The clamp circuit is responsible for adjusting the input voltage dc level. The video input is AC coupled to chip through a capacitor. Normally the clamp circuit is a current DAC, which charges or discharges the coupling capacitor. You probably will need some sophisticated digital system to control the clamp circuit and so is the AGC.

Maybe you can obtain more details on the hardware implementations in the following reference.

http://www.semiconductors.philips.com/acrobat_download/datasheets/SAA6713H_2.pdf...

With regards,
Jiawen Hu
Analog Design