[sdiy] Butterworth design

[Tom Wiltshire]

Csaba,

If it's an anti-aliasing filter that follows the DAC, then no, it  
isn't critical at all. It's there to remove digital noise from the  
DAC, so you want it to work as well as possible, but it won't matter  
if you change the cutoff frequency a bit. Like Scott said, if you  
lower it too much, the worst that can happen is that you'll lose a  
little brightness. You can just call that 'retro character' - who  
wants a brittle, digital sound anyway?! So, just use the nearest  
handy values on the lowside.

Let us know if you come up with something with the dsPIC digital  
oscillator. I've been working (fairly unsuccessfully) on this too.

Regards,
Tom



On 17 Jun 2008, at 14:57, Csaba Zvekan 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 Æ·-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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