SCS PTC-IIex, Manual

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8 FAX
For transmission use when using AM-FAX, refer to chapter 8.8 , page 105 and chapter
8.8.1 , page 105 .
8.7.2 Fmfax
Starts the FM-FAX-MODEM. The measured instantaneous signal frequency is sent via
the RS-232 serial interface, at the rate set by the
MBaud
command.
The time-resolution required, and filter bandwidth may be pre-set using the
FResolut
parameter. The steepness of the FM-detector may be pre-set using the Deviation
parameter. The output values reach from 0 to 255. Measured values smaller than 0 are
output as 0. Values greater than 255 are limited to 255 by the PTC-IIex. The center
frequency is exactly 1900 Hz. With a frequency of 1900 Hz as AF input, the PTC-IIex
always gives out the appropriate value of 128. With the standard setting of 400 Hz shift
(Deviation = 400), then an output value of 255 is obtained with an input frequency of
2300 Hz, and the input frequency of 1500 Hz gives an output value of 0.
The tuning indicator displays the output values. The left-hand limit represents 1500 Hz
(by the standard setting of 400 Hz shift) and the right-hand limit 2300 Hz.
For transmission use when in FM-FAX operation, refer to chapter 8.8 , page 105 .
8.7.3 Sstv
Starts the SSTV-MODEM. This MODEM resembles very closely the FM-FAX-
MODEM (see Fmfax- MODEM command). It consists of an FM detector with a center
frequency of 1900 Hz and a pre-set shift of 400 Hz. (Independent from the Deviation-
Parameter). The Black and White limit frequencies are appropriately 1500 and 2300 Hz.
The maximum possible resolution and input filter bandwidth may be pre-set using the
SResolut
parameter.
Unlike the FM-FAX-Modem, values smaller than 0 (measured frequency lower than 1500
Hz) or greater than 255 (measured frequency higher than 2300 Hz) are not just limited to
0 or 255. In SSTV, It has been found advantageous to
fold-back incorrect values into the
correct area. If the system, for example, measures a frequency of 2700 Hz, then it does
not give out a value of 255. Instead, the output value is calculated using the formula
255- (2700-2300)/800*128.
This folding-back has been shown to give less color errors in multipath propagation
conditions, and with other interference conditions. As the tuning indicator also displays
the output values in SSTV-Modem operation, this
folding-back can perhaps be somewhat
confusing during tuning operation.
The SSTV synchronization pulse using a frequency of 1200 Hz is filtered independently
from the FM-Detector, and is processed with the help of a relatively complex threshold
value method. Basically, the PTC-IIex interprets these pulses as being a separate
amplitude modulated signal, independent of the picture information. As a special new
development, the PTC-IIex can use a processing method especially designed for the line
synch-pulse, that virtually accumulates the information over a number of lines. As this
processing algorithm requires the line synch-pulse timing for correct operation, there
exists a special command (
SMode
), with the help of which the PTC-IIex is informed of
the current SSTV sub-mode being used. The multi-line check may be turned off, which
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