AN2834 Freescale Semiconductor / Motorola, AN2834 Datasheet - Page 4

• Manufacturer Part Number: AN2834
• Description: AN2834, Packet Telephony Automatic Level Control on the StarCore SC140 Core
• Manufacturer: Freescale Semiconductor / Motorola

Architecture
For example, the bias for a white Gaussian noise with zero mean and unit variance is as follows, which clearly
converges to one for values of a close to one.
2
One major G.169 requirement to constrain the gain adaptation algorithm is that the ALC should not enhance the
background noise. In the current design, the signal is considered background noise if P
background noise requirement can be met by stopping the gain adaptation and disabling gain amplification in the
presence of pure background noise. That is, if the gain g(n) is larger than one, temporarily set it to one; otherwise,
keep it unchanged.
In the G.169 terminology, x(n) is the send-in signal; the receive-in signal, denoted by z(n), is the source of echo
after it is reflected by a hybrid circuit [3]. To detect the presence of echo, the power level P
signal z(n) is estimated using the same single-pole low-pass filter that estimates P
with P
the current design assumes that echo is present. Therefore, the G.169 requirement that the ALC should not track the
level of echo signals is observed if the gain adaptation is stopped when echo is detected. Both the background noise
and the level tracking conditions can be handled at the same time using the following control signals:
where U ( ) denotes the standard step function, which is equal to one if its argument is positive and to zero
otherwise. Based on these control signals, e
adjusted by the ALC device; e
Since the ALC must be implemented in a fixed-point DSP, special signal conditioning must be defined. To keep the
dynamic gain within predefined bounds, the total ALC gain g
gain g(n), such that g
implementation. During the adaptation, lower and upper bounds g
unit total gain is obtained when g(n) = G
imposed on the output signal from the ALC.
The gain adaptation sequence after the G.169 requirements are met is as follows:
4
min
Architecture
1.
2.
3.
4.
5.
6.
. To take into account possible delay in the echo path, if P
Estimate the send-in and receive-in signal levels: P
Compute the control signals: e
Update the gain signal: g
Constrain the gain signal: g
Define an intermediate gain signal: g
Use the control signal e
Packet Telephony Automatic Level Control on the StarCore SC140 Core, Rev. 1
t
(n) = g
s
g(n). This step can also be viewed as a format-converter in a fixed-point
z
(n) = 1 indicates the presence of echo.
x
e
(n): g
1
z
(n) = g(n – 1)[1 + P
2
n
(n) = min {max{g
max
4
g
e
x
(n) = [g
x
(n) and e
b
=
x
(n) = 1 indicates that the send-in signal is strong enough to be gain
= 1 / g
a
n
U
3
=
=
k
(n) = min{g
2
s
=
. Finally, lower and upper bounds y
(n) – g
z
U P
(n).
n
n N
–
1
+
x
U P
1
3
(n),g
(n)]e
a
n
x
(n)e
t
(n) is decoupled into a static gain g
2
(n),G
z
–
x
min
(n) and P
x
x
k
3 a
P
(n) + g
min
(n)(P
},g
z
min
–
(n)
–
max
and g
max
P
ref
}.
min
3
}.
P
– P
(n).
z
max
(n).
min
y
(n – 1))].
at least once in the latest N samples,
are imposed on the dynamic gain. A
x
(n) and P
x
min
(n)
and y
z
Freescale Semiconductor
y
(n) of the receive-in
(n)) and compared
P
min
max
s
. Therefore, the
and a dynamic
are also
Equation 8