If it's any help, the following diagram is taken from some notes on Monochrome Composite video signals that I wrote whilst developing the TellyMate. It shows the PAL signal, but the NTSC signal is similar. Note that this isn't a proper interlaced signal - it's a fake-progressive signal - most TVs understand it though.
No matter what you do, you're always going to be up against timing issues when displaying TV signals - there's just not a lot of free time. There's a lot of time-sensitive code in the TellyMate to enable it to read and process serial data without corrupting the display at any time. For more details, see the article about the design of the TellyMate.
If you've got a 'spare' Arduino, here's a thread that details how you can turn an Arduino into a TellyMate and here's a diagram on flickr.
@digimike: you might find that the 75R resistor is not required - most TVs should already have this. Including the 75R on 'your side' effectively halves the voltages being output. See this AVRFreaks thread.
I hope this helps!