PAL Color Television System: Key Components & Functions

Color Subcarrier Oscillator Fundamentals

The color subcarrier frequency must be very precise. To generate this precise frequency, a crystal oscillator is used. Its features include:

• It operates at very specific frequencies.
• Frequencies are easily synchronizable.
• To correct the oscillator signal, the principle of correction by Phase-Locked Loop (PLL) is used.

The crystal circuit has three differentially-controlled phase parts:

• The crystal oscillator provides a frequency signal, which can be stable and regulated.
• The phase comparator provides the control voltage.
• Phase reactance varies the oscillator capacitance.

PAL ID Circuit Function

The PAL ID circuit provides essential information to both the color suppression circuit and the PAL switch.

PAL Decoder: Error Correction & Signal Recovery

The primary mission of the PAL decoder is to perform averaging of two lines, thereby removing the phase error introduced by the PAL system. It incorporates a delay line in the chrominance channel, which acts as a memory for one line's functions. Signal recovery with the PAL decoder is straightforward:

• For the recovery of the U vector, it is necessary to add two consecutive lines.
• For the recovery of the V vector, it is necessary to subtract two consecutive lines.

Synchronous Demodulators: RY & BY Signal Detection

Synchronous demodulators play a crucial role in detecting the R-Y and B-Y color difference signals. To perform this demodulation, it is essential to generate and restore the color subcarrier signal.

Color Suppressor (Killer) Circuit Explained

The color suppressor circuit, also known as the Killer circuit, is responsible for disabling or suppressing the action of the chrominance channels when no color signal is present.

Matrix Circuits: Deriving RGB Color Signals

Matrix circuits are responsible for deriving the RGB color information signals. There are two main types of matrix circuits, based on the excitation method used in the picture tube:

• RGB Final Stage
• Final Stage by Color Difference

RGB Final Stage: Bandwidth Considerations

In the RGB Final Stage, the color difference signals typically have a bandwidth of about 1 MHz, whereas the luminance (Y) signal has a bandwidth of 5 MHz. The resulting composite signal, after combining, therefore also exhibits a bandwidth of 5 MHz.

Color Difference Final Stage: R, G, B Generation

This stage retrieves the color difference signals (R-Y), (G-Y), and (B-Y). The primary color signals (R, G, B) are then produced in the picture tube by adding the luminance (Y) signal to these color difference signals:

• (R-Y) + Y = R
• (G-Y) + Y = G
• (B-Y) + Y = B

Integrated Circuits in Color TV Systems

Integrated circuits (ICs) in color television systems typically perform a wide range of functions, including:

• Processing of luminance (Y) signals.
• Processing of chrominance (color) signals.
• Control of analog functions (e.g., brightness, contrast, saturation).
• Color suppression (Killer function).
• Generation of CAC (Color Automatic Control) voltage.
• Matrixing of color information.
• Switching between internal signals or external signals from sources like teletext, On-Screen Display (OSD), or SCART connections.