Making video with a microcontroller is usually a tough thing to do, due to the limitations of speed and memory. This guy from NatureTM used the Texas Instruments MSP430 mixed signal microcontroller for his project. He divided his clock in a way that he has a good aspect ration (not taller than wider) with good resolution, but not run out of memory. So he came up with a resolution of 192×40 pixels. To further increase the data transfer speed, he used hardware SPI, as software data transfer is much slower. During his project making, he found a useful guide that explains the PAL Aand NTSC composite video (from this site). Visit his page for more info.