I was told to ask any video design questions in the Video Processors forum, so here it goes. I've read some good info in this thread
, but I'd still like more information on this topic.
I'm designing a circuit with a 480i YPbPr output. It's running off a single supply so there is a known DC offset added to the outputs which sets the blanking level at +0.8 volts. I've been reading all I can find about DC/AC coupling video inputs/outputs, among which are:http://www.maxim-ic.com/app-notes/index.mvp/id/3303http://www.maxim-ic.com/app-notes/index.mvp/id/3768http://www.fairchildsemi.com/product...eo_filters.pdf
What I gather from these documents is that most equipment AC couples their video inputs and then uses a clamp or DC restoration circuit to set its own optimal DC level. Thus, anything connected into these inputs can have any DC level, since it is removed by the input circuitry. This is confirmed in Video Demystified (4th ed.)
in Chapter 5
under SDTV YPbPr Interface
: "For receivers, video inputs should be AC-coupled and have a 75-Ω ±5% output impedance." "DC offsets up to ±1V may be present."
I have attached pictures of the quotes in case someone wants to look at them in their entirety.
According to this, everything should be fine if I leave my outputs DC coupled. However, when I connect my YPbPr outputs to a TV, the peak white level of the image appears to be clipping on every TV that I try. If I AC couple the Y output, by adding a 220µF series capacitor, the white level is normal. Adding the capacitor to the color difference outputs doesn't seem to change the resulting image. My guess is that the DC (average) value of the Y signal before considering the additional +0.8 volts, is greater than +0.2 volts. This would push the total DC value past the ±1V limit as stated in Video Demystified. Can anyone confirm this? If I readjust my circuit to lower the blanking level to +0.4 volts, then the white level looks fine even without the coupling capacitor.
What causes the actual clipping though? Figure 6a
of that 2nd Maxim link
seems to imply that it can handle any DC offset. Don't modern input stages include AC coupling followed by a clamp that forces the bottom of the sync pulse to a reference voltage? That shouldn't be affected by different DC offsets...or am I missing something?
This circuit I'm working on needs to be so small, that adding the coupling capacitor would result in me scrapping the project completely. Avoiding the field tilt issue is also desirable. I can redesign my entire circuit to use the minimum DC bias allowed to keep my signals within the linear operating range of my amplifiers. Hence, the signals will have the same voltage levels as shown in Figure 5.6
of Video Demystified (4th ed.)
. I've attached a picture of that figure as well.
My main question is...would this be sufficient in a compatibility or "good practice" sense? Or should I really be putting that coupling capacitor in there no matter what? Or am I understanding this whole AC coupling DC coupling thing completely wrong? Please set me straight!
Thanks for any help. Oh, and sorry about all the random questions scattered throughout the post. Answers to any of them will be most appreciated.