zl50211 Zarlink Semiconductor, zl50211 Datasheet - Page 10

• Manufacturer Part Number: zl50211
• Description: 256 Channel Voice Echo Canceller
• Manufacturer: Zarlink Semiconductor

•
1.1
The adaptive filter adapts to the echo path and generates an estimate of the echo signal. This echo estimate is then
subtracted from Sin. For each group of echo cancellers, the Adaptive Filter is a 1024 tap FIR adaptive filter which is
divided into two sections. Each section contains 512 taps providing 64 ms of echo estimation. In Normal
configuration, the first section is dedicated to channel A and the second section to channel B. In Extended Delay
configuration, both sections are cascaded to provide 128 ms of echo estimation in channel A. In Back-to-Back
configuration, the first section is used in the receive direction and the second section is used in the transmit
direction for the same channel.
1.2
Double-Talk is defined as those periods of time when signal energy is present in both directions simultaneously.
When this happens, it is necessary to disable the filter adaptation to prevent divergence of the Adaptive Filter
coefficients. Note that when double-talk is detected, the adaptation process is halted but the echo canceller
continues to cancel echo using the previous converged echo profile. A double-talk condition exists whenever the
relative signal levels of Rin (Lrin) and Sin (Lsin) meet the following condition:
where DTDT is the Double-Talk Detection Threshold. Lsin and Lrin are signal levels expressed in dBm0.
A different method is used when it is uncertain whether Sin consists of a low level double-talk signal or an echo
return. During these periods, the adaptation process is slowed down but it is not halted. The slow convergence
speed is set using the Slow sub-register in Control Register 4. During slow convergence, the adaptation speed is
reduced by a factor of 2
adaptation is halted completely.
Each echo canceller in the EVP has four functional states: Mute, Bypass, Disable Adaptation and Enable
Adaptation. These are explained in the section entitled Echo Canceller Functional States.
(channel N)
(channel N)
ST-BUS
PORT2
Adaptive Filter
Double-Talk Detector
Rout
Sin
µ/A-Law/
Linear
µ/A-Law
Linear/
Slow
Figure 4 - Functional Block Diagram of an Echo Canceller
0 to -12dB
Level Adjust
relative to normal convergence for non-zero values of Slow. If Slow equals zero,
Disable Tone
Programmable Bypass
Detector
Instability
Detector
0 to -12dB
Level Adjust
Offset
Null
Echo Canceller (N), where 0 < N < 31
Lsin > Lrin + 20log
Zarlink Semiconductor Inc.
-
ZL50211
Σ
Narrow-Band
10
0 to -12dB
Level Adjust
Detector
Non-Linear
Processor
10
(DTDT)
Microprocessor
Double - Talk
Detector
Interface
MuteR
Offset
0 to -12dB
Level Adjust
Null
Path Change
MuteS
Disable Tone
Detector
Detector
µ/A-Law/
Linear
µ/A-Law
Linear/
(channel N)
Data Sheet
(channel N)
ST-BUS
PORT1
Sout
Rin