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參數(shù)資料
| 型號(hào)： | ZL38002QDG |
| 廠商： | ZARLINK SEMICONDUCTOR INC |
| 元件分類： | 數(shù)字傳輸電路 |
| 英文描述： | Low-Voltage Acoustic Echo Canceller With Noise Reduction |
| 中文描述： | DATACOM, ISDN ECHO CANCELLER, PQFP48 |
| 封裝： | 7 X 7 MM, 1 MM HEIGHT, MS-026ABC, TQFP-48 |
| 文件頁數(shù)： | 8/47頁 |
| 文件大?。?/td> | 440K |
| 代理商： | ZL38002QDG |
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ZL38002
Data Sheet
8
Zarlink Semiconductor Inc.
that no reconvergence is necessary after gain changes, avoiding a burst of each overtime the speaker gain is
changed.
1.10 AGC
The AGC function is provided to limit the volume in the speaker path. The gain of the speaker path is automatically
reduced during the following conditions:
When clipping of the receive signal occurs
When initial convergence of the acoustic echo canceller detects unusually large echo return
When howling is detected
The AGC can be disabled by setting the AGC- bit to 1 in MC control register
1.11 Programmable Gain Pad
The ZL38002 has three gain pads located at Sin, Sout and at the adaptive filter (Xin). These gain pads are intended
to be set once during initialization and not be used as dynamic gain adjustments. The purpose of theses gainpads
are to help fine tune the performance of the acoustic echo canceller for a particular system.
For example, the gain pad can be used to improve the subjective quality in low ERL environments. The ZL38002
can cancel echo with a ERL as low as 0 dB (attenuation from Rout to Sin). In many hand free applications, the ERL
can be low (or negative). This is due to both speaker and microphone gain setting. The speaker gain has to be set
high enough for the speaker to be heard properly and the microphone gain needs to be set high enough to ensure
sufficient signal is sent to the far end. If the ERL (Acoustic Attenuation - speaker gain - microphone gain) is greater
than 0 dB, then the echo canceller cannot cancel echo. To overcome this limitation, the gain pad at Sin and Sout
can be used to lower the Sin level (and therefore the ERL) by 6 dB, perform the echo cancellation then amplify it at
Sout by 6 dB. This will have the effect having 0dB gain between Sin and Sout for double talk signals while injecting
a additional 6 dB attenuation for the echo return. It is important to reduce the DTDT threshold (Page 0 address 30)
to match the Sin/Sout gain settings.
The gain can be accessed through Customer Gain Control Registers 1 - 2 (Page 0, Address 1C
H
- 1D
H
).
1.12 Mute Function
A pcm mute function is provided for independent control of the Receive and Send audio paths. Setting the MUTE_R
or MUTE_S bit in the MC register, causes quiet code to be transmitted on the Rout or Sout paths respectively. The
ZL38002 has an optional DC offset control. The user can add a positive offset to the mute value. This is controlled
through the DC offset register (Page 0, Address 03h)
Quiet code is defined according to the following table.
LINEAR
16 bits
2’s complement
SIGN/
MAGNITUDE
μ
-Law
A-Law
80h
CCITT (G.711)
μ
-Law
A-Law
+Zero
(quiet code)
0000h
FFh
D5h
Table 1 - Quiet PCM Code Assignment
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