T7570 View Datasheet(PDF) - Agere -> LSI Corporation

Part Name

Description

Manufacturer

T7570

Programmable PCM codec with hybrid-balance filter.

Agere -> LSI Corporation 

T7570 Programmable PCM Codec

with Hybrid-Balance Filter

Functional Description (continued)

Hybrid-Balance Filter (continued)

Data Sheet

October 1996

TIP

ZL

RING

VFXI

ZL'

RVFXI

–

+

SEL

2.4

ZT

±1

INV

SEL 2

HYBAL2

1ST-ORDER

HI FREQ.

FILTER

(REG 3)

ATTENUATOR

GAIN

TO TX

GAIN BLOCK

SET IN REG 2

HYBAL1

1ST- OR

2ND-ORDER

FILTER

(REG 2)

VFRO

+2.4

Figure 3. Block Diagram Hybrid-Balance Filter Network

FROM RX

GAIN BLOCK

5-2788 (C)

Programming the Filter

On initial powerup, the hybrid-balance filter is disabled.

Before the hybrid-balance filter can be programmed, it

is necessary to design the transformer and termination

impedance to meet system 2-wire input return loss

specifications, which are normally measured against a

fixed test impedance (600 Ω or 900 Ω in most coun-

tries). Only then can the echo path be modeled and the

hybrid-balance filter programmed. Hybrid balancing is

also measured against a fixed test impedance, speci-

fied by each national telecommunication administration

to provide adequate control of talker and listener echo

over the majority of their network connections. This test

impedance is ZL in Figure 3. The echo signal and the

degree of transhybrid loss obtained by the programma-

ble filter must be measured from the PCM digital input,

DR0/1, to the PCM digital output, DX0/1, either by digital

test signal analysis or by conversion back to analog by

a PCM codec/filter.

Three registers must be programmed in the T7570 to

fully configure the hybrid-balance filter (refer to Table 2

for Byte 1 addressing):

Register 1: Select/deselect hybrid-balance filter;

invert/noninvert cancellation signal;

select/deselect Hybal2 filter section;

set attenuator.

76

SEL INV

Byte 2 of Register 1

5 43210

SEL2

GAIN (All “0” = MAX)

Register 2: Select/deselect Hybal1 filter;

set Hybal1 to biquad or first order;

select pole and zero frequency.

Register 3: Program pole frequency in Hybal2 filter;

program zero frequency in Hybal2 filter.

Standard filter design techniques can be used to model

the echo path (see Equation 5) and design a matching

hybrid-balance filter configuration. Alternatively, the fre-

quency response of the echo path can be measured

and the hybrid-balance filter designed to replicate it.

T7570 hybrid-balance software is available from your

Lucent Account Representative to aid in selecting the

best balance filter register settings.

12

Lucent Technologies Inc.

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