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From: "John Jardine"
Subject: Re: FILTER for Computer Phone Line
Date: Fri, 3 Jan 2003 22:38:53 -0000
NNTP-Posting-Date: 3 Jan 2003 22:25:19 GMT
X-Newsreader: Microsoft Outlook Express 5.00.2615.200
Howard Davis wrote in message
> I'm having dial-up ISP disconnect problems due to phone line noise, and
> to design a lowpass filter (3 pole; -18 db/octave) that will minimize
> out-of-band noise but not slow downloads or uploads. I would greatly
> appreciate the technical information I need to do this:
> Optimum cutoff frequency (-3db).
> Source impedance of phone line.
> Input impedance of a 56K hardware modem.
> Is a Butterworth (maximally flat) or Bessel (phase linear) filter response
> most desirable?
You won't be able to add any improvement to the digital filters already in
your modem as they are nearly ideal brick wall types.. If they can't cope
with the noise then nothing can.
The problem here seems to be that the signal to noise ratio coming off the
line is simply too bad for the information to be recovered effectively, no
matter how clever the electronics are. You're up at the theoretical Shannon
limit and any extra filtering can only reduce available bit transfer rates
(both for TX and Rx).
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