are expanded to 30 bits by adding six bits for errordetection and correction (EDAC). These six bits arealso called hamming bits. The value of these bits arebased on parity checks of specific combinations of the24-bit data word.During the receive cycle, the six EDAC, orhamming bits, are examined for errors. There isenough redundancy in the EDAC to allow forcorrection of a single bit error. The operator cancontrol the selection of the error correction mode. Ifthe data word is not a control word, the word isexamined to determine if it is error-free, contains acorrectable error, or contains uncorrectable errors. Ifthe DTS is in the error detection and label mode, adetected error is identified and labeled before the dataword is sent to the CDS computer. In the errordetection and correct mode, the DTS attempts tocorrect an error before sending the data word to theCDS computer. In both modes, the six EDAC bits aredeleted and replaced with two parity error status bits.These status bits are defined in table 4-1.Audio Tone Generation and CharacteristicsThe DTS converts the 24-bit data word, alongwith the six EDAC bits, into a composite audio signalconsisting of 16 tones. This composite 16-tone signalis the data frame. The tones range in frequency from605 Hz to 2,915 Hz and are the odd harmonics of 55Hz. The specific frequencies of the tones are shownin table 4-2. The 605-Hz tone is used for Dopplercorrection, and the 2,915-Hz tone is used for data andsynchronization. Each of the data subcarrier tones(tones 2 through 16 in table 4-2) represents two binarybits of differential quadrature phase-shift modulateddata.The Doppler tone (605 Hz) is not phasemodulated. It is used to correct for Doppler shifts inthe received tones caused by the relative motionbetween the transmitter and the receiver. It is alsoused to correct for the Doppler shift that may occurbecause of differences between the transmitter andreceiver frequency standards.The 2,915-Hz tone has two separate uses. Duringthe transmission of the preamble and Net Sync, the2,915-Hz tone is used to identify frame timing. Thistone is phase shifted 180 degrees at the end of eachframe. When detected by the receiving DTS, thephase shift indicates the start of a new frame. Whenthe DTS is in corrected timing, this information isused to set the timing for the data frames that follow.When stored timing is used, the frame timing that wasset during Net Sync is used.The Doppler and sync tones vary from each otherand the other data-carrying tones in amplitude. TheDoppler tone is 6 dB greater than the other tones.During the Net Sync and preamble frames, theDoppler tone is transmitted at 12 dB and the sync toneis transmitted at 6 dB. The Doppler tone istransmitted at 6 dB during the transmission of dataframes and the sync tone is used as a data tone. Datatones are transmitted at 0 dB.The audio tones are divided into data frames toidentify the separate parallel groupings of 30 bits. Itis the phase angle shift of each of the 15 data tonesthat conveys the digital information contained in thetone. During each frame, each data tone frequencyhas a particular phase. At each frame boundary, thephase of each data tone is shifted with respect to theprevious frame. The amount of this phase change, orphase difference, determines the value of a two-bitnumber Two data bits yield the following fourpossible combinations: 00, 01, 10, and 11. Eachcombination is associated with a phase difference ofone of four values: 45 degrees, 135 degrees, 225degrees, or 315 degrees from the previous position.4-11
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