The Cyber-Spy.Com Usenet Archive Feeds Directly
From The Open And Publicly Available Newsgroup
This Group And Thousands Of Others Are Available
On Most IS NNTP News Servers On Port 119.
Cyber-Spy.Com Is NOT Responsible For Any Topic,
Opinions Or Content Posted To This Or Any Other
Newsgroup. This Web Archive Of The Newsgroup And
Posts Are For Informational Purposes Only.
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 email@example.com,
> nonophil wrote:
>> On 1/11/03 7:03 AM, in article NsOT9.firstname.lastname@example.org, "Ban"
>>> email@example.com 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 ?
Go Back To The Cyber-Spy.Com
Usenet Web Archive Index Of
The sci.electronics.design Newsgroup