PAL COLOR TV RECEIVER:
A colour TV receiver is identical to a black-and-white receiver with the addition of a chroma section and a colour image tube. Figure 5.7 shows a block schematic of a PAL colour TV receiver. This receiver's many parts are detailed below.
1. VHF and UHF Tuner:
The TV receiver has a VHF and UHF tuner on the front. It has a tuned circuit that allows you to choose the channel you want. The antennal signal is amplified and transformed into an IF signal, which is then sent into the video IF amplifier. Tuners also include an automatic frequency tuning feature for accurate colour reproduction.
2. Video IF Amplifier:
Since the tuner's output signal isn't strong enough to drive the video detector, it's amplified to the appropriate level via cascaded IF amplifiers.
3. Video Detector:
From the modulated composite video stream, the video detector recovers the original video signal.
4. Sound Section:
Trap Circuit, Limiter, FM Detector, Audio Amplifier, Power Amplifier, and Loudspeaker are all included in the sound area. The original sound signal is initially recovered from the modified sound carrier signal at this step. The FM detector's output is suitably amplified and applied to the loudspeaker for sound reproduction.
5. Automatic Gain
Control Circuit:
Despite the intensity of the input signal, the Automatic Gain Control (AGC) circuit keeps the output signal at a consistent amplitude. The sync separator recovers the H and V sync pulses in the deflection circuit. Proper oscillators and amplifiers are used to process them. Finally, applied to the V and H deflection coils to concurrently deflect electron beams in the V and H directions.
Figure: Block Diagram of PAL Color TV Transmitter
6. Luminance Signal:
From the video detector, the chrominance and luminance signals follow independent routes before rejoining in the matrix portion. From the composite video stream, the luminance signal processing network recovers the luminance (Y) signal. The cathodes of colour picture tubes generally receive a negative-going Y signal (-Y).
7. Color Signal Processing:
The signal available at the output of the video detector is properly amplified before feeding it to the various sections.
a) Chrominance Band Pass Amplifier: The chrominance bandpass amplifier selects the chrominance signal while rejecting the composite signal's other undesirable components.
b) Burst Banking Circuit: During colour burst periods, this circuit blocks signal flow to the chrominance bandpass amplifier.
c). Burst Amplifier: The burst gate amplifier isolates the colour burst signal from the chrominance signal while also enhancing it to the appropriate level. This transmission has a frequency of 4.43 MHz.
d). Generation and Control of Subcarriers: The major goal of this section is to generate a subcarrier with the right frequency. The phase discriminator and variable reactance components combine to serve as an Automatic Phase Control circuit in the subcarrier oscillator, which is a crystal oscillator used for creating subcarrier signals with a frequency of 4.43 MHz. It picks up subcarrier and burst signals.
If the frequency of the subcarrier oscillator is exactly right, its phase is altered by 90o for the incoming burst signal. The APC circuit's output is fed into a 7.8 kHz tuned amplifier. The 7.8 kHz ac component was overlaid on the output signal in this circuit. The output of this circuit is sent into the colour killer and identification circuits.
Before applying the subcarrier output to the V demodulator, the identification circuit controls an electrical switch that alternately reverses the phase of the subcarrier output. The 7.8 kHz component is available at the APC circuit of the reference subcarrier oscillator when a colour signal is received. The chrominance bandpass amplifier now performs the usual operation.
There is no 7.8 kHz during monochrome signal reception. As a result, the chrominance bandpass amplifier is turned off by the colour killer circuits.
8. Separation of U
and V Modulation Products:
Here, the PAL delay line circuit, adder, subtractor, V and U sync demodulators, and difference signal amplifiers with matrix network are considered. The chrominance signal generated by the chroma bandpass amplifier is sent into one of the adder and subtractor circuits' inputs. Using a PAL delay line, the same signal is delayed and applied to the other inputs of adder and subtractor circuits.
The U information is the adder's output. Similarly, the subtractor's output is V data. Two separate double sidebands, suppressed carrier RF signals emerge from the adder and subtractor's output. These signals are sent to synchronous demodulators in the U and V bands, respectively. The concern oscillator's colour subcarrier signal is applied straight to the U sync demodulator.
Similarly, the same signal is delivered to the V sync demodulator through an electrical switch to create a + or -90o line by line phase-shifted signal. The original B-Y signal is recovered by the U demodulator. Similarly, the R-Y signal is recovered by the V demodulator. The G-Y signal is produced by combining the two signals in a matrix network. These colour difference signals are applied to the colour picture tube's matching grids.