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

Part Name

Description

Manufacturer

T7570

Programmable PCM codec with hybrid-balance filter.

Agere -> LSI Corporation 

Data Sheet

October 1996

T7570 Programmable PCM Codec

with Hybrid-Balance Filter

Functional Description (continued)

Programmable Functions (continued)

To set receive gain, first determine the gain required of

the codec. For line card use, determine the codec’s

allocation to set the overall transmission level point

(TLP) at Tip\Ring accordingly (usually 0 dBm or

–4 dBm).

Once the codec gain is chosen, determine the signal

level that would be delivered to VFRO when the refer-

ence TLP appears at DR. Take the antilog of the gain in

dB (GR) divided by 20 and multiply by the 0.7746 refer-

ence level to obtain the signal level at VFRO in Vrms. As

follows:

(3)

antilog10 (GR / 20) * 0.7746 = Vrms

Finally, convert the signal level output to a decimal

number (n) using the following formula:

(4)

200 * log10 (Vrms / 0.1045) = n

Round n to the nearest integer and convert to binary.

This is the code required by byte 2 of this instruction.

Some examples are given in Table 9.

Table 9. Byte 2 of Receive Gain Instructions

Bit Number

76543210

00000000

00000001

00000010

—

1 0 1 0 1 1 1 0*

—

11000010

1 1 0 0 0 0 1 1†

0 dBm0 Test Level (Vrms)

at VFRI

No Output (low Z to GND)

0.106

0.107

—

0.7746

—

0.975

0.987

* 0 dB path gain setting.

†Programming values greater than those listed in this table are

permitted. However, large signals may cause overload.

Hybrid-Balance Filter

The hybrid-balance filter on the T7570 is a programma-

ble filter consisting of a second-order section, Hybal1,

followed by a first-order section, Hybal2, and a pro-

grammable attenuator. Either of the filter sections can

be bypassed if only one is required to achieve good

cancellation. A selectable 180° inverting stage is

included to compensate for interface circuits that invert

the transmit input relative to the receive output signal.

The second-order section is intended mainly to balance

low-frequency signals across a transformer SLIC, and

the first-order section is intended to balance midrange

to higher audio-frequency signals.

As a second-order section, Hybal1 has a pair of low-

frequency zeros and a pair of complex conjugate

poles. When configuring the Hybal1, matching the

phase of the hybrid at low- to midband frequencies is

most critical. Once the echo path is correctly balanced

in phase, the magnitude of the cancellation signal can

be corrected by the programmable attenuator.

The second-order mode of Hybal1 is most suitable for

balancing interfaces with transformers having high

inductance of 1.5 H or more. An alternative configura-

tion for smaller transformers is available by converting

Hybal1 to a simple first-order section with a single real

low-frequency pole and zero. In this mode, the

pole/zero frequency can be programmed.

Many line interfaces can be adequately balanced by

use of the Hybal1 filter only, in which case the Hybal2

filter should be deselected to bypass it.

Hybal2, the higher-frequency first-order section, is pro-

vided for balancing an electronic SLIC and is also help-

ful with a transformer SLIC in providing additional

phase correction for mid- and high-band frequencies,

typically 1 kHz to 3.4 kHz. Such a correction is particu-

larly useful if the test balance impedance includes a

capacitor of 100 nF or less, such as the loaded and

nonloaded loop test networks in the United States.

Independent placement of the pole and zero location is

provided.

Figure 3 shows a simplified diagram of the local echo-

path for a typical application with a transformer inter-

face. The magnitude and phase of the local echo sig-

nal, measured at VFXI, are a function of the termination

impedance ZT, the line transformer, and the impedance

of the two-wire loop, ZL. If the impedance reflected

back into the transformer primary is expressed as ZL',

then the echo path transfer function from VFRO to VFXI

is the following:

(5)

H(W) = ZL' /(ZT + ZL')

The signal level returned at VFXI must be between

–2.5 dB to –10.25 dB over the voice band, relative to

the output at VFRO, in order for the hybrid balance

function to be effective. Signals outside this range

exceed the range of programmability of the hybrid

path, and the software will provide unacceptable hybrid

balance performance over the voice band.

Lucent Technologies Inc.

11

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