Edit


In one afternoon, an equipped hamradio operator can begin receiving SSTV. All that is needed is to make a connection cable between a transceiver and a sound card and download some SSTV related software. Tune into 14.230\,MHz USB for 24 hour-a-day SSTV activity.

Programs for SSTV operations are very similar and provides equivalent basic functions, of course with a different comfort. Some of them are intuitive and well-arranged, with another you need study manual. Everybody has possibility to choose from more variants and see what is best for him and what does provide requested functions. Every operator must know these basic functions:


  • \item to configure an accurate sampling frequency for reception and transmission,
  • \item to set proper volume levels of sound card,
  • \item to use tuning indicator,
  • \item manually change transmission mode,
  • \item to load and save images in common graphics formats,
  • \item to create image gallery for own transmission and
  • \item add text into transmitted images.


\section{Hardware configuration} Edit

Take a moment for the selection of a suitable configuration of your hamshack personal computer. The fact is that the requirements of an operating system and software are rising constantly based on the software. For SSTV operation, however, it is possible to use an obsolete PC with an old 386 processor, with {\em Hamcomm} modem and some of popular programs from the nineties running under DOS. In this case it's enough to use 4\,MB of RAM, a 100MB hard disk and an SVGA graphics card with 256 colors, but better results can be obtained with a graphics mode with 32 or 64 thousand color support

For the use of computer sound card it's sometimes better to use a better PC. The minimal configuration should be PC with 150MHz Pentium with 64\,MB of RAM at least, with few gigabytes hard disk. A graphics card needs to support 1024$\times$768 resolution and a mode with 64 thousands or 16.7 millions colors. The minimal operating system is Windows (version OSR\,2), but some new programs may not run under Win95. I recommend some better hardware but you don't need the latest mega-hyper model.

There are also software products for MacOS and for GNU/Linux, but the largest selection is for Microsoft Windows.

\section{Sound card as a modem} Edit

A sound card is standard equipment on PC nowadays. It can be used as a music player, for multimedia, games and recording. The main application of a sound card in hamshack is MODEM. The modem~-- MOdulator/DEModulator allows computer information to be transmitted and received over a physical media like radio waves or telephone lines. The modem translates analog signals to computer data and vice versa.

\subsection{Sound processing in PC} Edit

To allow the computer to work with a sound signal it must convert it into format suitable for data processing (digital/discrete signal). This is handled by the computer and software. The user must tune for a proper signal, make sure the timing settings for the sound card are correct and that the signal is getting to the sound card at a proper level. Too low and the image will be poor, too high and the sound card will overload. This will not harm the sound card but again the image will be poor.

\subsubsection{Sampling} Edit

In the first moment it is carry out a {\em sampling}. The sampling is activity which periodically scans current value of analog signal. This happens for example 11,025 times per second or it depends on sample rate configured by user and or supported by sound card. Sampling frequency of sound cards ranges from 8\,kHz suitable for internet telephony up to 96\,kHz designated for more exacting requirements of recording studios.

\placefigure[][none]{The conversion of an analog signal into numeric data.}

The answer to the question of what sampling frequency should be used gives us Shannon's theorem (also knows as Nyquist-Kotělnik-Shannon theorem). It defines that a signal continuous in time, containing spectral components with the highest frequency $f_{max}$, can be clearly reconstructed from a sequence evenly spaced samples with sampling frequency $f_s$ greater then double $f_{max}$:

$$f_{s} > 2f_{max}$$

The importance of Shannon's theorem you can see on following example. The signal in \in{figure}[pic:sampled]a expresses the dependency between time $t$ and amplitude $A$. We can find by using of Fourier's analyse (see \in{chapter}[bandwidth]), that the signal contains two harmonic components, showed in~\in[pic:sampled]b.

\placefigure[][pic:sampled]{Example of signal.} \startcombination[2*1] {\externalfigure[software/obr/signal-A.pdf]}{a) Surveyed signal} {\externalfigure[software/obr/signal-B.pdf]}{b) Spectral components} \stopcombination

By using Fourier's transformation the signal can be also expressed like a dependence between amplitude $A$ and frequency $f$~-- {\em signal spectrum}. There are evident both frequency components $f_1$ and $f_2=f_{max}$ of the signal in \in{fig.}[pic:spectrum].

\placefigure[][pic:spectrum]{The frequency spectrum of signal in \in{fig.}[pic:sampled]a} {\externalfigure[software/obr/spektrum.pdf]}

For explicit signal reconstruction there must be condition $f_{s} > 2f_{max}$ satisfied, see fig.~\in[pic:sampleok]. If a sampling frequency is lower then $2f_{max}$ then higher frequency components are lost. This error is called {\em aliasing}.

\placefigure[][pic:sampleok]{The signal sampled with frequency higher then $2f_{max}$} {\externalfigure[software/obr/signal-VZ.pdf]}

\placefigure[][pic:samplebad]{The sample rate doesn't meet the Shannon's theorem.} {\externalfigure[software/obr/signal-VZbad.pdf]}

For sampling of common narrow band signals like SSTV, RTTY, PSK31 or WEFAX transferred via SSB channel with bandwidth about 2\,500---3\,000\,Hz the sample rate 11,025\,Hz is enough.

\subsection{Analog-to-digital conversion} Edit

The next way of an analog signal continues to the analog-to-digital (A/D) converter. The current value of signal converted into digital data in this device. Some A/D converters works with a resolution 8 or 16 bits according to type or settings of a sound card. The resolution of A/D converter indicates the accuracy of signal amplitude scan in a defined range, for 8 bits it is $2^8=256$ values and for 16 it is 65,536.

Constrained resolution of A/D converter causes an {\em quantization} error. E.g. for 8 bit converter processing voltage range 0--5.0\,V it's the error $5.0/(2^8-1) \doteq 0.02$\,V. The 8bit converter cannon distinguish voltage levels lower then 0.02\,V. So for input voltage 3.111\,V it could find corresponding numeric value $10011110_2\,\approx 3.098$\,V or $10011111_2\,\approx 3.118$\,V because less significant bit is influenced by quantization error. The size of the error can be decreased by greater resolution of A/D converter. For our purposes the 16bit resolution is acceptable.

A modern sound cards could be equipped with digital signal processor (DSP), which extend card functions e.g. for filtering or data compression during recording so it can lighten load of main computer CPU. E.g. {\em Sound Blaster Live!} contains programmable DSP labeled {\em EMU10K1}.

The choice of sound card type depends only on user's preferences and his intends of use. Many PCs has integrated sound card directly on a motherboard.

\subsection{Interface between TRX and PC} Edit

The basic interface is made of shielded cables and 3.5mm jack plugs. A reception cable connects sound card input {\em Line In} and TRX headphones output or output for external speaker. For use of sound card microphone input can be used TRX output often labeled as {\ss AF OUT} with impedance about 10\,k$\Omega$ which gives max. output voltage 100\,mV. This output could be also used for interfacing tape deck or audio amplifier. Microphone input of sound card has automatic gain controller (AGC) for better recording and it is possible connect dynamics microphones with impedance from 600 to 10.000\,$\Omega$.

\placefigure[][none]{Basic interface between transceiver and sound card.} {\externalfigure[software/obr/sound_card.pdf][width=0.6\makeupwidth]}

For the transmission it is possible to use {\em Line Out} with impedance about 600\,$\Omega$. The {\em Line Out} can be connected to microphone input of TRX or a rear panel connector like {\ss PATCH IN}.

Some transceivers has a feature that microphone input and rear panel input are interconnected so it is necessary disconnect the microphone during AFSK transmission, because noise in hamshack could interfere with sound card signal! Check your TRX instruction manual for particular interfacing.

Last thing you need to set up is audio levels of received and transmitted signal. It can be made using operating system tools. The level of transmitted signal should be about 2/3 of max. level. The signal could not be too attenuated or over-excited and distorted. You can detect it by monitor of outgoing signals. For receiving signal you can set proper level directly on TRX and check the input level in your SSTV software.

\placefigure[][none]{Software volume control.} {\externalfigure[software/obr/hlas.png][width=.8\makeupwidth]}

After the audio mixer configuration it is useful to save your sound card settings (you can restore it every time before operations). A program {\em QuickMix} is can easily store your settings, because some other program can change it.

\URL{QuickMix V. 1.06}{}

\subsection{PTT control} Edit

The PTT (Push-To-Talk) button switch between reception and transmission. For its control there are several possibilities:

\startitemize[n] \item Manual PTT switching. This handy method is not very elegant, but for the first experiments can be used.

\item Automatic switching can provide TRX with a VOX feature, when the TRX is automatically keyed by signal in the input. An disadvantage of this method may be that its reaction is not immediate, so in case of digital modes the beginning states of transmission or SSTV vertical synchronization can be lost. Keep in ming that operating system often produce malicious sounds that could accidentally key the transmitter.

\item Automatic PTT switch can control a computer. All SSTV programs support PTT control over a simple serial port (COM, RS-232) circuit. The circuit contains one switching transistor or opto-isolator and few passive parts. See \in{schematic}[pic:trafo] for details. The control signal is connected to {\tt RTS} pin (7 at Cannon DB9 connector, 4 at DB25) or {\tt DTR} (4 at DB9, 20 at DB25), selected pin can be changed by software configuration. The ground is on serial port wired on pin 5 at DB9 or 7 at DB25.

The big amount of handheld TRXs has a similar pin for microphone input and PTT. In this case an audio signal should be galvanicaly separated by capacitor about 100\,nF and PTT signal is connected by resistor which resistance can by find in TRX instruction or you can connect trimming resistor about 15\,k$\Omega$ and test the max. value when TRX switching.

\item Some transceivers support control over serial port. This CAT (Computer Aided Transceiver) interface can provide PTT switching. Over CAT interface can be send commands e.g. for tuning, mode control, etc. This method must be supported by software, for example MixW can control some TRXes so it is not needed to practically touch the TRX buttons. \stopitemize

What to do if your computer is not equipped with serial port? Some motherboard manufacturers build only one serial port and notebook manufacturers doesn't provide any serial port. If this happens you can use VOX or obtain USB/RS232 interface. Some programs also support similar switching circuit as described before but on parallel port (LPT).

\subsection{Eliminate supply noise} Edit

A computer and a TRX can have slightly different electrical potential and in this case the direct connection causes annoying noise in communication channel. It is possible to remove noise witch galvanic separation of both devices. The path of audio signal should go throught galvanic transformer and PTT control switch with opto-isolator, e.g. 4N25, 4N33, etc. Maybe you will need to change {\ss R2} to lower value when the opto-isolator is not switched properly when serial port signal is on.

\placefigure[][pic:trafo]{The galvanic separation of transceiver and sound card.} {\externalfigure[software/obr/sound_if.pdf]}

\section{Timing oscillator configuration} Edit

There is description of synchronous (free-run) SSTV system in \in{section}[horsync]. Horizontal synchronization pulses (syncs) are detected only at the beginning of reception and after synchronization a reception device stops detect syncs and receive with free-run scan. Due to this there are excessive requirements for accurate timing of corresponding stations.

If the timing slightly differs then images are distorted~-- inaccurate timing causes image {\em slant}. You can see image slant for 0.01\,\% timing difference in \in{fig.}[pic:slant]. If a transmitting station has higher timing (and reception lower) the image slants to the right (\in[pic:slant]a) in opposite situation to the left (\in[pic:slant]b).

\placefigure[][pic:slant]{Image slant distortion when inaccurate timing is used for free-run modes.} \startcombination[2*1] {\externalfigure[software/obr/slant1.png][width=5cm]}{a) Opposite station has higher timing} {\externalfigure[software/obr/slant2.png][width=5cm]}{b) Opposite staion has lower timing} \stopcombination However timing derived from sample rate is not used to be exactly 11,025.00\,Hz, but often can differ up to few tenths of percent for each peace of hardware. For speech and music processing it doesn't matter, but in free-run transmission of SSTV it causes problems.

\warning{ The configuration of accurate timing/sample rate for reception and transmission apart must be done to meet the strict requirements for synchronous SSTV broadcast. Your signal must be acceptable for any SSTV device. }

All SSTV program are equipped with a tool for the timing configuration.

It is possible to receive the SSTV signal from any band and, by receivng an image, set the timing~-- the program will automatically compute any timing deviation. This way has a disadvantage, because not all SSTV stations have the proper transmit timing. This is caused by the offset between the receiving and transmitting timing.

A much more precise way is to use shortwave broadcasts with accurate timing signalsl. Programs are equipped with a special reception option, which displays spectrum in a second cycles. For timing setting just tune to the frequency of the broadcast and leave the received pulses on for several minutes. (End edit by N1YLE)

The usable transmitter is a Moscow station {\ss RWM} operating on frequencies 4,996.0, 9,996.0, 14,996.0\,kHz with 8\,kilowatts power. So it can be nicely received in Europe/Asia region. Select CW mode and tune your receiver directly to one of station frequencies. The unmodulated carrier is transmitted between 0. and 8. minute of an hour, telegraphy identification goes from 9. minute and then the timing signal will continue. Pulses in intervals 1/60 and 1\,Hz goes between 10. and 20. minute and 10Hz pulses goes between 20. and 30. minute. This is repeated every 30 minutes.

\placefigure[][none]{The configuration of accurate timing with {\ss RMW} reception in MMSSTV.} {\externalfigure[software/obr/rmw_nastaveni.png][width=.75\makeupwidth]}

The reception of {\ss WWV} station is the next possibility. This station broadcast timing pulses and announcement on frequencies 2,500.0, 5,000.0, 10,000.0, 15,000.0, 20,000.0\,kHz and uses double sideband (DSB) modulation. You can receive it with AM mode selected. The {\ss WWV} operates from the North America, Fort Collins in Colorado. The used power ranges from 2.5 to 10\,kW.

There is yet another way with WEFAX station reception, because these stations must have accurate timing too due to synchronous transfer.

The deviance error you should measure use to be expressed like absolute value of actual frequency, e.g. $f=11024{,}45$\,Hz or like deviance from $f_s$ the $\Delta_f=-0{,}55$\,Hz. Some program this measure in {\em parts per million (ppm)} unit. The ppm deviance can be computed:

$$\Delta = \frac{\Delta_f}{f_s}\cdot10^6.$$

For the $f=11024{,}45$\,Hz the deviance in ppm is:

$$\Delta =

\subsection{Transmit timing offset} Edit

There is necessity to configure transmit timing {\em TX offset} after the precise configuration of reception sample rate, when received SSTV images are not slanted. It is important for your own transmission, because inaccurate transmit timing causes image slant on reception side.

Some programs makes possible to monitor outgoing SSTV signals, so with this feedback it is practicable to check the TX offset~-- deviance between reception and transmission sample rate. The feedback can be internal or external. {\em Extenal feedback} needs to connect Line Out and Line In with cable and it requires a sound card with full-duplex mode enabled. By this way you can set {\em TX offset} precisely on your own.

The {\em internal feedback} doing almost the same, but no external cables is needed. But some sound cards support only software feedback, so you will find zero deviance, but it is not real fact! Then the TX offset setting must be done with external feedback or with opposite station help. You need to disable any automatic corrections of received signals in this way.

Anyway you need to make \uv{dry run} QSO before your first CQ. This helps you to uncover possible problems with TX offset, supply noise, audio level, etc.

The TX offset issue is often pretty messy. You can notice that some software running concurrent with your SSTV program can influence sound cart output and then the change of sample rate occurs. Even the simple Volume Control tool can do this. So it is useful to stop unnecessary program running in the background. Especially programs that can influence sound card output or decreases stability of Microsoft Windows.

You may notice a strange behaviour if you are user of modern sound card with full duplex mode enabled with several output channels with a support of different sample rates for each channel. This is for example {\em SB Live! Value}. I have noticed that my TX offset randomly changes! I have this experiences with SB Live! Value and I found that another radio amateurs has same. You can try to set other sample rate than 11,025.0\,Hz in this case, if this doesn't load your computer too much. For example try 48,000.0\,Hz, this value is fixed sample rate (see your card user's guide) and best results you can achieve with using of this value or its half or quarter~-- 24,000.0\,kHz or 12,000.0\,kHz. When you change this value you need to recalibrate your accurate timing again.

To avoid these problems you can constantly monitor the outgoing signal through the external feedback with the TRX monitor enabled and before your today first transmission you will check that everything is fine. It's unpleasant that problems often occurs during QSO and then you will stunned by counterpart replay images.

\section{SSTV tuning} Edit

First of all, we need to find SSTV stations by listening near calling frequencies. Thanks to typical SSTV sound and clattering of syncs it is not a problem to distinguish between SSTV and other communication modes.

Every SSTV program is equipped with precise tuning indicators~-- spectroscopes, see fig.~\in[pic:tuning]. The spectroscope shows frequency band from 1000\,Hz to 2500\,Hz with marks for critical frequncies~-- 1200\,Hz for syncs, 1500\,Hz and 2300\,Hz for the video signal.

Is is possible to simple detect band of video signal and syncs during clear reception. Rotate the tuning knob to achieve that all important frequencies are aligned in spectroscope display.

\placefigure[][pic:tuning]{Spectroscopes in common SSTV programs.}
{\externalfigure[software/obr/chromapix.png][width=.15\makeupwidth]}{Chroma Pix}

\section{Video digitalization} Edit

The video digitizer should be additional equipment of SSTV station. The device can convert output signal from camera into computer form. There is a great choice of many different devices with varying capabilities, parameters and price. You can choose some webcams, frame grabbers, TV cards or digital cameras. Then your broadcast will not be limited only to pre-prepared images and you will have a lot more fun with live transmission.

The cheapest option are web cameras, they are equipped with a low-resolution CCD and low-cost optics, but provided quality is suitable for SSTV.

An another option is a TV card with video input. This possibility is more expensive because you must connect an external camera where the choices ranges from cheap CCTV black and white or color CCD cameras up to professional studio equipment.

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