# POTSBAND Design filter

Started by August 3, 2015
```Hi,
Can anyone give introduction about POTSBAND Design filter? and what the purpose of this filter???

I found matlab code for this filter,
function [b,a]=bandfilter(fs, f1, f2)
%POTSBAND Design filter for 300-3400 telephone bandwidth [B,A]=(FS)
%Input: FS=sample frequency in Hz
%Output: B/A is a discrete time bandpass filter with a passband gain of 1
%The filter meets the specifications of G.151 for any sample frequency and has a gain of -3dB at the passband edges.

if( nargin<2 )
f1 =  300 ;
end
if( nargin<3 )
f2 =  3400 ;
end

szp=[0.19892796195357i; -0.48623571568937+0.86535995266875i];
szp=[[0; -0.97247143137874] szp conj(szp)];

zl=2./(1-szp*tan(f1*pi/fs))-1;

al=real(poly(zl(2,:)));
bl=real(poly(zl(1,:)));
sw=[1;-1;1;-1];
bl=bl*(al*sw)/(bl*sw);

zh=2./(szp/tan(f2*pi/fs)-1)+1;

ah=real(poly(zh(2,:)));
bh=real(poly(zh(1,:)));
bh=bh*sum(ah)/sum(bh);
b=conv(bh,bl);
a=conv(ah,al);

Thankyou
```
```On Mon, 03 Aug 2015 17:42:51 -0700, Biruntha Gnaneswaran wrote:

> Hi,
> Can anyone give introduction about POTSBAND Design filter? and what the
> purpose of this filter???
>
> I found matlab code for this filter,
> function [b,a]=bandfilter(fs, f1, f2)
> %POTSBAND Design filter for 300-3400 telephone bandwidth [B,A]=(FS)
> %Input: FS=sample frequency in Hz %Output: B/A is a discrete time
> bandpass filter with a passband gain of 1 %The filter meets the
> specifications of G.151 for any sample frequency and has a gain of -3dB
> at the passband edges.
>
> if( nargin<2 )
>   f1 =  300 ;
> end if( nargin<3 )
>   f2 =  3400 ;
> end
>
> szp=[0.19892796195357i; -0.48623571568937+0.86535995266875i];
> szp=[[0; -0.97247143137874] szp conj(szp)];
>
> zl=2./(1-szp*tan(f1*pi/fs))-1;
>
> al=real(poly(zl(2,:)));
> bl=real(poly(zl(1,:)));
> sw=[1;-1;1;-1];
> bl=bl*(al*sw)/(bl*sw);
>
> zh=2./(szp/tan(f2*pi/fs)-1)+1;
>
> ah=real(poly(zh(2,:)));
> bh=real(poly(zh(1,:)));
> bh=bh*sum(ah)/sum(bh);
> b=conv(bh,bl);
> a=conv(ah,al);
>
> Thankyou

POTS is Plain Old Telephone Service.

When a comment in some code says "XXXX specification", that means you're
supposed to USE GOOGLE to see if you can find the spec -- G.151 in this
case.  If you find a G.151 specification that's about the viscosity of
toilet bowl cleaners or the diameter of ramming rods for muzzle-loader
rifles, you most likely need to keep looking until you find one
pertaining to telephony.

--

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