video AGC and back porch

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.

On May 1, 5:05&#4294967295;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.

this is generally done by timing relative to the H sync

you can prob get a better answer here:

sci.engr.television.advanced

Mark

"Mark" <makolber@yahoo.com> wrote in message
news:5af89103-d406-4e8f-8962-cb23be5b6cde@27g2000hsf.googlegroups.com...
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.

this is generally done by timing relative to the H sync

you can prob get a better answer here:

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.