Section 6 – VC-20™ ATC-10b Vocoder Board Advanced Features
VC-20
ATC-10b Vocoder Board User’s Manual
page 8
The Speech Compression Specialists
D
IGITAL
V
OICE
S
YSTEMS,
I
NC.
3.4
VC-20-ATC-10b Connector Signals
TXD3-0 TXD3-0 are the voice data signals from the VC-20™ - ATC-10b encoder. These
signals changes state on each rising edge of TXCK. The 96 voice bits of each 20 ms
voice frame are transmitted in 24 TXCK cycles (TXCK = 9.6 kHz), resulting in an
effective data rate of 4800 bps. The first TXCK cycle of each frame is setup to allow the
VC-20™ - ATC-10b to operate in loopback mode (the decoder reads the first 4-bit
word as a control word). The first TXCK word contains 0000 binary, and then the next
24 TXCK cycles contain the 96 voice bits.
TXCK This is the encoder data clock which controls the transfer of the channel data out of the
encoder. This signal must be 9.6 kHz +/- .05%. TXD, FMEN, GO, and RUN are
synchronous with the rising edge of this clock signal to within 500 ns.
RXD3-0 RXD3-0 are the voice data signals input to the VC-20™ - ATC-10b decoder. These
signals are each clocked into the VC-20- ATC-10b on each falling edge of RXCK. The
96 voice bits plus 4 control bits of each 20ms voice frame are input in the first 25 RXCK
cycles (RXCK = 9.6 kHz). The first RXCK cycle contains a 4-bit control word, and the
remaining 24 RXCK cycles contain the voice bits, resulting in an effective data rate of 4.8
kbps. The four bits of the control word are organized as follows: RXD3 = LOST,
RXD2 = PARAMINIT, RXD1 = DESLNCE, and RXD0 is not currently used. LOST
signifies a lost frame of data (active high), PARAMINIT re-initializes the voice coder
parameters (active high), and DESLNCE signifies a frame of silence (active high).
RXCK This is the decoder symbol clock which controls the transfer of the channel data into the
decoder. This signal must be 9.6 kHz +/- .05%. RXD3-0, FMDE, and DESLNCE are
synchronous with the rising edge of this clock signal to within 500 ns.
FMEN This is the encoder frame flag which identifies the frame boundary of the
channel output data. FMEN must be active for one clock period of TXCK
every 20 ms.
FMDE This is the decoder frame flag which identifies the frame boundary of the channel input
data. FMDE must be active for one clock period of RXCK every 20 ms.
VRXI VFXI is the analog speech input which is filtered with a 100 Hz. - 3600 Hz. bandpass
filter and then input to the encoder via a 16 bit linear codec. In order to avoid clipping
VRXI must not /- 1.5 volts.
