On Sat, 14 Apr 2012 17:39:13 -0700, Fred Marshall wrote:> On 4/13/2012 2:20 PM, kaz wrote: >>> Try multiplying by exp(j*2*pi*(-15:15)*.13) instead of exp(j*2*pi* >>> (0:30)*.13). Assuming that you're starting with a filter that has >>> even symmetry around the middle, that will make your resulting filter >>> even- symmetric in the inphase and odd-symmetric in the quadrature, >>> which should make the phase delay constant. >>> >>> Tim Wescott, Communications, Control, Circuits& Software >>> http://www.wescottdesign.com >>> >>> >> Thanks for that. Indeed that is what I found out now. >> >> Kadhiem > > What's the purpose here? > > r b-j mentioned redesign. That's cheap, fast and effective. Why not? > Or is this an exercise in math? > > FredI don't know what Kaz is up to, but designing bandpass FIR filters by designing a lowpass prototype, then modulating it up to the desired center frequency, is a pretty common design technique. -- My liberal friends think I'm a conservative kook. My conservative friends think I'm a liberal kook. Why am I not happy that they have found common ground? Tim Wescott, Communications, Control, Circuits & Software http://www.wescottdesign.com
Shifting FIR frequency centre
Started by ●April 13, 2012
Reply by ●April 15, 20122012-04-15
Reply by ●April 15, 20122012-04-15
On 4/15/12 1:10 AM, Tim Wescott wrote:> On Sat, 14 Apr 2012 17:39:13 -0700, Fred Marshall wrote: > >>> >> >> What's the purpose here? >> >> r b-j mentioned redesign. That's cheap, fast and effective. Why not? >> Or is this an exercise in math? >>maybe it is, but i agree with Tim that if Kaz is trying to scoot his FIR up from being resonant at DC (i guess we call those "low-pass filters") to resonant at something that's higher than DC, you can do that effectively by simply multiplying by 2*cos(w*n). i just hadn't understood that specifically from what Kaz was saying.> I don't know what Kaz is up to, but designing bandpass FIR filters by > designing a lowpass prototype, then modulating it up to the desired > center frequency, is a pretty common design technique.i agree. i just never caught that from the OP. -- r b-j rbj@audioimagination.com "Imagination is more important than knowledge."
Reply by ●April 19, 20122012-04-19
>> by simply multiplying by 2*cos(w*n)... which is BTW very close to multiplication with the rotating exp(i...) phasor, if I use 2 cos(x) = exp(ix) + exp(-ix). That means, cos() will create mirror images at positive AND negative frequencies but real-valued coefficients remain real. And modulation with exp(i...) gives a one-sided filter with complex-valued coefficients.
Reply by ●April 21, 20122012-04-21
On Fri, 13 Apr 2012 12:11:05 -0500, Tim Wescott <tim@seemywebsite.com> wrote:>On Fri, 13 Apr 2012 11:56:40 -0400, robert bristow-johnson wrote: > >> On 4/13/12 4:26 AM, kaz wrote: >>> You design a linear FIR(symmetrical real coefficients) then you decide >>> to move its frequency centre. >> >> how are you doing that? by redesigning it (with all design parameters >> except one that are unchanged)? >> >> if you had an FIR, h[n], and you wanted to simply adjust the >> coefficients to bump up or down the whole frequency response, you can do >> something with the Hilbert transform and analytic signal (the same thing >> we do to make a "frequency shifter"), but there are all sorts of other >> things that happen in the chain, and it doesn't preserve phase >> linearity. > >I'm not sure what you're thinking, but you can easily shift a lowpass >FIR's response in frequency by multiplying it by a sinusoid.[Snipped by Lyons] Hi, Yep, you're correct. However, to ensure that the "shifted" filter is linear phase the sinusoid's time samples *MUST* be symmetrical. Which means the sinusoid's time samples will probably NOT start at zero phase. See Ya', [-Rick-]
