TE4201-Communication Electronics TV Receivers and Color System VHF-UHF reception stage VHF-UHF reception stageVHF-UHF reception stage Video stages Video stagesVideo stages Synchronization and Deflection Synchronization and DeflectionSynchronization and Deflection Color processing stage Color processing stageColor processing stage High Definition TV (HDTV) High Definition TV (HDTV)High Definition TV (HDTV)
TE4201-Communication Electronics 2 VHF-UHF reception stage The UHF signal is immediately stepped down to the IF frequency and then goes to the VHF RF amplifier and mixer, which provide the gain that makes the UHF and VHF signals of about equal strength when they go into the IF amplifiers. This is done because RF amplifiers and transistor mixers (that supply gain) tend to be more costly and also offer relatively poor signal-to-noise ratios at the high UHF frequencies. The large majority of tuners are synthesized, which allows for the remote control feature that is found on most sets.
TE4201-Communication Electronics 3 A TV receiver utilizes the superheterodyne principle, handle video and synchronizing signals as well as the audio signal 1. The incoming signal is selected and amplified by the RF amplifier 2. Stepped down to the IF frequency by the mixer-local oscillator blocks. 3. The IF amplifiers handle the composite TV signal, 4. The video detector separates the sound and video signals. 6. The other video detector output is the video (plus sync) signal. 5. The sound signal detected out of the video detector is receiver system in itself.
TE4201-Communication Electronics 4 7. The actual video portion of the video signal is amplified in the video amplifier and subsequently controls the strength of the electron beam that is scanning the phosphor of the CRT 8. The sync separator separates the horizontal and vertical sync signals, which are then used by the horizontal and vertical oscillators. 9. The oscillator outputs control the horizontal and vertical movement of the electron beam that is scanning the phosphor of the CRT 10 The high-voltage output is derived by stepping up the horizontal output signal (15.75 kHz) via transformer action. This transformer, usually termed the flyback transformer, has an output of 10 kV or more, which is required by the CRT anode to make the electron beam travel from its cathode to the phosphor
TE4201-Communication Electronics 5 Video stages Radio receivers require relatively high-Q tuned circuits since the desired bandwidth is often less than 10 kHz. A TV IF amp requires a pass band of about 6 MHz because of the wide frequency range necessary for video signals. Hence, the problem here is by using high Q components, how to get a wide enough bandwidth but still have relatively sharp falloff at the pass band edges. The use of a lower-Q tuned circuit in the middle helps provide a flatter overall response than would otherwise be possible. Most TV IF amplifiers solve this problem through the use of stagger tuning. Stagger tuning is the technique of cascading a number of tuned circuits with slightly different resonant frequencies The response of three separate LC tuned circuits is used to obtain the total resultant pass band with dashed lines.
TE4201-Communication Electronics 6 Note that frequency conversion reverse the positions of the frequency. When a channel frequency 82 MHz mix with an oscillator frequency 123MHz, IF for this is =41MHz. But a lower position frequency of 76MHz is mixed with the same oscillator 123MHz, it produce =47MHz. 82 MHz76 MHz 123 MHz oscillator 47 MHz41 MHz
TE4201-Communication Electronics 7 Surface Acoustic Wave (SAW) filters. Color television receivers require a very complex IF alignment procedure due to the critical nature of their required bandpass characteristic. Just as crystals rely on the effects in an entire solid piezoelectric material to develop a frequency sensitivity, SAW devices rely on the surface effects in a piezoelectric material such as quartz or lithium niobate. It is possible to cause mechanical vibrations (i.e., surface acoustic waves) that travel across the solid's surface at about 3000 m/s.
TE4201-Communication Electronics 8 The sync takeoff block is the point where the horizontal and vertical sync pulses arc extracted from the video signal. Once applied to the cathode, this signal varies the electron beam strength such that white and black spots appear on the CRT face The contrast control is analogous to the volume control of a radio receiver. It simply varies the amplitude of the signal applied to the CRT. THE VIDEO SECTION The video section takes the output of the video detector (0 to 4 MHz) and amplifies it to sufficient level to be applied to the picture-tube cathode.
TE4201-Communication Electronics 9 Synchronization and Deflection The sync signals consists of both vertical (at 60 Hz) and horizontal (at kHz) sync pulses, a means to separate one from the other is necessary. The vertical sync pulse is of long duration, while the horizontal sync pulse is of very short duration. The output of the low-pass filter will be the lower- frequency vertical sync pulse at 60 Hz since it is a wide pulse rich in low-frequency components. The output from the high-pass filter will be the horizontal sync pulse al kHz since it is a very narrow pulse that is rich in high- frequency content. The vertical and horizontal sync pulses are applied to the vertical oscillator and the horizontal oscillator respectively which controls the scanning of the electron beam on the CRT surface the sync separator clips the sync pulse off the video signal. This prevents the sweep instability by spurious video signals. Once the sync separator has clipped the sync signals from the lower-level video signal, the two types of sync pulses are applied to both low- and high-pass filters.
TE4201-Communication Electronics 10 Sawtooth waveform output of the horizontal oscillator which controls the scanning of the electron beam on the CRT surface should be linear sweep. Following figures show non-linear sweep of sawtooth waveform. Stretch at left and crowding at right Stretch at right and crowding at left Crowding at the center
TE4201-Communication Electronics 11 Color processing stage TV color system is compatible. a color transmission can be reproduced in black-and-white shades by a monochrome receiver and a monochrome transmission is reproduced in black and white by a color receiver. To remain compatible, the same 6-MHz bandwidth must be used including more information for the color. This problem is overcome by a form of multiplexing. It turns out that the video signal information is clustered at kHz intervals throughout its 4- MHz bandwidth. Midway between these kHz clusters (harmonics) of information are unused frequencies. By generating the color information around just the right color sub carrier frequency ( MHz), it becomes centered in clusters exactly between the black-and-white signals. This is known as interleaving.
TE4201-Communication Electronics 12 Three separate color signals are fed into the transmitter signal processing circuits (matrix) and create the Y. or luminance, signal and the chroma, or color, signals I and Q. The Y signal contains just the right proportion of red, blue. and green such that it creates a normal black-and-white picture. It modulates the video carrier just as does the signal from a single black-and-white camera with a 4-MHz bandwidth. The chroma signals. I and Q, are used to phase-modulate the 3.58-MHz color sub carrier, which then interleaves their color information in the gaps left by luminance Y signal's sidebands. This modulation by the I and Q signals is accomplished in a balanced modulator, thus suppressing the 3.58-MHz sub carrier, as it would cause interference at the receiver.
TE4201-Communication Electronics 13 The three color cameras scan the scene in unison, with the red, green, and blue color content separated into three different signals. At the receiver these three separate signals are made to property illuminate groups of red. green, and blue phosphor dots (called triads), and the original scene is reproduced in color.
TE4201-Communication Electronics 14 High Definition TV (HDTV) To develop standards for high-definition TV (HDTV) systems, the goal is to offer a TV picture that will rival the definition of motion picture film presentations. Principal barriers to implementation of HDTV include the need for wide transmission bandwidths. the lack of worldwide standards for both production and transmission of the signals, and the difficulty of achieving compatibility with existing TV receivers. It is generally felt that HDTV receivers will use digital signal processing that converts the received signal from analog to digital. The VSLI chips to accomplish this have been developed but are still being refined. Besides the higher overall picture quality, they promise such features as Freeze-frame, zoom focusing, and picture-within-a-picture.
TE4201-Communication Electronics 15 High-Definition TV (HDTV) systems uses the two-signal approach. When the two are transmitted, standard receivers display the 525- line program, HDTV receiver can display the 1125-line resolution of the combined signals. Notice that a different aspect ratio (16:9) is used for the HDTV picture. a second produces a narrowband detail output. One camera produces a standard 525-line output
TE4201-Communication Electronics 16 Horizontal Deflection and High Voltage The horizontal sync pulses are used to calibrate the horizontal oscillator, which is then amplified to a powerful level by the horizontal output amplifier and then applied to a high-voltage transformer commonly referred to as the flyback transformer. Its outputs drive the horizontal yoke windings and provide the high voltages for the CRT after rectification.