AN2832 Freescale Semiconductor / Motorola, AN2832 Datasheet - Page 3

• Manufacturer Part Number: AN2832
• Description: Packet Telephony Remote Diagnostics on the StarCore SC140 Core
• Manufacturer: Freescale Semiconductor / Motorola

The method discussed here for estimating and dynamically emulating impulse response of hybrid circuits is
motivated by channel estimation techniques in ADSL systems [5], which have been extensively employed in the
field and proven to be accurate and reliable. The proposed method is part of a hybrid circuit identification device
(HCID) illustrated in Figure 2.
The HCID unit is an intrusive measurement component of the remote diagnostic device that uses control signals to
specify how the identification process is performed. Upon activation, a periodic training sequence is transmitted to
the hybrid and its reflection is captured. During this process, uncorrelated spurious noise may be injected, such as
audio signals from a telephone unit connected to the hybrid, but the HCID can usually mitigate the noise, even if
the training sequence and the spurious noise coexist as double talk. The goal of the HCID is to provide accurate
impulse response estimates of the hybrid circuit.
The HCID unit can connect to multiple hybrid circuits, using time-division multiplexing (TDM) and different
channel data states to control its operation. Figure 3 shows a prototype of a multi-channel device using the
MSC8101ADS board [7]. Multiple hybrid circuits connect to the ADS board via a T1 line. Software is loaded
through the debug port from a personal computer; control signals are provided, and impulse responses are obtained
via RS-232. The MSC8101 DSP device digitally generates the training signal to estimate hybrid circuits. This
simple prototype can be used to develop an optimized software module for integration on the packet telephony
system illustrated in Figure 1.
A similar configuration for non-intrusive measurements uses an adaptive filter structure that operates in parallel
with an echo canceller infrastructure. Non-intrusive methods are not considered here, but alternatives are readily
available in the literature (see [2]). Echo cancellers can be equipped with a built-in debugging/testing feature that is
selectively enabled to estimate most required measurements during regular in-service operation and pass these
estimates to the media gateway as needed. After a physical hybrid impulse response is estimated, the user can
selectively emulate another hybrid circuit by compensating for the existing echo and injecting a desired echo
signal. Intrusive methods are usually required to perform hybrid circuit emulation, although non-intrusive methods
can be selectively employed. Hybrid emulation proceeds as follows:
Freescale Semiconductor
1.
2.
3.
4.
5.
2-Wire Side
Estimate the impulse response of a hybrid circuit.
Use the estimated impulse response to estimate the echo signal generated by the hybrid circuit.
Subtract the estimated echo signal from the actual echo signal to generate a residual echo signal.
Use a target impulse response to generate a target echo signal.
Add the target echo signal to the residual echo signal to generate an emulated echo signal.
Noise
Packet Telephony Remote Diagnostics on the StarCore SC140 Core, Rev. 1
Hybrid
Figure 2. Hybrid Circuit Identification Device (HCID).
Echo Signal + Noise
Training Sequence
4-Wire/2-Wire
HCID
4-Wire Side
Basics of Remote Diagnostics
Impulse Response
Control Signals
Estimate
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