AN84 datasheet(5/26 Pages) SILABS

5 / 26 page

AN84

Rev. 0.6

1.3. Conclusion

The Si305x DAA product family is designed to increase

the near-echo cancellation with an additional hybrid in

the digital path operating at 16 kHz. Near-end echo is

primarily caused by the mismatch between the ac

termination and the CO termination. The transmit and

receive signal path also affects the echo to a lesser

extent. By introducing a filter that models the near-end

echo 180 degrees out-of-phase to the receive path

echo, the hybrid response can be improved. This

improvement in the hybrid response results in greater

cancellation of the transmit signal when the near-end

echo and digital hybrid response are added together at

the digital hybrid stage.

To generate the coefficients, the 8-tap FIR filter

structure used in the digital hybrid must be taken into

account. This digital filter structure requires the hybrid

response to be represented in the z-domain.

“Appendix A—Sample MATLAB Code” contains sample

MATLAB code to calculate the hybrid coefficients. This

code should help in understanding the process of

calculating the hybrid coefficients.

A hybrid coefficient lookup table can be found in

" Appendix B—Hybrid Coefficient Lookup Tables" on

page 10. These tables provide a set of coefficients to

use for different line conditions.

Figure 6. Echo

AN84 Datasheet(PDF) 5 Page - Silicon Laboratories