Many people these days are getting apodizing and minimum phase filters confused. They are very different things. Neither of these requires any special algorithm, they can both be implemented in standard FIR filters. Nobody has intellectual property on either concept. Certain companies may claim IP for a specific implementation, but that does not mean they have claim to the basic concept which can be implemented by anybody.
If you already have an FIR filter mechanism adding an apodizing or minimum phase filter (or both) is trivial. Both apodizing and MP filters are compromises, you are trading off one aspect for another, the trick is to come up with exactly where you want to be in the compromise space, and THAT can only be done by listening. So on to implementing a custom filter in the Touch. Currently there is no way to replace the filter in the Touch with another filter. The chip MAY have a way to implement an external filter, but that definitely has not been implemented in the hardware. I don't think its possible to completely turn off the inbuilt filter and do it entirely in the CPU. For some reason when DAC chip makers disable the internal filter they use different pins for the inputs. So you COULD do it, but the hardware would have to be designed for it. I'll have to check on the specific DAC chip and see. You COULD implement an apodizing or MP filter in software with a 2X oversampling which converts 44.1 to 88.2, but then you are cascading two filters, the one in the software and the one in the hardware. This is fraught with peril. Its likely to result is something you do not expect. Even if you design the software filter for a specific DAC chip's filter, there is no guarantee the combination can get close to what you want in the first place if you were designing the hardware filter from scratch. Designing a software filter such that when it is cascaded with the hardware filer will produce the desired filter function is NOT trivial. In order to do this correctly you need the specific details of the filter already in the DAC chip. Nobody puts this in their spec sheets. So you will have to run some tests to try and figure out what it is. And just to make it more fun, most DAC chips already have cascaded filters, which is going to make the task MUCH more difficult. Of course you can try 5000 filters and see what sounds the best. And even that is a VERY tiny slice of the possible space. You might get lucky, but probably not. Less someone gets the wrong idea, I AM definitely in favor of custom DAC filters, I don't think ANY chip maker has correctly implemented their digital filters. I implement my own in my own DAC designs, BUT doing that in the Touch is going to be extremely difficult. The best bet is to just enjoy the Touch as is. John S. -- JohnSwenson ------------------------------------------------------------------------ JohnSwenson's Profile: http://forums.slimdevices.com/member.php?userid=5974 View this thread: http://forums.slimdevices.com/showthread.php?t=77869 _______________________________________________ Touch mailing list [email protected] http://lists.slimdevices.com/mailman/listinfo/touch
