Audio signal series' beginning points(about 50) distort after a bandpass filter I am using some bandpass filters to detect a certain freq of the sampled audio series in noisy environment. Everything is OK except this:
I found that for many bandpass filters I tried(butterworth, window based FIR etc.) The audio signals' beginning points(about 50~100) always distort after the filter. For example:
[![enter image description here][1]][1]

I am new to DSP so maybe I didn't express in an scientific way... What I mean by "distortion" is that in some cases these beginning points should have been filtered out but they were not, and in other cases vice versa.After these distorted points, everything becomes fine... 
Could u plz help me on this issue?
Thank you very much

To make this question more clear, below is a plot consists of 2 waveforms, the blue one is the original audio signal sampled, and the brown one is the sampled signal passed butterworth filter.It doesn't like filter delay stuff.
That's the filter's output transient, whose length is dependent on the effective length of the filter impulse response. If you are performing block based filtering, then only the very first block should exhibit it and later subsequent blocks should not display this transient, due to properly set initial values.