transmissions.   In   other   words,   both   speeds   are converted using the same reference signal. Synchro-to-Digital (S/D) Conversion Two  methods  are  currently  in  use  to  convert synchro data to digital words (BAMs): the sector method  and  the  octant  method.  Both  methods  of conversion  require  a  reference  voltage  input  for conversion to take place. SECTOR   CONVERSION.   —The   sector conversion  method  uses  the  reference  voltage  to determine the time to sample the stator voltages for conversion to take place. The ideal time to sample the stator voltages is when the reference voltage is at or near the positive or the negative peak of its cycle. Sixty-Degree Sector Determination.  —Once  the negative  or  the  positive  peak  of  the  reference  is detected, the sector in which the rotor is positioned may be determined. There are six 60-degree sectors within the 360-degree rotation of the rotor. The relationship of the stator voltages to the reference defines the sector. Table 13-2 shows the sector limits and the phase relationship of the stator voltages to the reference in each  sector. Stator  Voltage  Selection.  —When  the  sector  angle is determined, two of the three stator voltages are used to identify the ratio angle within the sector. The ratio angle is determined by a ratio between the two voltage samples. The two stator voltages selected depend on the sector. The appropriate voltages are gated to the conversion circuitry and converted to binary data. The sector angle and the ratio angle of the two stator voltages are summed to determine the binary angle of the rotor position in BAMs. Table  13-2.—Phase  Relationship  of  Stator  Voltages  to Reference OCTANT   CONVERSION.  —The   octant conversion method divides the 360 degrees of angular measurement   into   eight   45-degree   octants.   The conversion process first defines the octant and then the binary representation of the trigonometric angle within the octant. Octant  Determination. —The  5-wire  synchro signal (R1, R2, S1, S2, and S3) is first converted into two dc voltages representing the sine and cosine of the synchro angle. The polarity of the sine and cosine voltages and their respective amplitude to each other are used  to  select  the  octant  that  defines  the  three  most significant bits of the BAM word (figure 13-3). Successive  Approximations.  —The  remaining  bits of the BAM word are determined through a process of successive  approximations.  The  sine  and  cosine voltages are combined into a ratio voltage that is used to determine the condition of each of the remaining bit positions in the BAM word, starting at the MSB of the remaining bits. A trial and error method is used. A trial binary angle is generated and tested against the ratio angle  until  the  trial  angle  equals  the  ratio  angle, completing the conversion process. Single-Speed/Dual-Speed  Synchro  Conversions Synchro-to-digital  conversions  do  not  occur  on  a continuous basis. The synchro data is sampled as required by the controlling computer, usually on a periodic basis. A single BAM word is generated by the S/D  conversion  for  both  single-  and  dual-speed synchros. When dual-speed synchro data is being converted, two S/D conversions are required to generate one BAM word. The coarse synchro signal is converted immediately  before  the  fine  synchro  signal.  The summation of the two conversions is represented by a single binary word, indicating one angular value. Conversions for single-speed synchros are considered coarse  conversions  only. NOTE.  —For  more  detailed  information  on synchros and synchro systems, refer to NAVEDTRA 172-15-00-80,  NEETS,  Module  15,  Principles  of Synchros, Servos, and Gyros. DIGITAL-TO-ANALOG  CONVERSION Digital-to-analog (D/A) conversion is required when  digital  devices  must  communicate  with  an  analog system or equipment. Three types of D/A conversion are commonly encountered on shipboard systems: digital-to-linear,   digital-to-scalar,   and digital-to- synchro (D/S).  Linear signals are ac or dc voltages that 13-7