On Nov 20, 2010, at 12:43 PM, Wayne Burdick wrote: > Any difference must be attributed to noise and signal conditions, filter > bandwidth settings, or APF tuning vs. actual signal pitch. This is the nature > of APF; it is most effective under specific conditions. Try turning it on, > and if it doesn't help, leave it off.
Those running Mac OS X 10.6 (Snow Leopard) on your computers might want to play with this application to hear what changing filter parameters does to an "APF": http://homepage.mac.com/chen/w7ay/cocoaFilter/index.html The "stock" IIR filter in the program has adjustable Q and center frequencies, with a GUI so you can see the transfer function and compare different settings with what you "hear." There is also an experimental FIR filter in it that attempts to achieve better SNR than the IIR without introducing the "hollow" flat-topped narrow bandpass DSP sound. The IIR that is in the program is just a simple analog second order bandpass filter that is transformed into the digital domain. But you can change both the FIR and IIR filters with just a few lines of C code to experiment with something other than the "stock" filters. cocoaFilter is not intended as an end-user program, but as a sandbox for people to try other filter ideas without having to learn all of Cocoa and Core Audio. if you are familiar with DSP but don't want to deal with the Mac OS Core Audio or the Cocoa user interface, this framework should make it easy -- you only need to modify a few lines of the filter code. Just think of the "stock" filters as a starting point. Go build your own "INRADS" :-). Modern computers are so fast that you don't need a DSP chip to experiment with real time audio filters -- the "stock" filters in cocoaFilter use only about 2% of the processor load of *one* core of my Nehalem-based MacPro, and I didn't attempt *any* optimization (audio is filtered one sample at a time with the dynamic messaging of Objective-C; that is as inefficient as you can get). Only when you start building audio filters that are 10 times more complex, will you need to use Mac OS X's Grand Central Dispatch in your filters to spread the work among the cores of your processor. Audio Units and the AU Lab program in the Mac OS X Developer disk (it is on every Mac OS X Installer DVD) is another platform you can use to experiment with filters. It is probably a bit more complex than changing a few lines of code in cocoaFilter. cocoaFilter uses Audio Unit calls but you don't *need* to understand that code to experiment with other filters :-). 73 Chen, W7AY ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
