8 FAX
103
information is maintained. (If the wrong sideband is chosen then the pictures are
inverted
,
i.e. black lines are drawn white and vice-versa).
8.2.3
SSTV
With use of a computer creeping into virtually every shack, the SSTV mode has changed
from a technical challenge for a few specialists, to a relatively wide-spread and amusing
amateur radio pastime. Above all it is also interesting for
epicures in private
to observe
the large number of STTV transmissions on 20 and 80 m. In the early days of SSTV, it
was only possible to transmit relatively low definition Black and White pictures. The new
generation of SSTV standards, under good propagation conditions, offer an astoundingly
good, high resolution, true-color picture. In the last few years, two variants have
established themselves as de-facto standards.
MARTIN 1
in and around Europe, and
SCOTTIE 1
in the US and US-influenced areas of the world. Both standards are very
similar, and differ only in small details.
In order to allow high definition and also color transmissions to be made, the new
standards have a transmission time of approximately 2 minutes per frame (as against only
8 seconds for the original Black/White SSTV pictures).
One of the main problems of the old
steam SSTV
was the persistence of the CRT on
which the pictures were viewed. In this digital age, with electronic storage, the problem is
no longer there. The developer has more or less a free choice, and is virtually only
dictated by the wishes of the user.
Unfortunately, this freedom in transmission times has led to an unnecessary number of
different SSTV sub-standards, some of which are poorly documented. A real technical
requirement for these multiplicity of standards is not to be found. By limiting the SSTV
practice to a few modes, it is possible to keep a reasonable overview of the entire SSTV
scene.
Like in the early
steam SSTV
period, the modern systems still use FM as the means for
video transmission, very similarly to the FM-FAX standard (refer to chapter
, page
). The center frequency is usually 1900 Hz, with a shift of 400 Hz, so that as in FM-
FAX, the frequency limits are 1500 Hz and 2300 Hz representing the
black
and
white
respectively.
The difference between FM-FAX and SSTV is that SSTV uses picture and line
synchronization in the form of a special tone frequency which is
blacker
than
black
, that
is 1200 Hz. A tone-burst with a frequency of 1200 Hz and a duration of 30 ms signals the
start of a picture frame. At the start of every SSTV line, a tone of 1200 Hz with a duration
of 5 ms is inserted as a horizontal synch pulse, so that the exact start of a line can be
marked. The exact and rapid processing of this horizontal synch pulse is the key to
satisfactory SSTV reception.
The resolution of MARTIN 1 and SCOTTIE 1 is approximately 300 pixels per line. The
color palette is obtained by mixing the colors red, blue and green. Every line transmitted
actually consists of three lines, each containing the intensity components of the respective
colors. Effectively, each line contains three
sub lines
. When using this transmission
system, incorrect tuning does not lead to color errors. It leads instead only to changes in
color intensity.
