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Re: video AGC and back porch
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Mark
May 2, 2008, 8:57:37 AM5/2/08
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On May 1, 5:05 pm, "dtsao" <tsaod...@yahoo.ca> wrote:
> When designing a video AGC, I have read people use the back porch of the
> video signal as a reference and amplify accordingly. My question is, how
> can you know what the level of the back porch is in the first place?
> Firstly, I imagine the color information must first be filtered out, to
> get rid of the color burst on the back porch (right?).
> But what is next to determine the back porch level. Do you just determine
> the lowest level (corresponding to the sync), then measure the back porch
> from this? If you do this, you would need to somehow time when the back
> porch occurs, so how do you do this? If you wait too long, you would
> measure the luminance, and if not long enough, you haven't reached the back
> porch yet.
> I am not sure I am tackling this problem the right way, so any other
> suggestion is appreciated.
> When designing a video AGC, I have read people use the back porch of the
> video signal as a reference and amplify accordingly. My question is, how
> can you know what the level of the back porch is in the first place?
> Firstly, I imagine the color information must first be filtered out, to
> get rid of the color burst on the back porch (right?).
> But what is next to determine the back porch level. Do you just determine
> the lowest level (corresponding to the sync), then measure the back porch
> from this? If you do this, you would need to somehow time when the back
> porch occurs, so how do you do this? If you wait too long, you would
> measure the luminance, and if not long enough, you haven't reached the back
> porch yet.
> I am not sure I am tackling this problem the right way, so any other
> suggestion is appreciated.
this is generally done by timing relative to the H sync
you can prob get a better answer here:
sci.engr.television.advanced
Mark
Sal M. Onella
May 3, 2008, 1:11:08 AM5/3/08
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"Mark" <mako...@yahoo.com> wrote in message
news:5af89103-d406-4e8f...@27g2000hsf.googlegroups.com...
sci.engr.television.advanced
Mark
The term "keyed AGC" comes to mind. It's been a while, but I think a pulse
from the flyback temporarily forward biases a diode through which a cap
charges with the level of the back porch. This is in a wholly analog world.
In digital, you could establish the occurrence of back porch time from any
other event(s) in the signal that occured at a known time.
I quit playing with most discrete circuits a long time ago. Sorry.
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