AMBE-2000™ Vocoder Chip

User’s Manual Version 3.0

page 44

Confidential Proprietary, Subject to Change

0.25 > (1/N) > Average-Frame-Interval * Sample-Rate * (Frame-Drift + Sample-Drift)

For example with an Average-Frame-Interval = .02 (i.e. 20 ms) and a Sample-Rate = 8000, then with 100 parts per million

oscillator accuracy (i.e. Frame-Drift = Sample-Drift = .0001), then above constraint equates to 4 < N < 31.25, and N=30 would

be a reasonable selection. In this case the system would input the specified Slip Control Packet into the AMBE-20X0 vocoder

chip every 30’th frame enabling the vocoder chip to adjust for the actual clock drift.

***RECOMMENDED***

Method 3 - Extended Slip Control with as needed Slip Control Packets

The preferred method for using Slip Control Packets is to monitor the availability of data from the AMBE-2000 vocoder chip

and to only input Slip Control Packets into the data stream sent to the AMBE-2000 as needed. Note that for this method slip

control must be enabled (SLIP_EN is high) on the chip. This method provides compensation for the widest range of clock drift

(1.2% or 12000 ppm), with the greatest flexibility in frame-interval and sample-rate timing. In this method the same Slip

Control Packets shown in Table 7D are inserted into the data stream going to the AMBE-2000. However, unlike in the

Periodic method, the packets are not input at regular intervals but are instead only input to the chip when needed. The

recommended procedure for this Method of Slip Control is for the system to read a packet from the AMBE-2000 at regular

fixed frame intervals where the fixed interval must be within the range [19.875 – 20.125] ms for a sample rate = 8000 Hz. The

system application should check each word output by the AMBE-2000 and should continue requesting words from the chip

until the packet Header followed by 23 data words are received. If the words received before the Header word consist of only a

single termination word (0xfffe) then no further action is required. However if two or more termination words are received

prior to the Header then the system should input a Slip Control Packet to the AMBE-2000 on the next available transmission

into the chip (i.e. the next packet going into the AMBE-2000 decoder should be a Slip Control Packet). Once this Slip Control

Packet is input into the AMBE-2000 it will respond within 1-2 frames by advancing the time when packets are ready for

transmission by 125 microseconds. Note that this procedure also may require a small amount of buffering in the system to

account for the fact that the packet my not be ready for some small time (< 125 microseconds) after it is first requested.