极值点偏移,极值点

matlab 自带函数：function [upperEnv,lowerEnv] = envelope(x, n, method)
函数可使用三种方式计算：如下matlab帮助文档：

% [YUPPER,YLOWER] = ENVELOPE(X,N,ENVTYPE) specifies the type of envelope% to return. The default is 'analytic':% 'analytic' - returns the analytic envelope via an N-tap FIR filter% 'rms' - returns the RMS envelope of X over a sliding window% of N samples. % 'peak' - returns the peak envelope of the signal using a spline% over local maxima separated by at least N points.

方法一：
peak：使用样条返回信号的峰值包络线（由至少N个点分隔的局部极大值）
方法二：
对比直接利用findpeaks函数查找峰值，然后用插值interp1得到包络线；
…

N = length(x);
[pks,p]=findpeaks(x);
return=interp1([0 p N+1],[0 x§ 0],1:N,‘pchip’);