Q is centre frequency over bandwidth. Higher vales for q will make the
filter more selective. It can also be used to filter particular
musical intervals. A Q value of 1.5 will filter one octave. A perfect
fifth is 2.5. Some filters let you set the bandwidth, others let you
set the quality factor. It depends on the particular filter opcode you
use.



On 17 March 2011 08:50, Stefan Thomas  wrote:
> Dear community,
> I have a question about the mode opcode.
> I don't understand exactly what the kQ parameter does.
> Could be someone so kind to explain it to me?
> Thanks,
> Stefan
>


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1.5 represents a ratio of 2:1 which is an octave.

On 17 March 2011 19:15, Stefan Thomas  wrote:
> Dear Rory,
> thanks for Your explaniations.
> What I don't understand till now is:
> why is 1.5 an octave? Shouldn't it be 2?
>
> 2011/3/17 Rory Walsh 
>>
>> Q is centre frequency over bandwidth. Higher vales for q will make the
>> filter more selective. It can also be used to filter particular
>> musical intervals. A Q value of 1.5 will filter one octave. A perfect
>> fifth is 2.5. Some filters let you set the bandwidth, others let you
>> set the quality factor. It depends on the particular filter opcode you
>> use.
>>
>>
>>
>> On 17 March 2011 08:50, Stefan Thomas 
>> wrote:
>> > Dear community,
>> > I have a question about the mode opcode.
>> > I don't understand exactly what the kQ parameter does.
>> > Could be someone so kind to explain it to me?
>> > Thanks,
>> > Stefan
>> >
>>
>>
>> Send bugs reports to the Sourceforge bug tracker
>>            https://sourceforge.net/tracker/?group_id=81968&atid=564599
>> Discussions of bugs and features can be posted here
>> To unsubscribe, send email sympa@lists.bath.ac.uk with body "unsubscribe
>> csound"
>>
>
>


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How does 1.5 represent the ratio 2/1?

I would have thought that 1.5 = 1.5/1, not 1.5 = 2/1.

-Lou

On Mar 17, 2011, at 6:06 PM, Rory Walsh wrote:

> 1.5 represents a ratio of 2:1 which is an octave.
>
> On 17 March 2011 19:15, Stefan Thomas  > wrote:
>> Dear Rory,
>> thanks for Your explaniations.
>> What I don't understand till now is:
>> why is 1.5 an octave? Shouldn't it be 2?
>>
>> 2011/3/17 Rory Walsh 
>>>
>>> Q is centre frequency over bandwidth. Higher vales for q will make  
>>> the
>>> filter more selective. It can also be used to filter particular
>>> musical intervals. A Q value of 1.5 will filter one octave. A  
>>> perfect
>>> fifth is 2.5. Some filters let you set the bandwidth, others let you
>>> set the quality factor. It depends on the particular filter opcode  
>>> you
>>> use.
>>>
>>>
>>>
>>> On 17 March 2011 08:50, Stefan Thomas >> >
>>> wrote:
>>>> Dear community,
>>>> I have a question about the mode opcode.
>>>> I don't understand exactly what the kQ parameter does.
>>>> Could be someone so kind to explain it to me?
>>>> Thanks,
>>>> Stefan
>>>>
>>>
>>>
>>> Send bugs reports to the Sourceforge bug tracker
>>>            https://sourceforge.net/tracker/?group_id=81968&atid=564599
>>> Discussions of bugs and features can be posted here
>>> To unsubscribe, send email sympa@lists.bath.ac.uk with body  
>>> "unsubscribe
>>> csound"
>>>
>>
>>
>
>
> Send bugs reports to the Sourceforge bug tracker
>            https://sourceforge.net/tracker/?group_id=81968&atid=564599
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> To unsubscribe, send email sympa@lists.bath.ac.uk with body  
> "unsubscribe csound"
>



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Q is the ratio of the center frequency over the bandwidth. The formula for deriving Q (and the various other formulas that rely on that definition for Q) were designated by engineers to be useful in designing electronic circuits, not by or for musicians (at least not at first). If I recall correctly many digital filters are designed such that Q is linearly linked to a parameter of the algorithm. It may not even be possible to have octaves linked efficiently to those filters (though of course with hardware the definition of efficient has been changing).

http://en.wikipedia.org/wiki/Q_factor


here is a UDO that does that conversion, input is octaves, output is q:


       opcode ioct2q,i,i
ioct xin
i2n  pow 2, ioct
iq   = sqrt(i2n)/(i2n-1)
     xout iq
     endop

       opcode koct2q,k,k
koct xin
k2n  pow 2, koct
kq   = sqrt(k2n)/(k2n-1)
     xout kq
     endop


I can see the advantage of not doing all those exponentiations and square roots at k rate (not to mention audio rate), but probably the better way to do it would be an opcode that uses a lookup table like cpsoct and ampdb etc. do.


Louis Cohen wrote:
> How does 1.5 represent the ratio 2/1?
> 
> I would have thought that 1.5 = 1.5/1, not 1.5 = 2/1.
> 
> -Lou
> 
> On Mar 17, 2011, at 6:06 PM, Rory Walsh wrote:
> 
>> 1.5 represents a ratio of 2:1 which is an octave.
>>
>> On 17 March 2011 19:15, Stefan Thomas
>>  wrote:
>>> Dear Rory,
>>> thanks for Your explaniations.
>>> What I don't understand till now is:
>>> why is 1.5 an octave? Shouldn't it be 2?
>>>
>>> 2011/3/17 Rory Walsh 
>>>>
>>>> Q is centre frequency over bandwidth. Higher vales for q will make the
>>>> filter more selective. It can also be used to filter particular
>>>> musical intervals. A Q value of 1.5 will filter one octave. A perfect
>>>> fifth is 2.5. Some filters let you set the bandwidth, others let you
>>>> set the quality factor. It depends on the particular filter opcode you
>>>> use.
>>>>
>>>>
>>>>
>>>> On 17 March 2011 08:50, Stefan Thomas
>>>> 
>>>> wrote:
>>>>> Dear community,
>>>>> I have a question about the mode opcode.
>>>>> I don't understand exactly what the kQ parameter does.
>>>>> Could be someone so kind to explain it to me?
>>>>> Thanks,
>>>>> Stefan
>>>>>
>>>>
>>>>
>>>> Send bugs reports to the Sourceforge bug tracker
>>>>            https://sourceforge.net/tracker/?group_id=81968&atid=564599
>>>> Discussions of bugs and features can be posted here
>>>> To unsubscribe, send email sympa@lists.bath.ac.uk with body
>>>> "unsubscribe
>>>> csound"
>>>>
>>>
>>>
>>
>>
>> Send bugs reports to the Sourceforge bug tracker
>>            https://sourceforge.net/tracker/?group_id=81968&atid=564599
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>> "unsubscribe csound"
>>
> 
> 
> 
> Send bugs reports to the Sourceforge bug tracker
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> csound"
> 
> 



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As an example, assume the center frequency is 300. If Q is 1.5, then the 
bandwidth is 200, so the band goes from 200 to 400.

Louis Cohen wrote:
> How does 1.5 represent the ratio 2/1?
>
> I would have thought that 1.5 = 1.5/1, not 1.5 = 2/1.
>
> -Lou
>
> On Mar 17, 2011, at 6:06 PM, Rory Walsh wrote:
>
>> 1.5 represents a ratio of 2:1 which is an octave.



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To be pedantic, an octave is a Q of ~ 1.414

Mark Van Peteghem wrote:
> As an example, assume the center frequency is 300. If Q is 1.5, then the
> bandwidth is 200, so the band goes from 200 to 400.
> 
> Louis Cohen wrote:
>> How does 1.5 represent the ratio 2/1?
>>
>> I would have thought that 1.5 = 1.5/1, not 1.5 = 2/1.
>>
>> -Lou
>>
>> On Mar 17, 2011, at 6:06 PM, Rory Walsh wrote:
>>
>>> 1.5 represents a ratio of 2:1 which is an octave.
> 
> 
> 
> Send bugs reports to the Sourceforge bug tracker
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> Discussions of bugs and features can be posted here
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> csound"
> 
> 



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on 2011-03-18 at 09:19 Mark Van Peteghem wrote:

>As an example, assume the center frequency is 300. If Q is 1.5, then the 
>bandwidth is 200, so the band goes from 200 to 400.

i guess that some confusion might arise if you forget for a moment that, in
terms of pitch, the bandwidth is not symmetric. in your example, the
filter would go from the fifth below (2/3) the center frequency to the
fourth above (4/3), thus forming an octave. 

(i have no idea if this comment adds anything of interest to the thread...)



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