Using the 'Scope Simulation

The AVT system has a very useful Scope simulation available in the main
control panel. There are two controls that are directly associated with
the scope, both also in the control panel window, right next to it.

The first is the "tune" button. This activates the scope and locks the
mouse pointer. When you are done using the scope, simply press the left
mouse button and it will turn off.

The second is the "Trig" button. This causes the scope to re-trigger
when a negative edge, transiting the frequency region from black to
sync is detected. The net effect of this is that incoming SSTV of the
older format lines up left-to-right on the scope display, starting with
the sync pulse and continuing accross the display with the video
information for each scan line.

Generally, you use the scope to tune in other SSTV stations, but you
can also use it to detect the shift of RTTY signals, tune FAX, and other
interesting signals.

The scope is a two dimensional display, with time going positive from
left to right, and frequency going higher from bottom to top. There are
three red marks at the left edge of the dispaly area that indicate the
exact position where the standard black, white, and sync frequencies
are located. For your information, black is 1500 Hz, white is 2300 Hz,
and sync is 1200 hz. Additionally, there are marks on either side
of the sync mark, that show a '1' and a '0'. These are at 1100 and
1300 hz, which are the two frequencies that the Robot scan converters
use to send information as to what mode is coming. THis is done in the
main vertical sync pulse.


Tuning with another AVT station
When you are working with another station that has the AVT system,
you can ask them to open their SET window and press the BLK button.
This causes them to transmit a pure black tone - while you are
receiving this signal, ensure that the TRIG button is OFF (no light
showing) and press the tune button; tune your SSB receiver for a line
exactly on the graticule that is marked "black". once you have done
this, you are tuned exactly correctly - no guesswork.

Tuning with a station that is running a new Robot unit
Here, ask the other station to send you a grey scale, or failing that,
any other video. While receiveing this; make sure the TRIG button is
ON (light showing) and tune your SSB receiver so that the tips of the
sync pulses are exactly positioned next to the red arrow marked SYNC.

Tuning a station running other types of units
When tuning stations with older hardware, it is best to tune the
radio so that the black and white levels are correctly aligned -
not the sync. Some of the older Robot 400s, for instance, have
separate adjustments for sync, black, and white frequecies. We have
seen stations transmitting video where the black and white frequencies
were good, but the sync was too high or too low. In this situation, it
is best to tune for the video levels, because the object, after all,
is to receive an image. The newer Robot units like the 400c, 450c, and
1200c, do not have adjustments that can go out of (or be SET out of)
whack; instead, they use primitive digital synthesis techniques which
are quite reliable - the sync, black, and white frequencies are
always correct.

Sweep Rates
The Scopes sweep rate is set automatically when you press a recieve
mode. For instance, if you press the top left button, the 8 second
speed, and then the green memory, the AVT is set up to receive 8
second Robot-format black and white images. These inages have
scan line times of about 60 ms, and the scope is set for a sweep
rate of about 59 ms. Another example is pressing the CMP (composite)
button and the 72 second speed button. This sets the scope for
approximately 305ms per scan. For use with unusual signals (found while
SWL'ing, for instance) press one of these speed buttons to control the
sweep rate of the scope. 8 second, black and white is the
fastest sweep rate, and 72 second, color composite is the slowest
sweep rate.

The Filter Button
There is another button which affects the scope; This is the FILTER
button on the main control panel. This button runs a 2-pole
software low pass filter simulation in such a way as to have the
same effect one would have if you filtered the incoming video
with an op-amp circuit. You will notice that the incoming waveform is
smoothed considerably when the filter is on. Since the scope draws it's
waveforms by placing one, and only one, dot along the horizontal axis for
each measurement it takes, you may find that wildly varying frequencies
(like white noise) produce a display that makes little sense. If you
turn on the filter, you will see the low-frequency components that make
up the white noise. Experiment with this. Remember, though, that the
filter button affects not only the scope, but the receieved video as well;
If you notice that all your incoming video looks very smooooth, check the
filter button.