[SI-LIST] Re: AC coupling at video circuits
- From: "Orin Laney" <olaney@xxxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Mon, 7 Feb 2011 17:14:30 -0800
Analog video is typically capacitor coupled with DC restoration at the
receiver. The restoration typically comprises a transistor switch to clamp
the back porch of Hsync to ground. In professional equipment, it is true
that a video output might be DC coupled but consumer equipment almost never
is (for cost reasons), or might have a DC offset. A capacitor coupled input
with restoration handles any of these situations, making it universal.
Orin Laney
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Andrew Ingraham
Sent: Monday, February 07, 2011 4:29 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: AC coupling at video circuits
> Agreed the capacitor value needs to be much lower
(I think you meant to write 'higher')
> at the TX
> where it might need to drive 37.5 ohm (double terminated 75 ohm
> cable), as opposed to RX after term where the input impedance
> would be much higher.
Actually, if analog video is still done the way it used to be, the
coupling capacitor is in a 150 ohm circuit (in series with the
source's 75 ohms plus the cable/load's 75 ohms). It would be 37.5
ohms if you were looking at shunt capacitance.
The location of the capacitor affects its value only if it goes after
the RX end's 75 ohm termination. If before the load, then the
capacitor value would be the same as on the TX end.
I thought analog (RS170) video was typically DC-coupled, guess I was wrong.
Regards,
Andy
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