AMBE-2000™ Vocoder Chip

User’s Manual Version 3.0

page 43

Confidential Proprietary, Subject to Change

Word #

Value

Description

0

0x13ec

Header

1

0x07xx

Slip Control Packet ID, xx=CWD1 (See table 5-B) Typical value xx=00

2

0x1355

Slip Control indicator for AMBE-2000

3

0x009f

Slip Control data

4

0x0000

Must be 0x0000

5

0x0000

Must be 0x0000

6

0x0000

Must be 0x0000

7

0x0000

Must be 0x0000

8

0x0000

Must be 0x0000

9

0x0000

Must be 0x0000

10

0x0000

Must be 0x0000

11

0x0000

Must be 0x0000

12-23

Channel Data for AMBE® voice decoder

Table 7D: Slip Control Packet

Method 1 - Internal Slip Control

This is the simplest method of slip control and is the default method provided slip control is enabled (SLIP_EN is high) on the

chip. In this method the vocoder chip’s internal slip adjustment of 160 to 161 sample per frame is used. In order to work

properly, the system designer must set-up the sample rate and packet timing so that the following constraint is met:

160 < Average-Frame-Interval * (1 +/- Frame-Drift) * Sample-Rate * (1 +/- Sample-Drift) < 161

For example using an Average-Frame-Interval = 20 ms, and assuming 100 parts per million oscillator accuracy (i.e Frame-Drift

= Sample-Drift = .0001), then the above constraint requires:

8002 Hz < Sample-Rate < 8048 Hz.

In practice Sample-Rate=8002 Hz would be preferred since it is closer to the nominal value of 8000 Hz.

In another example the Sample-Rate = 8000 Hz. as provided by a PCM source. Again assuming 100 parts per million

oscillator accuracy for both clocks (i.e. Frame-Drift = Sample-Drift = .0001), then the above constraint requires:

20.0041 ms. < Average-Frame–Interval < 20.1209 ms.

This can be achieved by slightly decreasing the channel bit rate or adding an extra bit into the channel bit stream every several

voice frames.

Method 2 - Extended Slip Control with Periodic Slip Control Packets

The AMBE-2000 can provide extended slip compensation through the insertion of Slip Control Packets. One method of using

this capability is for the system to periodically insert these Slip Control Packets into the data stream sent to the AMBE-2000.

Note that for this method slip control must be enabled (SLIP_EN is high) on the chip. This approach gives the designer a way

to accommodate clock drift while providing very flexible frame-interval and sample-rate timing. Furthermore minimal system

overhead is required. In this method a Slip Control Packet is generated by the system by setting the Control Words as shown in

Table 7D above, where the Channel Data is the compressed voice data being sent to the AMBE decoder. In the Periodic Slip

Control method such a Slip Control Packet is input into the vocoder chip every N frames. The value of N must be selected by

the design engineer to meet the following constraint: