The ST monitor connector has all sorts of video signals, including composit video.
Monitorconnection pinout
1 AO Audio Out
2 CVIDEO Composite Video
3 CS Clock Select
4 MD Monochrome Detect / Clock In
5 AI Audio In
6 G Green
7 R Red
8 +12V +12 VDC (520ST has GND)
9 HSYNC Horizontal Sync
10 B Blue
11 MVIDEO Monochrome Video
12 VSYNC Vertical Sync
13 GND Ground
Fact is that composit is needed to make the antenna signal, so even if your ST doesn't have composit video, it should be available on the mobo. That is, if I remember correctly.