AMBE-2000 Digital Voice Systems, Inc., AMBE-2000 Datasheet - Page 45

• 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.

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:
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:
0.25 > (1/N) > Average-Frame-Interval * Sample-Rate * (Frame-Drift + Sample-Drift)
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20.0041 ms. < Average-Frame–Interval < 20.1209 ms.
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Word #
12-23
10
11
0
1
2
3
4
5
6
7
8
9
0x07xx
0x1355
0x0000
0x0000
0x0000
0x0000
0x0000
0x0000
0x0000
0x0000
0x13ec
0x009f
Value
Slip Control Packet ID, xx=CWD1 (See Table 5-B) Typical value xx=00
Table 7D: Slip Control Packet
AMBE-2000™ Vocoder Chip
User’s Manual Version 4.0
Slip Control indicator for AMBE-2000™
Channel Data for AMBE® voice decoder
Slip Control data
Must be 0x0000
Must be 0x0000
Must be 0x0000
Must be 0x0000
Must be 0x0000
Must be 0x0000
Must be 0x0000
Must be 0x0000
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