simple composite video circuits
An analog video signal is a very complex waveform.  Getting any sensible video display based on a guitar's audio signal would be very difficult.

Look on Wikipedia for "Analog Television" for a description of the signal format.

A video image is drawn on the screen line-by-line.  In North America, there are 525 horizontal line to make up an image (but some are hidden in the vertical retrace period).  The horizontal scan rate is 15,734 Hz, with a vertical scan rate of 59.94 Hz (NTSC colour)(if I remember the numbers correctly).  Odd lines of the image are displayed on one vertical scan, and even lines on the next.