Reply by Robert Orban May 23, 20072007-05-23
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.
Reply by May 22, 20072007-05-22
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