Digital Voice Systems VC-20-MR2, Руководство пользователя

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Section 2 - VC-20 AMBE™ Vocoder Board Overview
VC-20 AMBE™ Vocoder Board User’s Manual
page 6
The Speech Compression Specialists
D
IGITAL
V
OICE
S
YSTEMS,
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NC.
The VC-20-MR2 can be connected to a modem or similar device as shown in Figure
3. The modem or the VC-20-MR2 can generate the clocks (TXCK and RXCK) and
frame synchronization (FMEN and FMDE). The modem reads the data from the
encoder on the data line TXD. Simultaneously the decoder reads data from the
modem on data lines RXD, RXD1, RXD0, HD_SD, DESLNCE and LOST.
Note: if soft-decision information is available the HD_SD signal should be held low,
otherwise the HD_SD signal should be held high so that only the MSB (RXD) of the
data signal will be read by the decoder.
The frame signals FMEN and FMDE are used to signify the beginning of each data
frame for the encoder and decoder, respectively. The required timing relationship
between these signals is shown in Figure 5. In order to operate in this mode,
jumpers 5-10 and 12-14 should be disconnected. If the VC-20-MR2 is generating
the clocks and frames, then jumpers 1-4 should be connected, otherwise, they should
be disconnected. Jumper 11 is disconnected if the LOST signal is used, otherwise it
is connected.
A final operating mode allows two VC-20-MR2's to be connected together using
their on-board clocks to generate the signals TXCK, FMEN, RXCK and FMDE. In
this mode the encoder on the first VC-20-MR2 is connected to the decoder on the
second VC-20-MR2. Similarly, the decoder on the first VC-20-MR2 is connected to
the encoder on the second VC-20-MR2. This is shown in Figure 4.
Generally, the encoder signals on each board are synchronized with the oscillator on
that board, and the decoder signals on each board are synchronized with the
oscillator on the other board. This is done by connecting jumpers 1, 3 and 11 on
each board. All other jumpers should be disconnected. The HD_SD control signal
must be held high or left unconnected to disengage soft-decision decoding. Care
must be taken that only one clock generator is connected to each of the clock signals
TXCK, FMEN, RXCK and FMDE.
The ECHO signal should be held low to enable the onboard echo cancellor. The 5
ms echo cancellor included in the VC-20-MR2 is generally sufficient for canceling
electrical echo introduced by a near hybrid (4-to-2 wire converter). If the ECHO
signal is held high or left disconnected the echo cancellor is disabled.
The VAD signal should be held low to enable the onboard Voice/Silence detection
and Adaptive Comfort Noise features. When enabled the ENSLNCE signal is
switched to indicate the presence of Voice. If the VAD signal is held high or left
disconnected, both Voice/Silence detection and Adaptive Comfort Noise are
disabled, and the ENSLNCE signal always indicates voice.