AM79C02AJC datasheet(43/48 Pages) AMD

43 / 48 page

SLAC Products

In the continuous Adaptation mode, the algorithm is

switched on (via MPI Command 19) after a call is con-

nected and remains on until the call ends. In this way,

the B filter is continually being optimized to the re-

ceived signal.

In the Adapt and Freeze modes, the algorithm is used

only when a line is brought into service and the DSLAC

device is activated. The algorithm is switched on and is

allowed to converge with the received signal, which is

a bandlimited white noise signal generated in the ex-

change for this purpose. The noise signal need only be

injected for less than a second to yield converged coef-

ficients. The Adaptive mode then is switched off (via

Command 19).

The converged coefficients may be read out of the

DSLAC device (using MPI Command 31) and stored for

future reference. The DSLAC device is now optimized

for general input signals.

Adaptive Filter Programming

The purpose of the B filter is to cancel the received

signal that leaks across the hybrid into the transmit path.

The B filter transfer function must match (as closely as

possible) the transfer function of the echo path.

There are two programmable registers associated with

the adaptive B filtering. The Echo Path Gain (EPG) is a

programmable value that predicts the amount of the

receive signal leaking across the hybrid to the transmit

path. The EPG is used as part of an algorithm, which

stops the adaptive filter from iterating in the presence

of signals from the subscriber line (nearend talker).

The Error Level Threshold (ELT) is a programmable val-

ue that determines the transhybrid loss the adaptive

filter attempts to meet. The adaptive algorithm contin-

ues to iterate until it meets the loss requirement speci-

fied by the ELT. Both the EPG and ELT values are

generated by the WinSLAC™ software program (for-

merly AmSLAC2™ software). Please refer to the soft-

ware technical documentation.

User Test Modes

The DSLAC device supports testing by providing both

digital and analog loopback paths as shown in Figure 8.

In the TSA Loopback mode, the DR input is connected

to the DX output in the Time Slot Assigner circuitry. The

TSA Loopback mode is programmed via Command 21.

A different type of digital loopback is provided when the

AISN register is programmed with a value of 10000. In

this case, the AISN circuitry is disabled and the VOUT

pad is connected internally to VIN. This allows the D/A

and A/D converters to be included in the digital loopback

test. This mode is programmed via Command 13. Note

that the signal, which is connected internally from VOUT

to VIN, also is present on the VOUT pin.

The VIN input can be connected to the VOUT output

through the Z filter for analog loopback. The response

of the line to low frequencies can be tested by disabling

the high-pass filter. Additionally, the receive and trans-

mit paths may be cut off.

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