AMBE-2000 Digital Voice Systems, Inc., AMBE-2000 Datasheet - Page 44

• Manufacturer Part Number: AMBE-2000
• Description: 2.0 - 9.6 KBPS Single ic For Toll-quality, Low Data Rate, Real Time Speech Compression
• Manufacturer: Digital Voice Systems, Inc.

AMBE-2000™ Vocoder Chip
User’s Manual Version 4.0
7.7
Slip Enable
In any real time communication system, clock skew issues must be anticipated to keep the flow of data smooth from one end of
the system to the other. The SLIP_EN (pin 82) signal allows the encoder of the AMBE-2000™ to react to small slips in the
encoder channel signals. When the AMBE-2000™ is in active mode, the chip produces the signals internally for the transfer of
data. Because this timing will then likely drive the transmission channel, the necessity of controlling slip becomes a moot
point.
Any time the AMBE-2000™ encoder channel is in one of the passive modes and the channel timing is asynchronous to the
A/D-D/A clock (very rarely are these two interfaces coupled) then the SLIP_EN pin should be set active high.
The AMBE-2000™ Vocoder chip processes speech in voice frames that are approximately 20 ms in duration. When
configured appropriately the chip provides a slip control feature that automatically adjusts the frame size to either 160 or 161
speech samples per frame. This slip control feature allows the vocoder chip to compensate for drift between the frame and
sample rate clocks on the order of approximately 0.6% (6,000 ppm.) The vocoder chip also accepts Slip Control Packets that
extend the range of allowable frame sizes to be between 159 and 161 samples per voice frame. When properly used these Slip
Control Packets provide the designer with additional flexibility in dealing with clock drift.
There are three recommended methods for using slip control on the AMBE-2000™ Vocoder Chips, which are described below.
The system designer should select the method that best meets the needs of their system configuration. Also included is some
background information on the operation of the AMBE-2000™ in passive mode
In order to help understand the Slip control feature here is a brief description on reading encoder packets from the AMBE-
2000™ in passive framed mode.
When transmitting a packet, the AMBE-2000™ writes a Header = 0x13ec followed by 23 words of data, followed by 0xfffe
into the transmit (i.e. output) buffer. The terminating word 0xfffe is written into the transmit buffer by the AMBE-2000™ at
the end of each encoder packet. Normally in passive mode this terminating word is in the transmit buffer at the beginning of
each transmission cycle (from the previous frame) and so it is the first word output whenever a packet is transmitted. If the
encoder packet is ready, then the second output word will be the packet Header=0x13ec followed by 23 words of data.
However if the packet is not ready then the AMBE-2000™ will continue to output the terminating word (0xfffe) until the
packet is ready and placed in the transmit buffer. At this point the full 24-word packet beginning with the Header will be
output on subsequent transmissions. This process continues for each packet transmission, which occurs nominally every 20
ms, provided that each 24-word packet (Header + 23 data words) is read in full. If the full 24 words of the packet are not read
from the AMBE-2000™, then the chip’s transmit buffer will contain some words left over from a previous packet. Generally
these words would be 0x0000 for lower data rates, which don't use the last words of the packet. This case should be avoided.
It is recommended that in passive framed mode the system always read packets by requesting
words from the AMBE-2000™ until a packet Header is received and then continuing to request 23
additional output words from the AMBE-2000™ until a total of 24 words beginning with the
Header word = 0x13ec are received. Any words output by the AMBE-2000™ prior to the Header
should be ignored by the system (except for monitoring as discussed in Method 3 below).
DVSI Confidential Proprietary, Subject to Change
Page 44
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