Analog Front End
The following sections show the functional blocks that process the analog video signal.
Anti-Aliasing Filter
The analog front end of the video decoder includes an anti-aliasing low-pass filter that attenuates undesirable high frequencies in the video signal before converting to digital video signal.
Frequency response of the anti-aliasing filter
The filter attenuation is always set to -3 dB at 7 MHz, and -10 dB at 10 MHz.
Decimation Filter
The over-sampled digitized composite video data are decimated using a digital decimation filter.
Decimation filter
Automatic Gain Control (AGC)
The analog front end of the video decoder includes a variable gain amplifier and an automatic gain control loop that restore the nominal sync amplitude of the signal before converting to digital video signal. In other words, the video decoder can receive incoming signals at various strength and yet maintain a constant video signal by adjusting the weaker signal to receive more gain, stronger signals to receive less gain or none at all.
The AGC is always enabled. The gain range of the variable gain amplifier ranges from -6 dB up to 18 dB, allowing a wide range of video signal amplitudes to be correctly decoded.
DC Restoration
The DC level of the video is restored using back-porch clamping technique.