# Really simple DSP high-pass filter question

Good afternoon. I've worked with DSPs for some time now, but i'm scratching my head at a simple high-pass filter implementation that yields unexpectedly attenuated output.

My scenario is simple, i've used the fir1 function in MATLAB to generate 65 coefficients of a LPF. The freq response shows the passband is 0db, as it should be.

However when i use these coefficients in a HPF application and input a signal that should be well into the passband i get an output that's attenuated by almost 1000.

Has anyone else used MATLAB fir1() to generate HPF coefficients, is there a scaling factor or some application detail i've glossed over?

-Alexander