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Subject: Re: Building a simple video low pass filter
Date: Mon, 13 Jan 2003 09:48:58 +0000 (UTC)
NNTP-Posting-Date: Mon, 13 Jan 2003 09:48:58 +0000 (UTC)
User-Agent: tin/1.5.14-20020926 ("Soil") (UNIX) (Linux/2.4.20 (i686))
> On 1/11/03 7:03 AM, in article NsOT9.email@example.com, "Ban"
>> firstname.lastname@example.org wrote:
>>> nonophil wrote:
>>>> I am trying to build a simple video low pass filter. My goal is to
>>>> reduce the output bandwidth of a digibox because my TV can't handle
>>>> sharp signal transitions without ringing artefacts.
>>>> The filter should cut frequencies > 5Mhz without creating an
>>>> impedance mismatch (digibox, cable and TV are 75 Ohms).
>>>> I used an interactive web-based L-C filter designer to mock-up a 3
>>>> poles 75 Ohms low pass filter
>>>> Would that work or am I missing something ? Any other ideas ?
>>> Unless youve got miles of cable between the digibox and the
>>> TV, I'd try something simpler.
>>> A capacitor on the end of the cable will act as a lowpass filter.
>>> Yes, it will screw up the impedance, but for a couple of meters of
>>> cable it's not an issue.
>> Actually the proposed filter will work well and will also "screw up" the
>> impedance (200R at 2.7MHz,0R at 6+MHz), but since both sides are terminated
>> (hopefully) the reflections will not have too much of an effect.
>> I found 0.82uH 470pF and 5.6uH to give a very good pulse response. But try
>> the 1capacitor solution first.
> The 1 cap solution works with a slight ghosting. I don't know if it is a
> reflection. My cable is quite short (1m). BTW putting a 1 series inductance
> gives almost the same result as 1 parallel cap (including the ghosting).
1m cable, will probably have a propogation delay around 1.5*1m*3.3ns/m =
Assuming the video signal is up to 5Mhz, that's essentially nothing.
You are probably seeing artefacts from something else in the signal chain
that were not visible before. (PAL delay line?)
Also possible is that a video amp is reacting poorly to the extra capacitance
and ringing a bit.
http://inquisitor.i.am/ | mailto:email@example.com | Ian Stirling.
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