Implementation And Optimization Of Echo Cancel Algorithm In VoIP

With the advent of IP voice technology, VoIP (Voice over IP) receives more and more application in recent years. Compared with the traditional telephone, IP phones are widely used because its high network bandwidth utilization, low-cost and flexibility to provide value-added features. However, the quality of voice can not be guaranteed because Internet is the best-effort model. If IP phone wants to compete with traditional telephone, it must improve its voice quality. One of key technologies is echo cancellation in video conference.There are two types of echo in telephone network, the first type, line echo can be generated because of the mismatch between the subscriber loop and switch office where two wires and four wires are connected. The second type, acoustic echo arises with the invention of hand-free phones, in which the microphone and loudspeaker are coupled. Acoustic echo is characterized with long delay and complex and unstable environment, and need more genius algorithms.The core of echo cancellation is adaptive filter theory, according to which residual echo is got by subtracting the echo replica from the actual echo and is further used to adapt the coefficients of the filter dynamically. Traditional adaptive filter algorithm has achieved good performance in dealing with line echo which has short delay. To handle acoustic echo, large-taps filters in frequency domain are often used, which make good use of FFT's efficiency to meet the real time requirements.This thesis is to investigate the line echo cancellation and acoustic echo cancellation algorithm, multidelay block frequency domain adaptive filter (MDF) and expects to implement and optimization their codec on AR1688, a fixed-point digital signal processor. It is meaningful to realize the AEC codec in AR1688 because of the low cost and high precision. The implementation was in two steps:first, make the float point to fixed-point conversion in C with optimization. Then translate the C codecs to DSP assembly codes, which is compatible with ADSP2181, debug the echo cancellation codec, the next work is the optimization of the echo cancellation codec according to the hardware features and instruction characteristic of AR1688 chip in order to reduce the complexity of algorithm, improve the speed of program, adapt the requirements of processor, and reduce the power of equipment. Eventually, we successfully achieved the requirements of real-time AEC on IP phone which the core element is AR1688 chip.