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Re: digital Sound IF filter must be linear phase?

Started by Unknown May 22, 2007
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.