From: gangadhar.m on
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


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
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
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

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.
>¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
>