i'm sorry for not being clearer in my earlier post.
my knowledge in electronics is limited so i will try to explain where i am at:
the transmitter and receiver parts are pulled from a consumer version very similar to the one seen on the Rapid Electronics website (part no. 43-0308).
http://www.rapidelectronics.co.uk
the transmitter side is left pretty much untouched (although will eventually will be coupled with a 7" tft monitor). the receiver side is connected with a dual L386-amp circuit that allows me to monitor the incoming audio on a pair of headphones. this amp circuit works fine and i have made sure to keep the input grounds seperate from the output grounds to avoid oscillation.
in both the transmitter and receiver circuits, the video and audio channels share a common ground with the power supplies (both regulated 12V dc through mains adaptors). this is how they were in the original consumer version so i have kept it that way.
like i said, the hissy-hum dissappears when i pull out the video cable in the transmitter and varies depending on the image make-up (bright versus dark areas). also, if i were to pull the video cable out of the receiver, an extremely loud buzz is heard - i'm not sure if this is because i need termination...?
like i said, i don't know much but i am trying to learn and any help that you might be able to offer me would be most appreciated.