Hello, Back in the days of the analog snyth., there was an SSM 2040 chip that was designed as a tracking filter. I see these from time to time on E-Bay. A reference is Electronic Music Circuits by Berry Kline. Vern Jones Timothy Box wrote: > > Many thanks for all the replies > > The solution is not as simple as I had hoped. Still I know now what I have > to look for. I might reduce the freq. range to 250hz to 3khz which will make > things a lot easier. I will try the active filter route and may be the > passive with switched in caps. > > Cheers > > Tim > > -----Original Message----- > From: pic microcontroller discussion list > [mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Michael Rigby-Jones > Sent: Thursday, February 13, 2003 10:17 AM > To: PICLIST@MITVMA.MIT.EDU > Subject: Re: [EE]: Rounding the square wave > > > -----Original Message----- > > From: Olin Lathrop [SMTP:olin_piclist@EMBEDINC.COM] > > Sent: Wednesday, February 12, 2003 6:16 PM > > To: PICLIST@MITVMA.MIT.EDU > > Subject: Re: [EE]: Rounding the square wave > > > > > I need to be able to round off the square wave coming from the HWPM on > > my > > > PIC. The HPWM output is being used to generate sound. I am currently > > using > > > the standard 2 Rs + 2 caps which works in a fashion but is still rather > > > harsh. The freq range is potentionaly from 247 hz up to 12.5 kHz so an > > > active filter is needed (I think). Can any body point me in the right > > > direction? > > > > You shouldn't need an active filter, at least with a 20MHz PIC. In fact, > > you probably don't need much filtering at all. A 20MHz PIC can do 8 bit > > resolution PWM at 78KHz. That's over 6 times the highest frequency of > > interest, and certainly higher than anyone can hear. A passive two pole > > filter with both poles at 17.7KHz (half octave above 12.5KHz) will > > attenuate the PWM frequency by about 25dB. > > > > Note that all the PWM frequencies are well above the audible range, so the > > only reason for filtering is so that downstream active electronics doesn't > > do strange things due to the high out of band frequency content. Most > > decent amplifiers will have a low pass filter right on the input anyway, > > but I agree that filtering and buffering it yourself is a good idea. > > > > The "quality" of the sound is much more a function of the PWM resolution > > and the quality of the function to create the output samples. Doing a > > sine, for example, on the fly is too much for a simple PIC. This is best > > done with a lookup table. > > > Olin, I jumped to the same conclusion as you initialy which is why I asked > the OP for some more information. He is not using PWM to produce e.g. a > sine, but is simply setting the pulse width to 50% and using the CCP as a > simple square wave tone generator. The filter he requires is not there to > simply remove the PWM carrier, which would be trivial. Unfortunately the > desired range spans several octaves which requires a tracking filter as > described by other people. > > A possibility is to use a voltage controlled filter (VCF) and use another > (low pass filtered) PWM output as the control voltage. This would also let > you have some nice swept filter effects if you wanted to. Unfortunately > VCF's are not trivial to design, although I would expect there are some > dedictaed IC's. I'm sure any synth enthusiasts could give you more > information. > > The idea of a switched capacitor filter is nice, apart from the fact that > the clock you must supply is a fairly high multiple of the filters corner > frequency, i.e. 50 or 100 times, so driving such a filter directly from the > CCP module is not possible, you'd need some way of upping the frequency e.g. > phase locked loop, and it all starts getting a bit complicated for a simple > project. > > Regards > > Mike > > -- > http://www.piclist.com hint: The PICList is archived three different > ways. See http://www.piclist.com/#archives for details. > > -- > http://www.piclist.com hint: The PICList is archived three different > ways. See http://www.piclist.com/#archives for details. -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.