>On Mar 3, 7:48 am, Randy Yates <ya...@ieee.org> wrote: >> DSP_PHD2BE <krup...@gmail.com> writes: >> > On Mar 2, 7:42 pm, Randy Yates <ya...@ieee.org> wrote: >> >> DSPGURU <krup...@gmail.com> writes: >> >> > [...] >> >> > Hello Mr. Chicken, >> >> >> > If you read about halfband filter (books or old posts in comp.dsp) >> >> > you >> >> > see they have really bad transition and are EXACTLY NOT what you >> >> > want. >> >> > Keep looking. >> >> >> You're answering a question that was not originally asked. However, >> >> with >> >> further information Dave has provided from subsequent posts, it does >> >> appear that he's decimating and thus needs something better. >> >> -- >> >> % Randy Yates % "Ticket to the moon, flight leaves here today >> >> %% Fuquay-Varina, NC % from Satellite 2" >> >> %%% 919-577-9882 % 'Ticket To The Moon' >> >> %%%% <ya...@ieee.org> % *Time*, Electric Light >> >> Orchestrahttp://www.digitalsignallabs.com >> >> > Hello Randy, >> >> > In first post Mr Chicken expresses intrest in "perfect cutoff". >> > Halfband far from perfect. >> >> Yes, he did say that. And therefore he was wrong in an "absolute truth" >> sense since no filter in the real world is a "perfect" lowpass filter. >> >> So then the question was, "How far from perfect is acceptable for >> 'Mr. Chicken'?" Since he did not mention he was decimating, half-band >> filters seemed a perfectly reasonable compromise. >> >> My comment to you stands.>Mr Randy, >Halfband as lowpass filter is what to do AVOID perfect. For pass gain >of 1.0, gain at FS/2 = 0.5 !!! Transition band tend to be >WWWWIIIIDDDDDEEEE !!! Nuf said, but you can have last word. >Regards, >Kamar Ruptan >DSP Guru (not with DSPGURU of Grant Griffin)Hey guys, I'm using this http://users.cscs.wmin.ac.uk/~krukowa/Poly.html it's perfect for the job, and it's really, really fast! Cheers, VelociChicken (from before the dinosaur)
Low pass filter at half Nyquist
Started by ●March 1, 2008
Reply by ●March 3, 20082008-03-03
Reply by ●March 3, 20082008-03-03
On Mar 3, 7:43 am, "VelociChicken" <b...@yahoob.com> wrote:> > Hey guys, I'm using this http://users.cscs.wmin.ac.uk/~krukowa/Poly.html > it's perfect for the job, and it's really, really fast! > > Cheers, > > VelociChicken > (from before the dinosaur)So, if that is a good approach, would you please tell us what are your passband width, passband tolerance, stopband width and stopband attenuation requirements and what design did you get from that approach to meet them? Real numbers clarify the discussion faster than 'really, really', really, really. Dale B. Dalrymple http://dbdimages.com
Reply by ●March 3, 20082008-03-03
dbd wrote:> On Mar 3, 7:43 am, "VelociChicken" <b...@yahoob.com> wrote:>>Hey guys, I'm using this http://users.cscs.wmin.ac.uk/~krukowa/Poly.html >>it's perfect for the job, and it's really, really fast! >> > > So, if that is a good approach, would you please tell us what are your > passband width, passband tolerance, stopband width and stopband > attenuation requirements and what design did you get from that > approach to meet them?Dale, you are asking the very difficult questions. Be merciful :)> Real numbers clarify the discussion faster than 'really, really', > really, really.It shoud be really really fast and it should do a very very good job on filtering. What else do you need to know? Only geeks care about the numbers. Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com
Reply by ●March 3, 20082008-03-03
>> >> Hey guys, I'm using this >> http://users.cscs.wmin.ac.uk/~krukowa/Poly.html >> it's perfect for the job, and it's really, really fast! >> >> Cheers, >> >> VelociChicken > > So, if that is a good approach, would you please tell us what are your > passband width, passband tolerance, stopband width and stopband > attenuation requirements and what design did you get from that > approach to meet them? > > Real numbers clarify the discussion faster than 'really, really', > really, really. > > Dale B. Dalrymple > http://dbdimages.comWhat I meant by fast was, computation wise, it out performed all the other filters I've tried. If you google 'polyphase filter' you'll find better explanations than I can give. I've optimised this code http://www.musicdsp.org/archive.php?classid=3#39 which states that it can achieve up to '150db rejection' with a transition band of .05
Reply by ●March 3, 20082008-03-03
On 3 Mar, 18:36, "VelociChicken" <b...@yahoob.com> wrote:> >> Hey guys, I'm using this > >>http://users.cscs.wmin.ac.uk/~krukowa/Poly.html > >> it's perfect for the job, and it's really, really fast! > > >> Cheers, > > >> VelociChicken > > > So, if that is a good approach, would you please tell us what are your > > passband width, passband tolerance, stopband width and stopband > > attenuation requirements and what design did you get from that > > approach to meet them? > > > Real numbers clarify the discussion faster than 'really, really', > > really, really. > > > Dale B. Dalrymple > >http://dbdimages.com > > What I meant by fast was, computation wise, it out performed all the other > filters I've tried.I know of people who emphasized 'computationally fast' in just about everything they tried to do. While their programs certainly were *faster* than eveybody else's, it took them more than 15 years to get their programs to compute the *correct* numbers...> If you google 'polyphase filter' you'll find better > explanations than I can give. I've optimised this codehttp://www.musicdsp.org/archive.php?classid=3#39which states that it can > achieve up to �'150db rejection' with a transition band of .05150 dB attenuation, that's on the order of 10e-7, right? I'm not sure I believe that - one is pushing the limits of what can be done with single-precision floating point numbers... Rune
Reply by ●March 3, 20082008-03-03
On Mar 3, 9:24 am, Vladimir Vassilevsky <antispam_bo...@hotmail.com> wrote: ...> It shoud be really really fast and it should do a very very good job on > filtering. What else do you need to know? Only geeks care about the > numbers. > > Vladimir Vassilevsky > DSP and Mixed Signal Design Consultanthttp://www.abvolt.comAnd only trolls need fear the light of numbers:) Dale B. Dalrymple
Reply by ●March 3, 20082008-03-03
>> If you google 'polyphase filter' you'll find better >> explanations than I can give. I've optimised this >> codehttp://www.musicdsp.org/archive.php?classid=3#39which states that it >> can >> achieve up to '150db rejection' with a transition band of .05>150 dB attenuation, that's on the order of 10e-7, right? >I'm not sure I believe that - one is pushing the limits >of what can be done with single-precision floating point >numbers... >RuneSingle-precision floating point? *pfft* - nobody uses those anymore! : )
Reply by ●March 3, 20082008-03-03
On Mar 1, 10:02 am, "VelociChicken" <b...@yahoob.com> wrote:> Hello, I'm currently using an FFT to zereo out all the frequences above half > Nyquist for my application. It makes perfect cutoff, ...Unfortunately, zeroing the "upper" bins of an FFT does not make a perfect cutoff filter in the general case. This seems to be a common misconception (why?). The problem is that any sharp transistions between bins may cause the all the frequencies between nearby bins to have wildly varying gains, and for the ripple in your frequency response to not only cover your entire frequency range, but to wrap around (unless you use really long FFTs in relation to your allowed S/N or quantization error). IMHO. YMMV. -- rhn A.T nicholson d.0.t C-o-M
Reply by ●March 3, 20082008-03-03
"DSPGURU" <kruptan@gmail.com> wrote in message news:0f4608f3-0df6-4634-968a-5a725f74b750@m36g2000hse.googlegroups.com... On Mar 1, 3:00 pm, "VelociChicken" <b...@yahoob.com> wrote:> >> My question to you good folks - is there a clever 'trick' to cut > >> frequencies > >> off above this specific (SR / 4) point using time domain techniques? Or > >> a > >> more efficient frequency domain method? > > >> I've tried a cascade of Butterworth filters, but it just isn't steep > >> enough > >> to consider using it in my app. > > >> I know is only a small chance there is a trick for this, but I've got > >> nothing to lose asking! : ) > > >> Thanks, > > >> Dave > > > Hi Dave, > > > Google half-band filters. Every other coefficient (except the middle > > one) is zero. > > Hello, thanks Randy that looks likely to be perfect! I hope I can keep the > coef lengths short... > > D- Hide quoted text - > > - Show quoted text -Hello Mr. Chicken, If you read about halfband filter (books or old posts in comp.dsp) you see they have really bad transition and are EXACTLY NOT what you want. Keep looking. Hmmmmm.... in my world, the transition width of a halfband filter is something that you specify. So, I'm curious why you say this or where, exactly, you read it. Fred
Reply by ●March 3, 20082008-03-03
"DSPGURU" <kruptan@gmail.com> wrote in message news:0f4608f3-0df6-4634-968a-5a725f74b750@m36g2000hse.googlegroups.com... On Mar 1, 3:00 pm, "VelociChicken" <b...@yahoob.com> wrote:> >> My question to you good folks - is there a clever 'trick' to cut > >> frequencies > >> off above this specific (SR / 4) point using time domain techniques? Or > >> a > >> more efficient frequency domain method? > > >> I've tried a cascade of Butterworth filters, but it just isn't steep > >> enough > >> to consider using it in my app. > > >> I know is only a small chance there is a trick for this, but I've got > >> nothing to lose asking! : ) > > >> Thanks, > > >> Dave > > > Hi Dave, > > > Google half-band filters. Every other coefficient (except the middle > > one) is zero. > > Hello, thanks Randy that looks likely to be perfect! I hope I can keep the > coef lengths short... > > D- Hide quoted text - > > - Show quoted text -Hello Mr. Chicken, If you read about halfband filter (books or old posts in comp.dsp) you see they have really bad transition and are EXACTLY NOT what you want. Keep looking. Hmmmmm.... in my world, the transition width of a halfband filter is something that you specify. So, I'm curious why you say this or where, exactly, you read it. Fred
