zl50235 Zarlink Semiconductor, zl50235 Datasheet - Page 11

• Manufacturer Part Number: zl50235
• Description: 16 Channel Voice Echo Canceller
• Manufacturer: Zarlink Semiconductor

2.1
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.
2.2
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.
In the G.168 standard, the echo return loss is expected to be at least 6 dB. This implies that the Double-Talk
Detector Threshold (DTDT) should be set to 0.5 (-6 dB). However, in order to achieve additional guardband, the
DTDT is set internally to 0.5625 (-5 dB).
In some applications the return loss can be higher or lower than 6 dB. The ZL50235 allows the user to change the
detection threshold to suit each application’s need. This threshold can be set by writing the desired threshold value
into the DTDT register.
The DTDT register is 16 bits wide. The register value in hexadecimal can be calculated with the following equation:
where 0 < DTDT
Example:For DTDT = 0.5625 (-5 dB), the
2.3
Integrated into the ZL50235 is a Path Change Detector. This permits fast reconvergence when a major change
occurs in the echo channel. Subtle changes in the echo channel are also tracked automatically once convergence
is achieved, but at a much slower speed.
Adaptive Filter
Double-Talk Detector
Path Change Detector
hexadecimal value becomes
hex(
0.5625 * 32768
(dec)
< 1
Slow
)
= 4800
relative to normal convergence for non-zero values of Slow. If Slow equals zero,
hex
DTDT
Lsin > Lrin + 20log
(hex)
Zarlink Semiconductor Inc.
= hex(DTDT
ZL50235
11
10
(dec)
(DTDT)
* 32768)
Data Sheet