[sdiy] Butterworth design

[Scott Gravenhorst]

If I may do some guessing here - it seems like your sample rate is around 22 KHz. 
The filter is probably a anti-alias filter.  The 22 KHz cutoff is more restrictive
than the original 24.1 KHz cutoff.  I would say that it will work both ways, but with
possibly losing some audible "brightness" in the sound with the 22.5KHz cutoff.  

Here's an experiment you can do - try it both ways and compare with your ears.  You
may find it does matter or not, but then you'll know and it seems simple enough to
try.  Use sounds that have transients like square, rectangle, pulse or sawtooth. 
Listen for missing harmonics at the high end, for missing "brightness".

Csaba Zvekan <czvekan at gmail.com> wrote:
>Sorry,
>
>It's true it's probably off topic. But then again maybe not .I am  
>trying to get some DCO's with a dsPIC and a PCM1753. That one requires  
>that filter design. I guess I could use the two resistors in series.  
>But my question is more if it is even relevant to the sound? Is it  
>critical?
>
>Csaba
>
>
>On Jun 17, 2008, at 3:46 PM, Dave Magnuson wrote:
>
>> Hi Csaba,
>>
>> This is an off-list reply
>>
>> Why not use a series combination?   4K7 + 560R gets you really close  
>> (5260 ohm)
>>
>> Dave
>>
>> ----- Original Message ----- From: "Csaba Zvekan" <czvekan at gmail.com>
>> To: <Synth-diy at dropmix.xs4all.nl>
>> Sent: Tuesday, June 17, 2008 9:24 AM
>> Subject: [sdiy] Butterworth design
>>
>>
>>> Hi group,
>>>
>>> I am gonna have to design a Butterworth 2nd Order Low Pass Filter  
>>> for  the Audio Output after a 24-bit Burr&Brown DAC PCM1753.
>>> Now my little software for filter design from Texas Instrument is  
>>> is suggesting a 5.23K resistor. If my cutoff frequency is supposed  
>>> to be 24.1 kHz -60dB. Of course I don't have that value handy.
>>> My question is if I'd use a 5K6 1% resistor it filters something  
>>> like 22.5 kHz almost 2k off .
>>> Is that very critical to the sound & distortion? Or are these just  
>>> standard values /recommendation that have somewhat of a toleranc.
>>> I mean the frequency is not even audible up in that range. Or am I  
>>> loosing valuable high end frequencies with it.But still, I  
>>> understand  it causes some digital noise from the ∆∑-DAC that  
>>> needs filtering.  Does it have to be exact the frequency 24.1kHz?
>>>
>>> Csaba
>>>
>>>
>>>
>>> _______________________________________________
>>> Synth-diy mailing list
>>> Synth-diy at dropmix.xs4all.nl
>>> http://dropmix.xs4all.nl/mailman/listinfo/synth-diy
>>
>
>
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-- ScottG

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