AN2834 Freescale Semiconductor / Motorola, AN2834 Datasheet - Page 2

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

Basics of Automatic Level Control
1
ALC, sometimes called automatic gain control (AGC), is a well-known application in communication systems with
a given input signal conditioned to produce an output signal as constant in amplitude as possible, while supporting
a wide gain range and controlled gain-reduction and gain-recovery characteristics. A nonlinear feedback loop
controls the signal level by adjusting a linear gain in either the receive or transmit paths of the system. For
example, an early hardware implementation includes a variable resistor, voltage amplifier, and feedback loop for
adjusting a resistor control amplifier [1]. In most systems, the target signal level is a user-defined constant value.
ALC has become an important application in digital telephony systems using network-based equipment, where a
restricted gain or loss is introduced in the transmit path to maintain the transmit signal level at a predetermined
value. In this context, ALC is part of a broader class of voice quality enhancement (VQE) devices [2], which may
include network echo cancellation [3], noise reduction, and other related signal enhancement processing blocks.
When ALC is used in these systems, the overall speech quality should be kept within acceptable mean opinion
score (MOS) levels [4], [5].
The International Telecommunication Union (ITU) developed the G.169 Recommendation [6] for network ALC
devices to ensure network stability and minimize degradation of speech or voice-band signaling tones when the
ALC device is in the communication network. However, the ITU does not specify a standard ALC algorithm,
planning rules, or target levels for ALC devices. Furthermore, some critical deployment aspects, as well as possible
extensions on either transmit or receive path directions, are still under development. The main G.169 requirements
are as follows, and specific G.169 tests are defined to ensure that these three requirements are not violated:
An ALC device can be modeled mathematically as a linear gain function g(n) that multiplies an input signal x(n)
and generates an output signal y(n) = g(n) x(n). The input and output signal levels are estimated by filtering |x(n)|
and |y(n)|
transform H(z) = (1 – a)/(1 –az
derivation uses a second-order model (that is, k = 2). The input and output signal levels are defined in Equation 1.
An ALC device can be designed by minimizing the mean squared error signal e(n) = P
reference (target) signal level for y(n). The gradient of the instantaneous squared error signal with respect to the
gain g(n) is given in Equation 2.
Where we use the approximation P
Equation 3.
2
Basics of Automatic Level Control
•
•
•
k
, k = 1, 2, . . . , with a low-pass filter. For simplicity, a standard single-pole low-pass filter with a Z
The ALC should not enhance the background noise.
It should not track the level of echo signals.
It should not distort signaling tones, such as DTMF, voice band data, and facsimile transmission.
Packet Telephony Automatic Level Control on the StarCore SC140 Core, Rev. 1
g n
–1
--------------- -
), 0 < a < 1 is employed to estimate signal levels. The analysis in the following
e
g n
2
y
(n)
n
=
P
P
x
y
g n 1
=
n
n
g
2
2 – e n
(n)P
–
=
=
aP
aP
x
(n). The resulting LMS adaptation algorithm is defined in
1 + P
x
y
---------------- -
P
n 1
n 1
g n
y
–
–
n
x
n P
+
+
4 – g n e n P
1 a
1 a
ref
–
–
–
P
x
y
2
2
y
n
n
n 1
–
x
n
ref
– P
Freescale Semiconductor
y
(n), where P
Equation 1
Equation 2
Equation 3
ref
is a
k