Figure 4-19.—Link-11 frame boundary phase shifts.
During receive operations, the tone package is
received from the radio set. The 30 bits of data are
extracted from the tone package by determining the
phase shift of each data tone with respect to the
previous frame. The 30 bits, which contain 24-data
bits and six-EDAC bits, are examined for errors. The
six-EDAC bits allow for the detection of errors and
provide enough redundancy to allow for correcting a
single bit error.
The operator can select whether or not the DTS
attempts to correct detected errors, as explained
earlier in this chapter. In the error detect (label)
mode, a detected error is identified and labeled before
it is sent to the CDS computer. In the error correction
(correct) mode, the DTS attempts to correct a detected
error, labels the error, and sends the data word to the
CDS computer.
The DTS is capable of receiving and processing
both the upper sideband and the lower sideband when
using a HF radio, depending on the position of the
sideband select switch. When you are using a UHF
radio, only the upper sideband is received and
processed.
If the sideband select switch is in the USB or the
LBS position, only the designated sideband is
processed. In the diversity (DIV) mode, the 30-bit
word is generated by adding the relative phase angles
of the USB and the LSB.
Because of propagation anomalies, noise, and
interference, the AUTO mode can be used to select
the sideband (USB, LSB or DIV) that yields the most
correct data automatically. In the AUTO mode the
DTS processes a word from each sideband and the
diversity combination. The decoded words are
examined for errors in the following order or priority:
DIV, USB, and LSB. A search of the three words is
made to find a data word with no error. If one is
found, it is selected for input to the CDS computer. If
none is found, the RCV DATA ERR indicator is lit
and the diversity combination data word is sent to the
CDS computer.
Radio Set Interface
The DTS generates the following outputs to the
radio set: upper sideband composite audio, lower
sideband composite audio, and key line. It receives
upper sideband composite audio and/or lower
sideband composite audio. UHF radio sets use only
the upper sideband signal and the key-line signal.
The key-line signal controls the transmit and
receive state of the radio set. The key line is set to
transmit Link-11 data. When the key-line is cleared,
the radio set returns to the receive mode.
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