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參數(shù)資料
| 型號： | T7570 |
| 元件分類： | Codec |
| 英文描述： | T7570 Programmable PCM Codec with Hybrid-Balance Filter |
| 中文描述： | T7570可編程PCM編解碼器與混合平衡濾波器 |
| 文件頁數(shù)： | 12/28頁 |
| 文件大?。?/td> | 567K |
| 代理商： | T7570 |
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12
Lucent Technologies Inc.
T7570 Programmable PCM Codec
with Hybrid-Balance Filter
Data Sheet
October 1996
Functional Description
(continued)
Hybrid-Balance Filter
(continued)
5-2788 (C)
Figure 3. Block Diagram Hybrid-Balance Filter Network
TIP
RING
Z
L
Z
L
'
Z
T
VF
X
I
VF
R
O
–
+
TO TX
GAIN BLOCK
HYBAL2
1ST-ORDER
HI FREQ.
FILTER
(REG 3)
HYBAL1
1ST- OR
2ND-ORDER
FILTER
(REG 2)
±
1
FROM RX
GAIN BLOCK
ATTENUATOR
GAIN
R
VFXI
2.4
+2.4
SEL
INV
SEL 2
SET IN REG 2
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
or 900
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 Z
L
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,
D
R
0/1, to the PCM digital output, D
X
0/1, 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.
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.
Byte 2 of Register 1
5
4
SEL2
7
6
3
2
1
0
SEL INV
GAIN (All “0” = MAX)
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