Jvfax - SCS PTC-IIIusb User Manual

8 FAX
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-IIIusb interprets these pulses as being a separate
amplitude modulated signal, independent of the picture information. As a special new
development, the PTC-IIIusb 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-IIIusb is informed of
the current SSTV sub-mode being used. The multi-line check may be turned off, which
deteriorates the synch recognition, but however, does allow unknown SSTV-
transmissions to be processed without problem when a good signal to noise ratio is
available.
The PTC-IIIusb uses the standard given in the JVFAX documentation, to pass the
recognized synchronization pulses to the PC program. The system uses the lower two bits
of every byte to indicate the synch pulse. Bits 0 and 1 are set to 1 in the resting state. As
soon as the PTC-IIIusb recognizes a correct synch-pulse on the 1200 Hz frequency, it sets
the bits 0 and 1 to 0 for the time the synch-pulse occurs. (Note: the VIS code is at present
not processed by the PTC-IIIusb, and therefore bit 2 is actually redundant for the
signaling condition.) The small decrease in color resolution caused by only having 6 bits
to signal brilliance information can, in practice, be ignored.
The JVFAX sync method used by the PTC may be turned off using the JSynch
parameter command. In theory, the PTC-IIIusb may be used with the JVFAX in the
operational condition LSB-SSTV-SYNCH NO. Then the system uses the full 8 bit data
width for picture information. The practical results however speak against using this
method.
With the JSynch parameter turned on (default setting), the recognized synchronization
pulse is signaled to the user with the CON LED flashing green.
A small time offset of the line-synch pulse may be set by use of the HSynch parameter.
This allows the whole picture to be shifted slightly left or right.
For transmission use when in SSTV-MODEM operation, refer to chapter 8.7.1, page 110.
8.6.4

Jvfax

Starts the JVFAX mode. This mode does not contain its own Modem as such. Instead, it
offers a sort of "springboard" to FM-FAX, AM-FAX and SSTV by using special control
sequences, that are recognized in this mode by the PTC-IIIusb. Directly after switching to
JVFAX operation, the PTC-IIIusb is in Modem mode FM-FAX, using a shift which has
been preset using the Deviation parameter (normally 400 Hz).
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