Wavecom W61PC V7.5.0 - Manual (Page 161)

WAVECOM Decoder W61PC/LAN Manual V7.5 Transmission Modes  151
Parameter Value
Operation modes Broadcast/Simplex FEC
Modulation FSK
Symbol rate 96, 144 and 192 Bd, variable 30-650 Bd
Receiver settings DATA, CW, LSB or USB
Input format(s) AF, IF
Additional Info ARQ1A with convolutional coding
The FEC-A mode is started by selecting a standard baudrate from the Baudrate item in the Demodulator
menu. Frequency shift and baud rate are determined using FSK Analysis. For automatic tuning Auto may
be selected.
The synchronization or idle state is easily recognized by its sound. It is an alternating mark-space keying
sequence (mark-space ratio approx. 40% - 60%).
Shift register length is set using the S-Reg item in the Options menu. Incorrect selection of the S-Reg
parameter causes incorrect error correction,and the data output rapidly becomes corrupted. If error cor-
rection is disabled (ECC off), the length of the shift register will not affect decoding (S-Reg not visible).
This feature allows any FEC-A signal to be decoded.
FEC-A will detect and correct transmission errors up to a certain limit; in the case of extreme interference,
error correction may worsen the situation, so reception without error correction may improve perfor-
mance.
FEC-A uses the ARQ-1A alphabet. Every second bit of the bit stream is used for the convolutional error
correction and thus each codeword consists of 14 bits.
FELDHELL
Feld-Hell is a synchronous picture telegraph system invented in the 1930s. It uses a virtual matrix laid
down on the character to be transmitted. The pixels of the matrix are then sent, scanning the matrix from
the bottom of the first column (left) to the top of the last column (right), covering a matrix of 7 columns x
14 lines. The first and last columns are blank, as are the top and bottom lines, giving an effective matrix
of 5 x 10 pixels. Pixels are always sent in pairs.
Parameter Value
Frequency range HF
Operation modes Graphic mode, Broadcast/Simplex
Modulation CARRIER KEYING AM
Symbol rate 122.5 Bd
Receiver settings DATA, CW, LSB or USB
Input format(s) AF, IF
Additional Info Synchronous matrix picture telegraphy
The original Hell system was a very simple mechanical one with an indented wheel for each character used
to generate the transmit pulse trains via a contact.
In the receiver, the pulses activated a printing magnet with a writing edge, which pushed a paper tape
towards a helix inked by an ink roller.
No means of synchronization besides nominal helix speed was used - speed differences showed up as ris-
ing or falling lines of letters - but as the pitch of the helix was designed to print a double row of charac-
ters, one complete character would always be displayed on the tape.
Hell utilizes AM in the form of CW or A2. When receiving Feld-Hell the decoder will emulate the original
Hell tape printer and print each character twice making exact phasing unnecessary as at least one charac-
ter will be complete and unbroken.