Understanding Video Signal Processing: Luminance and Chroma
Video Signal Processing
The video detector provides the composite video signal, which includes sync information, luminance information, and chroma information. The video detector output is separated to recover the luminance signal and the chroma signal. Both signals are processed separately and perform the following basic functions:
- Luminance Signal: Adapts the signal to the level needed for the circuit matrix.
- Chroma Signal: Amplified and then undergoes PAL decoding and demodulation to recover the color difference signals applied to the circuit matrix.
- Matrix Circuit: Recovers the original RGB signals.
Processing the Luminance Signal
In black and white sets, the output signal is applied directly to the image tube. In color receivers, the recovered luminance signal is delivered to the circuit matrix.
Luminance Processing on Color Receivers
The circuits extract luminance information (gray levels) from the composite video signal. The basic structure of the luminance processing system includes three blocks:
- Chroma Elimination
- Delay
- Luminance Signal Amplifier
Processing the Chroma Signal
It is necessary to remove the chroma signal through a bandpass filter centered at the color subcarrier frequency (4.43 MHz). The chrominance signal is amplified by the chroma amplifier. The PAL decoder separates the color difference signals. These difference signals are demodulated to obtain the color difference signals without modulation. The sync signal is extracted from the chrominance signal through the burst amplifier. This signal is sent to the chroma suppressor circuit, which regulates the chroma amplifier gain using a color machine to maintain a constant output level. The recovered sync signal, sent by the sender, is necessary for synchronization of the local oscillator color subcarrier.
Separation of the Color Signal
There are two ways to separate chrominance and luminance:
- Using a low-pass filter for the luminance signal and a band-pass filter for the chroma signal.
- Using comb filters.
Filters
It is possible to separate the luminance information from chrominance.
For luminance, the subcarrier frequency is eliminated through a low-pass filter or a trap filter. To remove the chroma signal, low-frequency information is eliminated using a bandpass filter.
Comb Filters
The best way to separate the luminance and chrominance signals is by using a comb filter.
Chroma Amplifier
It amplifies the weak chrominance output from the detector. The chroma amplifier includes an automatic chroma control (ACC) system to ensure a constant chroma signal level. The ACC is obtained by checking the received chroma level. When the chroma amplifier ceases to function correctly, the chroma suppressor circuit disables the chroma amplifier.
Burst Amplifier
The burst is separated from the rest of the video signal using an amplifier with a gate that only conducts the burst signal. This requires a processor that receives the delayed sync pulse line return.