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參數(shù)資料
| 型號(hào)： | 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(yè)數(shù)： | 11/28頁(yè) |
| 文件大?。?/td> | 567K |
| 代理商： | T7570 |
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11
Lucent Technologies Inc.
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 VF
R
O when the refer-
ence TLP appears at D
R
. Take the antilog of the gain in
dB (G
R
) divided by 20 and multiply by the 0.7746 refer-
ence level to obtain the signal level at VF
R
O in Vrms. As
follows:
(3)
antilog
10
(G
R
/ 20) * 0.7746 = Vrms
Finally, convert the signal level output to a decimal
number (n) using the following formula:
(4)
200 * log
10
(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
* 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
Bit Number
7 6 5 4 3 2 1 0
0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 1
0 0 0 0 0 0 1 0
—
1 0 1 0 1 1 1 0*
—
1 1 0 0 0 0 1 0
1 1 0 0 0 0 1 1
0 dBm0 Test Level (Vrms)
at VF
R
I
No Output (low Z to GND)
0.106
0.107
—
0.7746
—
0.975
0.987
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 VF
X
I, are a function of the termination
impedance Z
T
, the line transformer, and the impedance
of the two-wire loop, Z
L
. If the impedance reflected
back into the transformer primary is expressed as Z
L
'
,
then the echo path transfer function from VF
R
O to VF
X
I
is the following:
(5)
H(W) = Z
L
'
/(Z
T
+ Z
L
'
)
The signal level returned at VF
X
I must be between
–2.5 dB to –10.25 dB over the voice band, relative to
the output at VF
R
O, 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.
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參數(shù)描述 |
|---|---|
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