探索卷积的不同的实现方法，而非用matlab中的卷积命令(explore how this can be calculated efficiently using the FFT. This entails understanding the
difference between linear and circular convolution. We will explore using the FFT and compare it with a conventional
direct implementation of the convolution. To obtain the greatest advantage of the fft all the filters that we will use
have a sample length that is an integer power of 2. In general, it is always possible to pad an FIR filter with zeros to
achieve this.)