Re: Xlating FIR filter delay

[CEL]

Hi Angilberto,

On Thu, 2019-12-19 at 15:13 +0000, Angilberto Muniz Sb wrote:
> In the docs it says the xlating fir filter is a symmetric one, 

certainly not in general. The FIR filter can whatever filter you put in
there. You're thinking of an example somewhere!

So, what docs *specifically* say that? 

> has linear phase shift and causes no phase distortion.

I'd agree with "if it's symmetric, it's got a linear phase shift". I
wouldn't fully agree with "no phase distortion", because it's still a
frequency-dependent shift.

> Lets say I have the following setup:
> 
> x(t) = cos(wt),
> s1(t) = x(t) upshiffted to w1, s2(t) = x(t) upshiffted to w2
> 
> s(t) = s1(t) + s2(t).
> 
> Now, I apply two xlating filters to s(t):
> 
> r1(t) = s(t) filtered and downconverted to w
> r2(t) = s(t) filtered and downconverted to w
> 
> I see a phase shift (or time delay) between x(t) and r1(t) or r2(t).

Yes, that's *group delay*.

Best regards,
Marcus

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