help needed for Butterworth crossover response
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From: gangadhar.m on 16 Jul 2007 08:09 Hi all, I implemented a butterworth crossover with the matlab (code given below). The code is working perfectly for a 4th order butterworth crossover(i.e. 24 dB/Octave). Th added response for the lowpass and high pass filters of the butterworth crossover gives 3 dB increase at the corner frequency which is expected. But the same is not obtained when i use 2nd and 3rd order butterworth filters. Instead of having a 3 dB increase at the cutoff i am getting all pass response for 3rd order and a dip at cutoff for 2nd order butterworth filter. Can anybody help me with this? your help is very much appreciated. See the code below for your reference. %Butterworth crossover filterorder=4; cutoff=1000; fs=44100; [b,a] = butter(filterorder,cutoff/fs,'low'); h1=freqz(b,a,44100); semilogx(10*log(abs(h1)),'r'); hold [b,a] = butter(filterorder,cutoff/fs,'high'); h2=freqz(b,a,44100); semilogx(10*log(abs(h2)),'b'); h=h1+h2; semilogx(10*log(abs(h)),'g'); grid Regards Gangadhar
From: Vladimir Vassilevsky on 16 Jul 2007 11:04 gangadhar.m wrote: > Hi all, > > I implemented a butterworth crossover with the matlab (code given below). > > The code is working perfectly for a 4th order butterworth crossover(i.e. > 24 dB/Octave). Th added response for the lowpass and high pass filters of > the butterworth crossover gives 3 dB increase at the corner frequency which > is expected. > > But the same is not obtained when i use 2nd and 3rd order butterworth > filters. Instead of having a 3 dB increase at the cutoff i am getting all > pass response for 3rd order and a dip at cutoff for 2nd order butterworth > filter. The phase shift at the cutoff frequency depends on the order of the filters. You should combine the filter outputs so they add in phase. > Can anybody help me with this? your help is very much appreciated. I like this phrase. Anybody can certainly help you with this depending on how much is the real appreciation. Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com
From: gangadhar.m on 17 Jul 2007 03:15 Hi, As you said, the phase shift is playing a major role. Can you please tell me how should I proceed now to achieve 3 dB increase at the cut off frequency for all butterworth crossovers(2nd,3rd and 4th orders) irrsepective of change in the order. Actually same kind of behaviour is observed in implememntation of LR Crossovers also (i.e i got correct flat all pass response for LR 4 Crossover and a big dip at cutoff in case of LR 2 Crossover). Is there any way for the phase adjustment while combining the filter responses? I really appreciate your help in this. Thanks and regards Gangadhar > > >gangadhar.m wrote: >> Hi all, >> >> I implemented a butterworth crossover with the matlab (code given below). >> >> The code is working perfectly for a 4th order butterworth crossover(i.e. >> 24 dB/Octave). Th added response for the lowpass and high pass filters of >> the butterworth crossover gives 3 dB increase at the corner frequency which >> is expected. >> >> But the same is not obtained when i use 2nd and 3rd order butterworth >> filters. Instead of having a 3 dB increase at the cutoff i am getting all >> pass response for 3rd order and a dip at cutoff for 2nd order butterworth >> filter. > >The phase shift at the cutoff frequency depends on the order of the >filters. You should combine the filter outputs so they add in phase. > >> Can anybody help me with this? your help is very much appreciated. > >I like this phrase. Anybody can certainly help you with this depending >on how much is the real appreciation. > > >Vladimir Vassilevsky > >DSP and Mixed Signal Design Consultant > >http://www.abvolt.com >
From: Jerry Avins on 17 Jul 2007 09:16 gangadhar.m wrote: > Hi, > > ... Can you please tell > me how should I proceed now to achieve 3 dB increase at the cut off > frequency for all butterworth crossovers(2nd,3rd and 4th orders) > irrsepective of change in the order. ... Butterworth is Butterworth. If you change it in any way, it's not Butterworth any more. Try Linkwitz-Reilly. Jerry -- Engineering is the art of making what you want from things you can get. ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
From: gangadhar.m on 18 Jul 2007 02:11
Dear Jerry, As i mentioned earlier i get the same kind of response in case of Linkwitz Riley filters also..... In net, i find papers where they have flat all pass response for Linkwitz Riley crossovers(LR-2, LR-3 and LR-4 filters) but for me, all pass response is observed only for the LR-4 crossover and big dig at cutoff for the other LR Crossovers. Similar cases are observed for Butterworth crossovers also. Since we are having butterworth crossovers available, i assume that all butterworth crossovers shoud be able to exhibit 3dB increase at the cutoff irrsepective of their order. How can I achieve that? Please help me if my understanding is not correct. Regards Gangadhar >gangadhar.m wrote: >> Hi, >> >> ... Can you please tell >> me how should I proceed now to achieve 3 dB increase at the cut off >> frequency for all butterworth crossovers(2nd,3rd and 4th orders) >> irrsepective of change in the order. ... > >Butterworth is Butterworth. If you change it in any way, it's not >Butterworth any more. Try Linkwitz-Reilly. > >Jerry >-- >Engineering is the art of making what you want from things you can get. >¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ > |