Slow Scan Television - The Modern Way
Slow Scan Television has never been more popular than it is
at present, and it continues to gain more recruits every day. This is a
fascinating aspect of our hobby, both for the short wave listener and the
licensed operator. This interest is probably due in part, to the advances in
technology and computer software, combined with reports of excellent
pictures which can be received from exotic places all over the world. Reports
of excellent pictures direct from the MIR space craft have also heightened the
general interest.
When I was a short wave listener, Top Band or 160 meters was where my appetite was whetted for becoming licensed. I still remember taking my accumulator along to the local iron monger to have it charged, so that I could listen to top band on our normal broadcast receiver! These days the listener probably spends as much time in front of a computer as actually listening on the bands. Some may think that this is a bad thing, but our hobby has to move with the times if it is going to attract the youth of tomorrow and computers are becoming more popular every day. Having said that, it is not only the younger generation that find slow scan appealing. When I wrote an article for RadCom on slow scan that was published in April 1996, I received many letters from all age groups, including two gentlemen in their eighties. They wanted to know what type of computer to go out and buy! The aim of this article will be to demonstrate how easy it is to get going on slow scan, whether it be to simply receive pictures, or if you have worked for that license, to be able to send pictures as well. I will highlight the progress that has been made, both to the hardware and software, and give an indication of where things can go wrong. I have given talks to many of the local amateur radio clubs on the subject and can thus anticipate many of the things that you may wish to know.
So what is involved? These days, if you have a
modern computer and a short wave receiver, you can be receiving images within
ten minutes of loading the software.
That is not an exaggeration, as if your computer has a sound card, this eliminates the need for an interface and all that is required is a screened lead form the receiver head phone socket to the line input of the sound card. Obviously if you want to send as well as receive there is another lead required from the sound card output to the mic. input socket on the transceiver. Even with an older computer it should only take an evening to build an interface, or indeed, these can be bought from most amateur equipment retailers for about twenty-five pounds. I will discuss how to wire up the equipment in more detail later, as this is very important if you intend to transmit. We will also look at the software required, which will be compatible with your computer.
Let us assume first that you do not have a
computer and are looking for the most economical method for receiving images
off air. The minimum hardware I would recommend would be an IBM clone, with a
386 processor running DOS. You would want a good video card capable of
displaying at least 32,000 colours and a 14 inch VGA monitor. It should be
possible to run some of the software with a 286 processor and a basic video
card. As you should be able to pick up a second hand 386 for around 100 pounds,
it hardly seems worth buying something, that you will almost certainly want to
upgrade later. You would require to build or buy your own interface and,
circuits are included with most of the software programmes that require an
interface. The cost to build your own interface would be around 5 pounds. Of
course, if you already have a basic computer that meets the above
specification, then your work will be limited to building or buying the
interface, along with the interconnecting cables. The simple HAMCOM type
interface is all that is required.
If you want to set your sights a bit higher you may wish to look for something that will run the best software available and a 100 MHz or faster Pentium, running Windows 95, or 98, will allow you to do this, providing it is fitted with a 16 bit sound card. A reasonable specification computer these days should cost less than 500 pounds when new, and if you look at the cost of the other hardware in your shack, I think you may very well consider this a small price to pay. Even more so when you appreciate that you can keep the station log, run great circle map programmes, run propagation forecasting software and many, many other things, that it is possible to do with a computer. You will also want a decent size hard disk and I would recommend at least one GB. It is amazing how quickly the hard disk will fill up when you start saving pictures!
Now let as have a look at the software that you
will require. Over the past five years I have evaluated all the software that
is available for the IBM P.C., and although you may wish to try the share ware
versions yourself, I can certainly save you some time by recommending what to
look for. With a very basic computer running DOS, you will have to
confine yourself to one of the non multi-tasking programmes. Pasokon Lite
(EZ-SSTV) will meet your requirements nicely and the shareware version of this
software, along with many of the other SSTV programmes, can be downloaded from
the internet. If you have a computer with a fast 386 processor and a video card
capable of displaying 2 million colours you can run one of the multi-tasking
programmes. These allow you to prepare your picture for transmitting at the
same time as you are receiving a picture over the air. DL4SAW (GSH-PC) and
MSCAN 2.3 are both excellent, with the synchronization on MSCAN being
particularly good. You may have to inhibit smart drive if you have problems
with the picture jumping sideways during transmission. The best way is to try
things first and if you do have problems, then try to resolve them in a
systematic manner. You will find that, in most cases, the text help files will
provide trouble shooting information along with a circuit diagram for the
required interface.
If you have at least a 100 MHz Pentium and Windows
95 or 98, then you can go for one of the top SSTV programmes. JVComm 32 is very
good indeed and is not expensive to register. However, the top programme at the
moment has to be ChromaPIX. This programme has a full picture editor included
and will allow you, among other things, to tailor the brightness, contrast,
hue, saturation and sharpness before sending the image. The sharpness control
is particularly important, as the slow scan mode induces some softening of the
image during transmission and if you deliberately over sharpen your picture
before sending, the image received by the other station will be very much
better. The latest version of this software, 1.0, which came out last year is
really excellent and is vastly improved over earlier versions. A further update
is due very shortly, so keep a look out on the ChromaPIX site. I have carried
out extensive tests on this software, along with Danny, ON4VT, Danny runs the
SSTV DX Bulletin and his Web site is well worth a visit. This software is
continually under development and many more features are planned. I particularly
like the duplex feature, which allows transmit and receive at the same time and
eliminates the need to change between RX and TX screens. You will require a 16
bit duplex sound card to use this feature and these are available for around 25
pounds. The registered version of this software is available from Roger
Wilkins, G3XFA, and the cost is £84:50, including postage. I have
included a list of some of the more interesting web sites at the end of the
article, for those who have Internet access. There are of course many other
SSTV programmes available, but I have pointed you towards what I consider to be
the most appropriate for a given purpose. Just before we leave software I
should just mention one of the latest SSTV programmes to become available
called MM-SSTV. This software has been produced by JE3HHT and many of you will
be aware of his excellent RTTY programme called MMTTY. At the moment the
software is still in the early stages of development with Version One
having just been released. Synchronisation needs to be improved and
placing random text quickly also needs further work. Having said that, I
believe that it will be worth while keeping an eye on this software, as apart
from anything else it is completly free!
I mentioned that I would come back to the problems
that can be associated with inter connecting wiring and this is a very
important area for those contemplating transmitting. If you have RF in the
shack, you can be certain that slow scan will find it! You may hear slow scan
signals that sound as though they are being accompanied by a machine gun and
receive pictures that have a mass of little dashes on the received image. This
is a sure sign that R.F. is getting into the computer via the associated wiring
of the sending station. It is imperative that all interconnecting wiring is
screened and securely bonded to earth. I also bond the case of the computer to
earth. In addition, you may have to place ferrite rings at the output from the
computer and at the input mic. plug on the transceiver. You will often find
stations on the air exhibiting this type of problem, so some time spent here
will save you a lot of embarrassment later. Talking about embarrassment the
other thing to watch out for is how you connect in your microphone. With the
addition of a simple circuit, the software will automatically put your
transceiver into transmit and return you to receive on completion of
transmission. If your mic. has only a PTT switch, then you could find that any
audio in the shack would be transmitted along with your picture. Not the sort
of thing you want if you happen to be commenting to your friend about the
quality of the other person's picture! I use a small alloy box that takes the
signal from the microphone and the computer, then feeds the audio from the box
to the transceiver. The box can also accommodate the interface where this is
needed. If required, a simple switch can be included on the box to open circuit
the audio from the mic. You will find that some of the software authors have isolating
transformers in their inter connecting circuits. I personally would not
recommend this approach, as the small transformers can easily become saturated
at audio frequencies and will also tend to pick up any stray R.F., resulting in
degradation of the transmitted image.
I do not use any form of isolation when using the sound card, but if you feel that you would be happier with D.C. isolation to protect your sound card, then I would recommend using capacitors rather than transformers for this purpose.
Now a few hints about operating practice. Use SSB to return a CQ call. If you just send a picture, your chances of getting a contact will be greatly reduced. This is primarily due to a number of stations all replying with pictures at the same time and the chances of the person who sent the CQ receiving a good picture is dramatically reduced. Much better to establish contact in SSB, then move to a quiet frequency. Make certain that you can receive straight pictures before attempting to transmit. If at all possible make your first attempts on VHF or 80 metres. Much better to have a slanted picture transmitted to local stations, rather than to send it for thousands of miles! Try to make your own pictures rather than just lift them from a CD. You will find that the station you are operating would much rather have a picture of yourself, the shack and the family, rather than an image from a CD. With the cost of digital cameras and flat bed scanners dropping all the time, making your own pictures has now become a viable option. You can even get your own photographs transferred to CD and Kodak provide this service through many of the larger pharmacist shops that do developing and printing. Once you become more accomplished, you may also wish to try operating through one of the SSTV repeaters. These receive your picture and after a short delay will re-transmit your picture. ON4VRB runs on 28.700 MHz and K3ASI runs on 28.690 MHz.
In conclusion, it is not easy in a short article like this to cover everything that you may wish to know, but if you read the help files included with the software, you should not have too much difficulty. The band plan frequencies are shown in the table and this will provide you with all the information that you need to find SSTV signals. In Europe the normal mode used is Martin One and this is the mode to use when initially looking for pictures. Later you may wish to try to receive pictures from further afield and Scottie is the most popular mode in the USA and Canada. All the programmes will change over to the mode being sent, so you should not have too much difficulty finding signals that can be resolved.
Have fun and enjoy your slow scan. I certainly
have over the past five years. If you do have a problem that you can't resolve,
then by all means drop me an E-mail or a S.A.E. I will do my best to help you
to get going.
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SSTV band plan of preferred operating frequencies: VHF 144.500 FM 80m 3.730 - 3.740 LSB Calling frequency - 3.730 20m 14.225 - 14.235 USB Calling frequency - 14.230 17m 18.160 - 18.160 USB Calling frequency - 18.160 15m 21335 - 21.345 USB Calling frequency - 21,340 10m 28.660 - 28.700 USB Calling frequency - 28.680 Some recommended interesting web sites: |
John Cramond QTHR E-mail:
john@gm4nhi.freeserve.co.uk