solution: AGC
Hi H_u_n_t_e_r,
What you are looking for is an AGC (automatic gain control). Since you are talking about an analog signal, there is a very easy way to do automatic gain control: look at the sync tip.
In NTSC, the sync tip should 'always' be -286 mV while the active signal on the line is 714mV (for the 100% white level). But when a video signal is attenuated, the sync tip is generally attenuated as well. Hence, if the sync tip is 10% higher than normal, there is good chances that the active portion and the burst are 10% higher as well. So don't worry about the signal amplitude, just look at the sync.
How to find the sync tip? Use a regular sync extractor like the inexpensive GS1881 family (SOIC-8). It will give you the sync position and the back porch position (0V level). If you do some 'sample and hold' at both positions and substract the 2 levels, you have the sync amplitude.
Then, using an amplifier which gain is controlled by a voltage, you can increase or decrease the gain of the overall signal when the sync amplitude is lower or greater than 286mV. Just remember this: you don't always want you AGC to change gain every lines. An AGC often have a specs for slow or fast response time.
Many decoders (analog to digital) like the Phillips SAA7114 have an automatic AGC that will do the job. Look closely on the internet and you may find something that will do the job for you.
Hope this helps. TurboPC