[Freeswitch-dev] why does app "record_session" record extra frequency range

Hi all, I have a bridged call and a home brew module which adds a media bug to the local and partner session before the bridge app in the dial plan. The original speech codec used is G722, which sampling rate is 16KHz. I add a media bug which does low pass filtering the speech to 4KHz. But the spectrum of the recorded .wav file showed that it always contains the freq range from 0K to 8K. The desired spectrum should contain only 0 to 4KHz. I am confused, can anyone show me some hints? Thanks in advance. Best Regards, Han --