Originally Posted by nycap
Thanks DSheirer! I know these are real amateur questions but I know nothing of signal processing and have to learn so. . . So your saying that the goal of the filters is to lower the sample rate to between 2 and 4.5 times the highest frequency component. What is the "frequency component"?
So, let your learning begin ... Smith posted his Scientists and Engineer's Guide to DSP online ... and you can read it at:
The Scientist and Engineer's Guide to Digital Signal Processing
Read the whole thing, but specifically read chapter three where he describes sampling rates.
If you find that you really like DSP, I've learned a lot from Lyon's book: Understanding Digital Signal Processing. I've been studying it for 6 or 7 months and find that I keep learning something new every time I reread each chapter 7 - 15 times over ... it eventually sinks in :-)
To answer your question ... filtering reduces/eliminates the higher frequency components before you decimate, so that through decimating (reducing) the sample rate, those higher frequency components don't fold down (alias) on top of your lower frequency components, where your P25 signal frequency components are ... you'll have trouble decoding the signal if it's corrupted by aliasing.