14

DA9191A.000

July 31, 1997

FUNCTIONS

Forward voice channel data format for narrow band.

DSAT

Sync word

Data

DSAT

24

30

40

24

Sync word = 011001010110101001100110100110

DSAT = Digital Supervisory Audio Tone is one of seven 24-bit digital sequences added to the voice transmission.

DSAT is transmitted at 200 NRZ bits/second. The following is a list of the seven DSAT sequences.

DSAT sequence

0

1

2

3

4

5

6

The 40-bit long data sequence is generated at a 100 Manchester bits/second rate. The data sequence contains 28

bits of data and 12 parity bits.

The incoming data is captured by two 8-bit shift

registers. When one shift register is full the RXWRD

flag is set and an interrupt is generated. The

captured data must be read by the micro controller

within 20ms after the interrupt. Meanwhile, the other

shift register is being filled and when it is full a new

interrupt is generated. The shift registers are clocked

in at 400 Hz. Two samples of each state of both the

200 NRZ bits/second data and the 100 Manchester

bits/second data are loaded into the registers. Note

that there are as many transitions in the 200 NRZ

bits/second data as in the 100 Manchester

bits/second data. The micro controller will then be

used to filter the digital bit sequence and detect the

DSAT, SYNC WORD and DATA out of the bit

stream. The DATA must be checked with following

algorithm:

G(x) = x

8-bit

shift register

8-bit

shift register

STxD

SCL

SCL

400 Hz

400 Hz

C1

Rx