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According
to my copy of the ARRL Handbook 1990, Slow Scan Television (SSTV)
had its beginning back in 1958 as the result of the effort of a team
of Amateur operators led by Copthorne Macdonld, VE1BFL. The aim of
their effort was to transmit television pictures over a HF radio
circuit. The result was a system that allowed the transmission of a
singe television frame of 120 lines within the 3kHz voice frequency
range typically associated with HF phone operations.
Slow
Scan Television (SSTV) was in fact a cumbersome and technically
involved process using long persistence cathode ray tubes and
related electronics to decode and display a crude black and white
trace image.
The
first time I witnessed a demonstration of Slow Scan Television was
back in 1978. The thrust of the demonstration was to show how the
transmission of colour images can be achieved by transmitting a
series of scans through colour filters and photographically
recovering the image through multiple film exposures. It did work
and I was suitably impressed, however I concluded that this was for
the seriously dedicated SSTV / Photographer.
Software
A
lot has changed since those
days and most SSTV transmission and reception is now generated and
decoded by computer software that results in a highly acceptable
colour image being exchanged. The SSTV goal is still the same; that
is to transmit and or receive a single television frame over a voice
channel constant with the band and the mode in use. A typical HF
single side band channel would allow approximately 2.5kHz of band
width and therefore the SSTV picture will require about a minute
build up line by line.
There are number of modes of SSTV
transmission, but the two enjoying the most favor with operators are
Scottie-1 and Martin-1. Scottie-1 is the popular mode in
Australia
,
Japan
and the
United States
while
Martin-1 has been more favored by European operators.
There are a variety of SSTV software packages available, most of which are
very good. My preferred application is MMSSTV, partly as it is free,
but mostly as it is a very impressive package with many features and
above all is user friendly.
MMSSTV will operate quite comfortably on a relatively old computer of the
133MHz Pentium under Windows 95 ilk.
MMSSTV Version 1.11 is produced by Makoto Mori JE3HHT can be down
loaded from MM Hamsoft at http://mmhamsoft.amateur-radio.ca/
A search of the internet will uncover a large amount of SSTV related
information and programs. Many hams will passionately advocate MSCAN SSTV V2.2 or
another of the host of other programs out there, but MMSSTV is my
favorite!
See screen dump of
MMSSTV with image loaded and ready for transmission.
Interface
The
interface between the radio equipment and the computer is very
straight forward. The audio output and input from the radio can be
directly plugged into the relevant input and output of the
computer's sound card. While many operators have had success with
hard connection method I have chosen to use isolation transformers
to ensure that stray R.F. can not find its way into un-wanted areas
by way of ground loops.
Feeding
the signal into the microphone socket of the transceiver will
require a substantial attenuation of the SSTV generated signal to
prevent over driving the transmitter. This can be achieved by
feeding the signal via a simple resistor network and fine tuning
with the transceiver mic gain while monitoring the RF power level.
A
search of the internet will find a large range of circuits to
achieve the interface between computer sound card and the
transceiver. There are also a number of commercial interfaces
available for the task.
The
critical interface issue to watch for when transmitting is over
driving the transmitter which will result in a lot of adjacent
channel interference and possibly damage to the transmitter.
Duty
Cycle
The
SSTV generated signal is a frequency modulated tone causing the
transmitter to work hard. Therefore it is recommended that the
transmit power be wound back to around twenty watts for the average
100W HF SSB transceiver or to about the AM rating of the
transceiver. The duty cycle should not be an issue for the typical
VHF/UHF FM transceivers.
SSTV
Frequencies
The
official Australian WIA band plan designates a number of Slow Scan
Television call channels. There are to my knowledge no official
designations below the 20metre band although I have monitored SSTV
activity on 3.698MHz and 7.040 MHz from time to time.
SSTV
is particularly susceptible to noise, so my guess is that pictures
on the 40 and 80 metre band would be fairly degraded unless the band
was particularly quiet or signals were very strong. SSTV can be
enjoyed on 2 metres FM were even moderate signal strengths produce a
glass hard quality picture.
A
feature of MMSSTV is it can automatically store up to 32 received
images. I have found it interesting to leave MMSSTV and the radio
run over night on 14.230MHz and the next morning check to see what
parts of the world were open during that period.
WIA
band plan for SSTV activity:
14.230MHz SSTV International
calling frequency (Very active)
14.233MHz
SSTV (Also active)
21.340MHz SSTV International
calling frequency
28.680MHz
SSTV International calling frequency
145.625MHz
SSTV (Can be active)
The
Future
SSTV has moved from being an
expensive technically demanding experimental fringe activity to
being a low cost and technically accessible mode for all amateur
operators.
It interesting to note that this
has largely happened in the last ten years and the development of
High Definition Slow Scan Television (HDSSTV) and Digital Slow Scan
Television which is more to do with file transfer is starting to become
more main-stream leaving the future of transmitting pictures over HF
radio circuits bright and evolving.
I'm far from a dedicated SSTVer
I simply dabble, but if you have any questions about SSTV please do
not hesitate to give me a call!
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