zl50235 Zarlink Semiconductor, zl50235 Datasheet - Page 27

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

10.0
The Exponential Decay registers (Decay Step Number and Decay Step Size) and Flat Delay register allow the LMS
adaptation step-size (MU) to be programmed over the length of the FIR filter. A programmable MU profile allows the
performance of the echo canceller to be optimized for specific applications. For example, if the characteristic of the
echo response is known to have a flat delay of several milliseconds and a roughly exponential decay of the echo
impulse response, then the MU profile can be programmed to approximate this expected impulse response thereby
improving the convergence characteristics of the Adaptive Filter. Note that in the following register descriptions, one
tap is equivalent to 125 µs (64 ms/512 taps).
FD
SSC
NS
7-0
7-0
2-0
Amplitude of MU
Flat Delay
Decay Step Size Control
Decay Step Number: This register defines the number of steps to be used for the decay of MU where each
Functional Description of Register Bits
1.0
2
-16
is defined as FD
canceller FIR filter. The valid range of FD
extended-delay mode. The default value of FD
of MU. The decay rate is defined as a decrease of MU by a factor of 2 every SS taps of the FIR filter,
where SS = 4 x2
the FIR filter. The default value of SSC
step has a period of SS taps (see SSC
(512 or 1024) - [Decay Step Number (NS
For example; If NS
512 - [4 x (4x2
Flat Delay (FD
:
This register defines the flat delay of the MU profile, (i.e., where the MU value is 2
4
)] = 256 taps for a filter length of 512 taps.
SSC
7-0
7-0
7-0
)
x 8 taps. For example; If FD
2-0
=4 and SSC
:
. For example; If SSC
This register controls the step size (SS) to be used during the exponential decay
FIR Filter Length (512 or 1024 taps)
Figure 12 - The MU Profile
2-0
Zarlink Semiconductor Inc.
=4, then the exponential decay start value is 512 - [NS
2-0
ZL50235
2-0
). The start of the exponential decay is defined as: Filter Length
7-0
7-0
is 04
2-0
27
) x Step Size (SS)] where SS = 4 x2
is: 0 ≤ FD
7-0
= 4, then MU is reduced by a factor of 2 every 64 taps of
hex
7-0
.
is zero.
= 5, then MU=2
7-0
≤ 64 in normal mode and 0 ≤ FD
Step Size (SS)
-16
for the first 40 taps of the echo
Number of Steps (NS
SSC
2-0
.
-16
Data Sheet
7-0
Time
). The delay
7-0
7-0
)
≤ 128 in
x SS] =