1. Introduction
On the Android platform, the microphone and speaker environment creates the acoustics. Two acoustic echo paths can be determined. The first path is the air, and the second is the handset case. In the traditional
2G/3G circuit switch communication mode, the acoustic echo cancellation function is built into the GSM chip. The LMS algorithm is commonly used for echo cancellation. However, Android-based VoIP is a pure software app. The communication protocol, SIP/RTP, the speech codec, G.711/G729, and acoustic echo cancellation must all be implemented by software. Acoustic echo cancellation is difficult for two reasons: First, the LMS-based echo cancellation algorithm requires high computational power. Second, the record/play process on the Android platform is conducted frame by frame. The minimal frame size is 10 ms (80 sample points), thus lengthening the filter length and computation of the LMS algorithm. Previous echo cancellation methods that have been proposed for the Android platform are listed as follows