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Subject: Re: Building a simple video low pass filter
Date: Mon, 13 Jan 2003 17:52:59 +0100
Organization: Wanadoo, l'internet avec France Telecom
NNTP-Posting-Date: 13 Jan 2003 16:56:17 GMT
On 1/13/03 10:48 AM, in article firstname.lastname@example.org,
> nonophil wrote:
>> 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 =
> around 5ns.
> 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.
Parallel cap = ghosting
Series inductance = ghosting
Parallel cap + series inductance = NO ghosting !
What could it mean ?
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