AMBE-2000™ Vocoder Chip

User’s Manual Version 3.0

page 42

Confidential Proprietary, Subject to Change

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 (parallel or serial) mode, the chip produces the signals

internally for the transfer of data. Because the transmission channel will then likely be driven by this timing, 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 Slip control feature here is a brief description on reading encoder packets from the AMBE-2000 in

passive formatted 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 formatted 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).