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ZL38003 Datasheet(PDF) 11 Page - Zarlink Semiconductor Inc |
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ZL38003 Datasheet(HTML) 11 Page - Zarlink Semiconductor Inc |
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11 / 51 page  ZL38002 Data Sheet 11 Zarlink Semiconductor Inc. 2.8 Limiters To prevent clipping in the echo paths, two limiters with variable thresholds are provided at the outputs. 2.9 User Gain The user gain function provides the ability for users to adjust the audio gain on all paths. This gain is adjustable from -24 dB to +21 dB in 3 dB steps for the Sout and Rout paths. It is important to use ONLY this user gain function to adjust the speaker volume. The user gain function in the ZL38002 is optimally placed outside the echo path such that no reconvergence is necessary after gain changes, avoiding a burst of each overtime the speaker gain is changed. 2.10 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 2.11 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 1CH - 1DH). 2.12 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. |
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