Wavecom W61PC V7.5.0 - Manual (Page 228)

218  Transmission Modes WAVECOM Decoder W61PC/LAN Manual V7.5
The selective FEC is used when the receiving station must operate under "radio silence" conditions and
hence cannot transmit.
In most cases SITOR-FEC transmissions uses a rate of 100 Baud using the standard shift of 170 Hz.
Each character is transmitted twice with an interval of 35 bits. In the case of error-free reception of both
characters, the character is displayed on the screen. The underscore (_) character is displayed if both
characters are received in error.
SP-14
SP-14 is an AM modulated sequential 14 tone mode, which is optimized for the transmission of numeric
codes.
Parameter Value
Frequency range HF
Operation modes Broadcast/Simplex
Modulation AM, SUB MFSK-14
Speed 133.3 ms
Receiver settings AM
Input format(s) AF, IF
Additional Info 14.891 MHz, 14.888 MHz, 15.791 MHz, 16.291 MHz.
The tone separation is 16 Hz. The center frequency is 409 Hz and the bandwidth is approximately 210 Hz.
Each tone has been assigned a character:
 10 tones are assigned to the numerals 0...9
 One tone is used as a start character
 One tone is used as a stop character
 One tone is used as a space character
 One tone is used as a repetition indicator
A transmission is initiated with a start sequence, which identifies this mode and may be used for accurate
tuning. This sequence is transmitted at 1 Baud, which makes it readable even during very unfavorable
conditions.
The data proper is transmitted at a rate of 7.5 Baud. A sequence of 19 'P' characters is initially transmitted
followed by the five digit code words. The transmission is terminated by transmitting 20 'K' characters. As
the transmission rate is very low this mode is insensitive to fading and multipath propagation.
To decode a SP-14 signal it must first be demodulated using an AM demodulator in order to retrieve the
MFSK signal. The simplest way is to use the AF output of the receiver and select receiver AM demodula-
tion. If a receiver IF output is used, AM demodulation is not available and decoding becomes difficult.
However, using the MFSK demodulator the transmitted symbols may be retrieved. A MFSK IAS using 16
bit over sampling will retrieve symbol clock. In order to process both baud rates used by SP-14, the IAS
has to be configured to a baud rate of 15 Bd. Thus at a rate of 1 Bd 15 symbols per transmitted symbol,
and at 7.5 Bd two symbols per transmitted symbol must be received with a certain tolerance. Even if for
instance the same character is received 15 times in succession, error correction is impossible due to the
fact that in case of repetition of numerals the numeral itself is followed by the repetition indicator 'R'.
Tuning a SP-14 Signal
Using the WAVECOM decoder as an example the following shows how decoding SP-14 is performed:
After tuning the receiver to the selected frequency and turning AM demodulation on, the receiver AF out-
put (Line Out) is connected to the AF input.
If the receiver has been correctly tuned the following FFT display is shown: