Chrominance, Sync & Deflection Circuits in Video Monitors
6.13 Chrominance Process:
To start, the signal is given by the video control block. This signal goes through a 4.43 MHz bandpass filter, which removes luminance and sync components. The filter output will only signal corresponding to the color information of the image.
Once extracted, the chroma information is taken to the chroma control block, which controls the signal’s amplitude, resulting in varying saturation.
The PAL demodulator consists of a 64 microsecond delay line, an adder, and a subtracter. It separates the modulated color difference signals (U and V) that were combined at the station for broadcast, enabling their individual demodulation.
These U and V signals are amplitude modulated, requiring two independent demodulators to obtain the original shape. Since the transfer is made with the suppressed subcarrier format, a 4.43 MHz oscillator is needed to regenerate the subcarrier for demodulation.
Finally, the PAL system transmits color information with quadrature amplitude modulation. The signals must pass through a phase control to compensate for phase shifts introduced during transmission and avoid color errors.
At the exit of the demodulators, we have the color difference signals. If we play them on the cathode ray tube (CRT), we can reconstruct the three basic colors (RGB). To do this, we apply the color difference signals to the matrix, where the luminance information also arrives. The matrix will produce three signals corresponding to the three basic colors, which will be applied to the RGB control block.
The RGB control block, through an electronic switch in every line, can select either internal RGB signals or signals from an external source, such as an internal character generator (e.g., a teletext processor or a receiver with OSD).
Once the signals to display are selected, they need sufficient amplification to be applied to the cathodes of the picture tube. This is achieved using three color amplifiers. The signals from these amplifiers will be applied to the cathodes of the CRT, concluding the image signal processing.
7.13 Sync and Deflection Circuits:
The video signal from the control block reaches the sync separator block. This block separates the sync signals from the luminance and chrominance information, providing separate outputs for line sync (H output) and field sync (V output).
A) Vertical Circuits:
The vertical sync pulses from the video signal are used to synchronize the internal vertical oscillator, ensuring its frequency is controlled by the received impulses. This ensures the information displayed on the CRT is evenly distributed vertically. The oscillator signal is also applied to the vertical blanking circuit. These pulses eliminate the electron beam path during field retrace periods.
The oscillator output signal is shaped like saw teeth, suitable for producing continuous vertical scrolling from top to bottom of the screen, with a quick return to the starting point after the movement. The shape of this signal can be modified to suit the circuit’s needs.
Varying the sawtooth amplitude changes the voltage in the vertical deflection coils, adjusting the height of the reproduced image. To control the height adjustment, a time constant is applied to the oscillator signal, modifying the curve’s slope and adjusting the linearity of the vertical screen scan.
After these corrections, the signal is ready for the vertical output amplifier, which increases the voltage and current levels to match the needs of the vertical deflection coils. These coils generate the magnetic fields needed for vertical displacement of the CRT’s electron beam.
B) Horizontal Circuits:
We start with the H impulses (15,625 Hz frequency). These separated horizontal sync pulses from the video signal synchronize the horizontal oscillator. The next step is to amplify the sync pulses to sufficient levels to drive the deflection coils. This is achieved by the horizontal output amplifier. The output signal from this block has the appropriate level.
The output signal is also applied to the line transformer, which generates control signals for the picture tube, ensuring proper display.
8/13 Video Monitors:
In television studios, video monitors are used to display various image sources and the results of applied processes.
A video monitor can be defined as a TV without tuning circuits, intermediate frequency, and demodulation. This means it cannot receive radio frequency signals but can accept composite video, separate luminance and chroma signals, or RGB component signals. The function of these monitors is to present the image in one of the formats used in the processes leading to broadcast, with the highest possible quality. To obtain the highest resolution, the signal applied to the monitor should have undergone minimal processing.
Features:
- Several inputs and outputs in each format, selectable by the user.
- High impedance connection to avoid loading the circuit.
- 75 ohm load insertion if the connection is at the end of the line.
- Ability to adjust screen sweep (underscan or overscan) compared to the standard.
- Ability to receive an external sync source.
- Ability to view RGB components separately.
- In some models, self-adjustment of the RGB amplifiers ensures white balance.
- Tally indication to facilitate source identification in a production system.