Re: digital Sound IF filter must be linear phase?

On May 22, 8:51 pm, cincy...@gmail.com wrote:
> On May 22, 5:02 pm, "dtsao" <tsaod...@yahoo.ca> wrote:
>
> > Hi,
>
> > I need to build a digital filter for a sound signal. The SIF filter must
> > remove the video portion of the channel and be compatible with worldwide
> > standards and have good rejection.
> > From Matlab, it looks like this requires a tremendous amount of
> > coefficients if built using FIR. So I wonder, does anyone know if a SIF
> > filter MUST be linear phase? Or can it be done using an IIR? And what
> > would be the effect if the sound was not passed through a linear phase
> > filter.
> > Thanks.
>
> I don't think anyone can tell you what your filter's requirements are.
> Linear phase is nice, but as you said, you can trade it off for a
> lower-order IIR filter. What format is the sound signal in? Is it
> digitally-encoded, or is it an analog signal? If your sound is analog,
> then you might be in luck; the human ear doesn't care too much about
> distortion of phase response; in many cases, you can design a filter
> that will meet your magnitude requirements and still have normal-
> sounding output.
>
> Jason

If the sound signal is modulated, for example FM, then couldn't
nonlinear filter phase produce distortion in the demod output?

John

In article 
<1179882655.209966.36080@q66g2000hsg.googlegroups.com>, 
sampson164@gmail.com says...
>
>
>On May 22, 8:51 pm, cincy...@gmail.com wrote:
>> On May 22, 5:02 pm, "dtsao" <tsaod...@yahoo.ca> wrote:
>>
>> > Hi,
>>
>> > I need to build a digital filter for a sound signal. The SIF filter must
>> > remove the video portion of the channel and be compatible with 
worldwide
>> > standards and have good rejection.
>> > From Matlab, it looks like this requires a tremendous amount of
>> > coefficients if built using FIR. So I wonder, does anyone know if a 
SIF
>> > filter MUST be linear phase? Or can it be done using an IIR? And 
what
>> > would be the effect if the sound was not passed through a linear 
phase
>> > filter.
>> > Thanks.
>>
>> I don't think anyone can tell you what your filter's requirements are.
>> Linear phase is nice, but as you said, you can trade it off for a
>> lower-order IIR filter. What format is the sound signal in? Is it
>> digitally-encoded, or is it an analog signal? If your sound is analog,
>> then you might be in luck; the human ear doesn't care too much 
about
>> distortion of phase response; in many cases, you can design a filter
>> that will meet your magnitude requirements and still have normal-
>> sounding output.
>>
>> Jason
>
>If the sound signal is modulated, for example FM, then couldn't
>nonlinear filter phase produce distortion in the demod output?

Yes; it will produce nonlinear distortion. This is why linear phase IF 
filtering has been a design goal in FM receiver design for a long time 
and why devices like SAW filters supplanted LC transformer-coupled 
filters in high-quality designs. IIRC, a bit of amplitude rolloff (making 
the shape of the IF filters Guassian, for example) usually causes less 
nonlinear distortion than does phase distortion in the IF domain.