Keyed AGC, deal breaker?

[old_tv_nut]

Originally Posted by JohnCT

The first is keyed AGC. I've seen a bunch of early 50s TVs that seem to use the contrast control as the AGC. Do these TVs exhibit contrast shifting with picture content or signal strength? Is it annoying?
John

1) It's the other way around: they use AGC as the contrast control.
2) Where are you going to find a variable analog signal to worry about? You will be feeding these with some local signal source or digital/analog converter, and the signal will be constant.
3) Shifting black level may occur if the video is not DC coupled. Circuits for adding DC restoration to certain sets have been published here.

Further edit: do you want the DC coupling or lack of it to be like the original set, or want to fix it? Either philosophy seems to be reasonable to me, but it's definitely a choice. Also note, that the 1940s iconoscopes in cameras were by nature AC coupled, plus had shading issues that could wash a variable fog over some areas of the image and not others. Again, you need to consider if you want to show the set with authentic warts or modern fixes.