[SI-LIST] Re: AC coupling at video circuits
- From: steve weir <weirsi@xxxxxxxxxx>
- To: Roy M <roymesi@xxxxxxxxx>
- Date: Mon, 07 Feb 2011 05:01:32 -0800
Roy M wrote:
> 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?
>
In NTSC, black is the higher voltage and white is the lower voltage.
> 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?
>>
The transmission line and/or load impedance bleeds the capacitor.
>> 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
>>> Reno, NV 89501 www.ipblox.com
>>>
>>> (775) 299-4236 Business
>>> (866) 675-4630 Toll-free
>>> (707) 780-1951 Fax
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>>>
>>>
>>>
>
>
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--
Steve Weir
IPBLOX, LLC
150 N. Center St. #211
Reno, NV 89501
www.ipblox.com
(775) 299-4236 Business
(866) 675-4630 Toll-free
(707) 780-1951 Fax
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