Re: Filter design.



The amount of the distortion due to aliasing equals to the sum of power for all components higher then Fs/2.

Indeed the speech spectrum envelope falls at about 12dB/oct at high frequencues. The amount of the high frq content in the clear voice is very low. So just the direct sampling at 8kHz will be generally OK for the phone-like quality speech.

The first order filter is just the first order filter. It can't be Butterworth or anything else. Of course it is going to be of very little help for antialiasing.

Vladimir Vassilevsky

DSP and Mixed Signal Design Consultant

http://www.abvolt.com

tanmay.zargar@xxxxxxxxx wrote:

On the system that I am working with right now, I cannot afford to
sample at 16kHz (in fact any frequency higher than 8kHz) since I am
running out of MIPS. So I HAVE to sample at 8kHz and sadly I do not
have very good analog pre-filtering for this sampling rate. Change of
processor/clock rates/bus speeds is also not an option.

Now I know that there is going to be 'some' energy in the signal at
frequencies higher than 4kHz before sampling it (at 8kHz). This energy
at frequencies higher than 4kHz will cause aliasing after sampling. I
have implemented a simple first-order Butterworth low pass filter with
a cut off of 3.6kHz. I don't think that this filter will help since the
aliasing damage is already done. I just need to figure out how much the
damage is. Is there a way to measure the distortion due to aliasing
(say by viewing the spectrum of the signal before sampling)?

.