[SI-LIST] Re: AC coupling at video circuits
- From: Roy M <roymesi@xxxxxxxxx>
- To: steve weir <weirsi@xxxxxxxxxx>
- Date: Mon, 7 Feb 2011 13:44:03 +0200
And another question:
If a black video will enter AC coupling capactior - will it go up or down
(along the Y scale)? I mean after the AC couple - what will I see in the
scope?
and what about white video?
Roy
On Mon, Feb 7, 2011 at 1:42 PM, Roy M <roymesi@xxxxxxxxx> wrote:
> * If you set the high-pass cut-off frequency too high, you will see a
> shift in the reference luminance level across the upper left to lower right
> diagonal of each frame.*
>
> Why is that?
>
> On Mon, Feb 7, 2011 at 1:01 PM, steve weir <weirsi@xxxxxxxxxx> wrote:
>
>> Analog NTSC video is encoded into 59.94Hz frames. If you set the
>> high-pass cut-off frequency too high, you will see a shift in the reference
>> luminance level across the upper left to lower right diagonal of each frame.
>>
>> The coupling capacitor value is determined by the transmission line
>> impedance for long cables, or the receiver load impedance for localized
>> circuits. There are video line drivers that use feedback to correct for a
>> certain amount of DC drift, allowing smaller than normal capacitor values to
>> be used.
>>
>> Steve.
>> Roy M wrote:
>>
>>> Hi,
>>> How do I know which is the correct value for AC coupling in video
>>> interfaces
>>> (i mean the cap value)?
>>> I saw some circuits using 10uF while others use much less... both worked,
>>> by
>>> the way...
>>> is there some kind of rule of thumb for that issue?
>>> Regards,
>>> Roy
>>>
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>>
>>
>> --
>> Steve Weir
>> IPBLOX, LLC 150 N. Center St. #211
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>
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