Wavecom W74PC V8.7.0 - Manual (Page 270)

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Transmission Modes WAVECOM Decoder W74PC, W-PCI/e, W-CODE, W-CLOUD Manual V8.7.0
By selecting the Auto function, the software waits for the IOC identifier and synchronization sequence.
These signals are only transmitted once at the beginning of an image implying that capturing of the signal
will only take place at the start of the next image.
By selecting WEATHER-FAX the application starts immediately. The drum speed and the IOC may be en-
tered manually using the Drum speed and IOC items available from the Demodulator menu. The se-
lected values are displayed in the decoder status bar. In the short-wave band, weather charts are trans-
mitted almost exclusively with speeds of 60, 90 or 120 rpm. In the case of 60 rpm the drum rotates once
per second, for 90 rpm at one and a half time and for 120 rpm at a speed of twice a second. The index of
co-operation (IOC) is readily recognizable from the screen display when images are spread out too wide.
An IOC of 576 is mostly used on short-wave.
The decoder allows a continuous and smooth alignment of the image as it is being received from Op-
tions/Fine Speed. By using the Phase item in the Options menu the image can be shifted into the correct
position. Flip Left-Right and Flip Top-Down will reverse the image or turn it upside down.
To enhance the interpretation of gray-scale images, a false color option is available. Right-click on the im-
age to open the zoom menu and from the bottom part selects a color scheme. A checkmark is placed to
the left of the selection.
X.25
X.25 can be monitored in the VHF and UHF bands. This mode uses indirect FM (sub carrier) modulation.
Parameter Value
Frequency range VHF/UHF
Operation modes Duplex FEC TDMA
Modulation FM, SUB FSK
Symbol rate 300, 600 and 1200 Bd
Center frequency 1700 Hz
Shift 1000 Hz
Receiver settings FM BW = 12 kHz
Input format(s) AF, IF
Additional Info ITA-5 with block coding
To start X.25, select 1200 Bd, 600 Bd or 300 Bd. The correct shift or center may be set in the Demodula-
tor menu. Standard values for the center frequency and the shift are 1700 Hz and 1000 Hz.
X.25 uses a HDLC computer network protocol. X.25 is a synchronous system in which data is transmitted
in packets of 8 bit (octets).
Frame Format
X.25 provides three frame formats, the difference being the size of the sequence number.
The Basic frame has a frame sequence number size of 3 bits, so that the increment of the sequence num-
ber is a modulo 8 operation. In case of the Extended frame, the increment is a modulo 128 operation and
in case of the Super frame a modulo 32768 operation.
Packet structure
A X.25 packet constitutes the data field of a LAPB (HDLC) frame with a size of 64 to 4096 bytes.