Bill Sloman wrote:
At 22:59 17-3-2006, you wrote:
On Fri, Mar 17, 2006 at 10:13:59AM +0000, Dieter wrote:
>> Assuming for the moment that most video displays take 75 Ohm inputs,
>> does someone make 75-to-50 Ohm matching transformers for BNC?
>
> I don't think that would be possible for video. You need flat
>response down to DC, and transformers can't do that. Matching
transformers
>are mostly RF devices, and good ones work well over a few octaves.
You don't need flat response down to DC - video is designed to be
AC-coupled, which is why you have the back porch on the line synch
signal for your black level clamp to find and clamp to the local 0V. TV
line rate is 15kHz, which sets the lowest frequency where you need a
flat resonse.
We are talking about monitors with 5 inputs Red, Green, Blue, H-Sync,
V-Sync. So all the video signals (RGB) have is blanking (no porches).
Now, it is quite possible for monitors to have DC restoration based on a
blanking level of 0 Volts (which is more than just clamping). The
question is, do these monitors have DC restoration for the video signals?
--
JRT
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