digital Sound IF filter must be linear phase?

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.

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dtsao wrote:
> Hi,
> 
> I need to build a digital filter for a sound signal.

A sound signal where?  Sound signals can occur in a lot of different 
places -- perhaps someone will pick up on the 'SIF' as being a keyword 
(do you mean sound intermediate frequency, as in TV, perhaps?).  The 
rest of us, not being omniscient, may not know what you mean.

> 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?

Surely looking at the applicable standards would answer this?  If you're 
talking about the FM sound subcarrier in analog television a simple 
example would answer your question: if IIF filters wouldn't work, then 
TVs wouldn't have had sound until about 10 years ago.

> Or can it be done using an IIR?

And who says you can't be linear phase enough using IIR filters?  After 
all, FM radios were standing by ready to blast out rock & roll since 
before it was even invented.  Until recently they _all_ used IIR 
filters, implemented with coils & caps or with piezoelectric resonators, 
and they work just fine.

There are many things that you can do with IIR filters to increase their 
phase linearity.  You pay in complexity vs. rolloff, but perhaps not as 
much as you would with an all-FIR filter.

> And what
> would be the effect if the sound was not passed through a linear phase
> filter.

That depends on what form the sound is encoded in, and how you are 
decoding it.  Perhaps if you gave us that background...

-- 

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Posting from Google?  See http://cfaj.freeshell.org/google/

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html

yes, i mean sound intermediate frequency, as in for TV signals.

_____________________________________
Do you know a company who employs DSP engineers?  
Is it already listed at http://dsprelated.com/employers.php ?

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

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

Tim Wescott wrote:
(snip)

> And who says you can't be linear phase enough using IIR filters?  After 
> all, FM radios were standing by ready to blast out rock & roll since 
> before it was even invented.  Until recently they _all_ used IIR 
> filters, implemented with coils & caps or with piezoelectric resonators, 
> and they work just fine.

I thought that there were some things you could do with analog
filters that you couldn't do with digital IIR due to quantization
and sample rates.  Though with a sufficiently high sample
rate you may not notice the difference.

-- glen