CMX649 CML Microcircuits, CMX649 Datasheet - Page 12

• Manufacturer Part Number: CMX649
• Description: ADM Codec
• Manufacturer: CML Microcircuits

The de-scrambler receives the scrambled data from the data slicer and de-scrambles it to the original
Nine example maximal length codes are represented below through their polynomial coefficients which
5.1.10 Voice Activity Detector (VAD)
ADM Codec
data as long as the selected LFSR maximal length sequence is the same as that in the transmitting
scrambler. The de-scrambler block has the same configuration as the scrambler and is self-
synchronizing. Both the scrambler and de-scrambler can be bypassed.
can be directly programmed in Bits 9-0 of the SCRAMBLER CONTROL Register ($71):
The VAD function is implemented with an energy detector circuit. This circuit consists of an absolute
value function, an integrator and a threshold detector. The threshold detector level and the integrator
time constants (i.e. attack and decay time control) are user programmable via the DECODE and
ENCODE VAD THRESHOLD Registers ($D2 and $E2) and the DECODER and ENCODER MODE AND
SETUP Registers ($D0 and $E0). Referring to Figure 11, the input to the VAD comes from the PCM
signal. The signal is rectified and averaged with a lossy integrator. The output of the integrator is
compared to the VAD threshold to derive the logic signal VAD_OUT. If VAD_OUT is a logic one, signal
energy greater than the threshold is present. If VAD_OUT is a logic zero, signal energy is below the
threshold. Attack and decay times for the decoder VAD and encoder VAD can be independently
controlled via the DECODER and ENCODER MODE AND SETUP Registers ($D0 and $E0). Typical
attack and decay times used for detecting voice activity are 5ms and 150ms, respectively. The energy
levels may be read from DECODE and ENCODE VAD LEVEL OUTPUT Registers ($D4 and $E4) for the
decoder and encoder and used to adaptively set the detector threshold value by observing the energy
level of background noise.
2003 CML Microsystems Plc
$D0 & $E0
Constants
Time
constant factor
Length
attack time
SIGNAL
10
PCM
2
3
4
5
6
7
8
9
decay
Figure 11 VAD Block Diagram
Polynomial coefficients in hex format
C
1/16
1/32
1/4
1/8
| y |
12
0x00C
0x08E
0x003
0x006
0x014
0x030
0x060
0x110
0x240
0.5
VAD LEVEL
+
$D4 & $E4
-
$D2 & $E2
Threshold
VAD
VAD
OUT
CMX649
D/649/2