Audio Tone Generation and Characteristics

are expanded to 30 bits by adding six bits for error detection and correction (EDAC). These six bits are also called hamming bits. The value of these bits is based on parity checks of specific combinations of the 24-bit data word. During the receive cycle, the six EDAC, or hamming bits, are examined for errors. There is enough redundancy in the EDAC to allow for correction of a single bit error. The operator can control the selection of the error correction mode. If the data word is not a control word, the word is examined to determine if it is error-free, contains a correctable error, or contains uncorrectable errors. If the DTS is in the error detection and label mode, a detected error is identified and labeled before the data word is sent to the CDS computer. In the error detection and correct mode, the DTS attempts to correct an error before sending the data word to the CDS computer. In both modes, the six EDAC bits are deleted and replaced with two parity error status bits. These status bits are defined in table 2-1. Audio Tone Generation and Characteristics The DTS converts the 24-bit data word, along with the six EDAC bits, into a composite audio signal consisting of 16 tones. This composite 16-tone signal is the data frame. The tones range in frequency from 605 Hz to 2,915 Hz and are the odd harmonics of 55 Hz. The specific frequencies of the tones are shown in table 2-2. The 605-Hz tone is used for Doppler correction, and the 2,915-Hz tone is used for data and synchronization. Each of the data subcarrier tones (tones 2 through 16 in table 2-2) represents two binary bits of differential quadrature phase-shift modulated data. The Doppler tone (605 Hz) is not phase modulated. It is used to correct for Doppler shifts in the received tones caused by the relative motion between the transmitter and the receiver. It is also used to correct for the Doppler shift that may occur because of differences between the transmitter and receiver frequency standards. The 2,915-Hz tone has two separate uses. During the transmission of the preamble and Net Sync, the 2,915-Hz tone is used to identify frame timing. This tone is phase shifted 180 degrees at the end of each frame. When detected by the receiving DTS, the phase shift indicates the start of a new frame. When the DTS is in corrected timing, this information is used to set the timing for the data frames that follow. When stored timing is used, the frame timing that was set during Net Sync is used. The Doppler and sync tones vary from each other and the other data-carrying tones in amplitude. The Doppler tone is 6 dB greater than the other tones. During the Net Sync and preamble frames, the Doppler tone is transmitted at 12 dB and the sync tone is transmitted at 6 dB. The Doppler tone is transmitted at 6 dB during the transmission of data frames and the sync tone is used as a data tone. Data tones are transmitted at 0 dB. The audio tones are divided into data frames to identify the separate parallel groupings of 30 bits. It is the phase angle shift of each of the 15 data tones that conveys the digital information contained in the tone. During each frame, each data tone frequency has a particular phase. At each frame boundary, the phase of each data tone is shifted with respect to the previous frame. The amount of this phase change, or phase difference, determines the value of a two-bit number. Two data bits yield the following four possible combinations: 00, 01, 10, and 11. Each combination is associated with a phase difference of one of four values: 45 degrees, 135 degrees, 225 degrees, or 315 degrees from the previous position. Each of these angles marks the center of a quadrant, as shown in figure 2-14. Each 90-degree quadrant is assigned a two-bit binary value. Any phase difference falling within that quadrant represents that binary value. This system of data encoding can tolerate an error in the prescribed phase shift of up to ±44 degrees before a single bit error will occur. An error in phase shift that is greater than 45 degrees, but less than 135 degrees, will cause the phase angle to fall into an adjacent quadrant. Notice that the values are assigned to each quadrant in such a way that if a phase shift error occurs, only one bit error will be introduced as long as the quadrant into which it falls is adjacent to the target quadrant. 2-11