Err, AV1 stands for Audio/Video input channel 1, as always seen on TV's.
So if you take the Amiga Video signal from the composite, then you have to switch to Composite from Antenna/Air. There you don't have to search any channels, it'll just show up, and better quality than Antenna/Air.
If you wanna try using CH36 on your TV-Card, then take the Antenna cable from the RF-Output from your A1200 and plug in the TV-Cards Antenna/Air IN plug, it'll work then. :-)
Sorry if i was a bit confusing... But it's very simple. I have a Mediator with TV card aswell and i use Composite OUT from my A1200 to show AGA modes on my SVGA Monitor. ;-)
Just switch to Composite or AV or AV1 on your TV software.
If you use TvR, then click on the LINE button (the one between FS and the Speaker button) at bottom. :-)