At 09:17 AM 10/29/2003 -0500, you wrote:
Might want to write a new > "energy detector" algorithm in dsp.c though based on a wideband/low Q > resonator approach (move the pole way in towards the origin) > as opposed to > narrow band goertzels (pole on the unit circle). More robust > for this type > of work.
Where does one go to learn this terminology and the math to implement it?
Caltech. Take PP's class. Just don't fail.
Seriously, any introductory book on filter design. When you design a filter, you write out the transfer function. zeroes in the numerator are called "zeroes". Zeroes in the denominator are called "poles". A filter will pass frequencies near its poles. It will suppress frequencies near its zeroes. As you move poles out towards the unit circle, the filter becomes less stable. You can imagine a "hitting" a filter with a burst at a single frequency. A filter with poles near the origin will die off very quickly. As you move towards the unit circle, the filter will "ring" longer and longer. On the unit circle there is no dampening at all. Outside of the unit circle, the filter becomes divergent. The order of the filter is simply the number of zeroes.
Goertzel (named after the guy who invented them) filters have their poles on the unit circle. Asterisk uses these to recognize DTMF and other tones. They are easy to calculate mathematically, but they have terrible properties if you need a wide frequency response over a sustained period. A resonator is essentially the same filter but moved inside the unit circle so that it will relax over time. It takes a few more operations to calculate, but you have alot more control over the final shape.
If you are interested, I would suggest getting a book on the subject. Just go to Amazon and find one you think meets your requirements. There are several. Any book on Digital Signal Processing will have at least a few chapters on filter design. Pay specific attention to the chapters on IIR filters. They will teach you about poles, zeroes and stability.
I would also recommend this tutorial on the web if you're interested:
http://www.dewtronics.com/tutorial.html
Chris
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