The NTSC receiver consists of five sections:
• Super-heterodyne section (pick up antenna, RF amplifier, local oscillator, mixer and IF stages).
• Video section (video detector, video pre-amplifier, video power-amplifier and delay line).
• Colour decoder (bandpass amplifiers, sub-carrier generator, balanced demodulators, de-weighting circuits, adders and colour drivers).
• Deflection circuits and picture tube (sync generator, saw-tooth current generators for V and H-deflections and picture tube).
• Sound section (inter-carrier frequency amplifier, FM demodulator, audio amplifiers and loudspeaker).
These sections are shown in the below block diagram:
Super-heterodyne section:
• It is similar to the super-heterodyne section in a monochrome receiver except that the inter-carrier frequency signal is taken out (for sound section) from the last IF stage instead of from the output of the detector or video pre-amplifier.
• The reason for this is to save the picture from the beat signal produced by mixing chroma signal (3.58MHz) with inter-carrier frequency (4.5MHz).
• Beat of the two signals would be 920 kHz in NTSC system which falls within the video bandwidth and would cause interference known as sound in picture.
Video section:
• Video detector recovers CCVS signal. Luminance signal (Y) is amplified and is delayed by a delay line so that it reaches the adder simultaneously with the chroma signal.
• Chroma signal propagated slowly in the colour decoder circuit due to lower bandwidth and hence Y signal has to be delayed.
NTSC colour decoder:
• Detailed block diagram of the NTSC colour decoder is shown in the below figure
• Chroma signal (C) is separated from the Y signal by bandpass filters and amplifiers shown in two blocks BPA-I and BPA-II.
• Colour burst signal is also separated from the CCVS signal and amplified.
• The colour burst signal is used to synchronize the locally generated sub-carrier with the help of an automatic frequency and phase-control circuit.
• Output of the bandpass amplifiers (C signal) goes to the synchronous BM detectors I and II.
• BM detector I also receives the synchronised sub-carrier.
• The sub-carrier going to BM detector I have a phase of 57o with (B-Y) axis.
• The sub-carrier’s phase is further changed to +90o and this then go to BM detector II.
• Thus sub-carrier is in phase with signal I at the first BM detector (synchronous) and therefore it detects the I phasor and not Q.
• The sub-carrier is in phase with Q signal at the second BM detector and therefore it detects Q phasor only and not I phasor.
• Thus the two decoders detect I and Q phasors which are then de-weighted to obtain basic colour difference signals B-Y and R-Y. A resistive matrix gives G-Y signal.
• The Y signal is added to the three colour difference signals in an adder and we get R, G and B signals as follows:
(R-Y) + Y = R
(G-Y) + Y = G
(B-Y) + Y = B
• These three signals are amplified by respective colour drivers and then fed to the control grids of the colour picture tube.
• They change the strength of the respective electron beams and finally produce the original picture on the screen with the help of the scanning process identical to the scanning that was done at the transmitter.
Deflection circuits:
• Deflection circuits for colour receiver are similar to that in a monochrome system except that additional currents for removing pincushion effect are made available for the deflection yoke.
• Colour picture tube required for this section is totally different from the monochrome picture tube.
• It consists of three guns which receive three respective colour signals (R, G and B) and three beams strike the three respective phosphor dots (in delta gun picture tube) or stripes (in precision in-line picture tube) to reproduce the original colours on the screen.
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