COMPOSITE VIDEO

Objectives:

1.1 Getting to know the basic composite video.

1.2 Measuring composite video and voltage.

1.3 Determining the parameters of composite video.

Equipment  In Used:

1 VCD / VTR

1 Oscilloscope 40 MHz and passive probe

An RCA cable connector - BNC (75 Ω) 

Circuit diagram:

Introduction:

Composite video is the format of an analog television (picture only) signal before it is combined with a sound signal and modulated onto an RF carrier.

Composite video is often designated by the CVBS initialism, meaning "Composite Video, Blanking, and Sync."

It is usually in standard formats such as NTSC, PAL, and SECAM. It is a composite of three source signals called Y, U and V (together referred to as YUV) with sync pulses. Y represents the brightness or luminance of the picture and includes synchronizing pulses, so that by itself it could be displayed as a monochrome picture. U and V represent hue and saturation or chrominance; between them they carry the color information. They are first modulated on two orthogonal phases of a color carrier signal to form a signal called the chrominance. Y and UV are then combined. Since Y is a baseband signal and UV has been mixed with a carrier, this addition is equivalent to frequency-division multiplexing.

Composite video can easily be directed to any broadcast channel simply by modulating the proper RF carrier frequency with it. Most analog home video equipment records a signal in (roughly) composite format: LaserDiscs store a true composite signal, while VHS tapes use a slightly modified composite signal. These devices then give the user the option of outputting the raw signal, or modulating it onto a VHF or UHF frequency to appear on a selected TV channel. In typical home applications, the composite video signal is typically connected using an RCA jack (called a phono plug in the UK), normally yellow (often accompanied with red and white for right and left audio channels respectively). BNC connectors and higher quality co-axial cable are often used in more professional applications.

In Europe, SCART connections are often used instead of RCA jacks (and to a lesser extent, S-Video), so where available, RGB is used instead of composite video with computers, video game consoles, and DVD players.

Some devices that connect to a TV, such as VCRs, older video game consoles and home computers of the 1980s, naturally output a composite signal. This may then be converted to RF with an external box known as an RF modulator that generates the proper carrier (often for channel 3 or 4 in North America, channel 36 in Europe). Sometimes this modulator was built into the product (such as video game consoles, VCRs, or the Atari, Commodore, or TRS-80 CoCo home-computers) and sometimes it was an external unit powered by the computer (in the case of the TI-99 or some Apple modulators) or with an independent power supply. In the USA, using an external RF modulator frees the manufacturer from obtaining FCC approval for each variation of a device. Through the early-1980s, electronics that output a television channel signal were required to meet the same shielding requirements as broadcast television equipment, thus forcing manufactures such as Apple to omit an RF modulator, and Texas Instruments to have their RF modulator as an external unit, which they had certified by the FCC without mentioning they were planning to sell it with a computer. In Europe, while most countries used the same broadcast standard, there were different modulation standards (PAL-G versus PAL-I, for example), and using an external modulator allowed manufactures to make a single product and easily sell it to different countries by changing the modulator.

The argument has been made that the point of removing the RF modulator to an external box was to prevent RF interference with the home computers, but as the modulator ran in the range of >50MHz in all countries, and the computers ran in the range of 1-4MHz, the possibility of significant interference is debatable, and on a 5V TTL logic computer, it is hard for the weak output of an RF modulator to cause interference. Since the RF modulator was sealed inside a metal can (though more to protect it from the computer noise), there was little RF to interfere with the computer. Finally, the same interference would propagate down the composite video cable or the power lead cable into the computer in any case.

The process of modulating RF with the original video signal, and then demodulating the original signal again in the TV, introduces several losses. RF is also "noisy" because of all of the video and radio signals already being broadcast, so this conversion also typically adds noise or interference to the signal as well. For these reasons, it is typically best to use composite connections instead of RF connections if possible. Almost all modern video equipment has at least composite connectors, so this typically isn't a problem; however, older video equipment and some very low-end modern televisions have only RF input (essentially the antenna jack); while RF modulators are no longer common, they are still widely available to translate baseband signals for older equipment.

However, just as the modulation and demodulation of RF loses quality, the mixing of the various signals into the original composite signal does the same, causing a checkerboard video artifact known as dot crawl. Dot crawl is an infamous defect that results from crosstalk due to the intermodulation of the chrominance and luminance components of the signal. This is usually seen when chrominance is transmitted with a high bandwidth, and its spectrum reaches into the band of the luminance frequencies. This has led to a proliferation of systems such as S-Video and component video to maintain the signals separately. Comb filters are also commonly used to separate signals, and eliminate artifacts, from composite sources.

When used for connecting a video source to a video display that supports both 4:3 and 16:9 display formats, the PAL television standard provides for signalling pulses that will automatically switch the display from one format to the other. The Composite video connection supports this operation. However the NTSC television standard has no such provision, and thus the display must be manually switched.

Composite Video Signal Construction

 

Composite video signal containing variations of the camera signal (image information), blanking pulses (blanking), and synchronization pulses (sync).

Figure 1 Three sets of composite video signal is a variation of the camera signal, blanking pulses, and synchronizing pulses.   (A) camera signals (image information) to a single horizontal line, (b) H blanking pulse signal is added to the camera, (c) Toll-alignment of H added to the pulse discharge. 

Figure 2 composite video signals for two horizontal lines 

In figure 2, the amplitude of voltage and current are shown sequentially for MRV two horizontal lines in the shadows, as time increases horizontal direction, the amplitude is changed to white shade, gray, or black in the picture. Starting from the far left at time zero, the signal at the level of white and MRV file located on the left image (the image). Once the first line from left to right, found different cameras with different amplitude signal corresponding to image information is required. After trace (trace) horizontal camera produces the desired signal for one line, MRV file located on the right image (image). Then the discharge pulse is inserted in order to restore the video signal amplitude to the top to the black level, so that the loop trail can be left empty.

After emptying time long enough to cover the trail repetition, emptying the voltage is removed. Then file located on the left, ready to cruise back  next line. In this way each horizontal line forth  respectively. Note that the second line shows the dark image information near the black level.

With regard to time, the amplitude of the signal-amplitude right after emptying in Figure 2 shows the information in accordance with the left side at the start line of cruise back. Just before discharge, the signal variation corresponds to the right side. Appropriate information in the middle line of cruise back  is half the time between discharge pulses. 

Figure 3. Details of horizontal blanking and synchronizing pulses.

Details of the horizontal blanking period as figure 3. Intervals marked H is the time required to cruise back a complete line including tracking and loop trail 

 

Figure 4. Details of the alignment pulses and discharge for successive fields within cruise back vertical.

Pulse-Pulse Alignment in Time Decommissioning V

 

Sync pulses are inserted in the composite video signal during vertical blanking pulse width shown in Figure 4. This includes pulses to equalize, pulses vertical alignment and horizontal alignment of multiple pulses. Signal-signal is shown at intervals of time at the end of the field and the next one, to describe what happens during the vertical blanking time Both signals are shown one above the other are the same, except for half-line shift between successive fields are required for MRV intertwined odd lines.

Starting from the left in Figure 4, the fourth-last line of MRV horizontal raster shown on the basis of joint discharge pulses and horizontal alignment is needed. Immediately after following the last visible line, the video signal is made into the black by the vertical blanking pulse in preparation for the repetition of vertical trace.

Vertical blanking period begins with a group of six pulses cruise back, which separate the half-line intervals. Next is the vertical alignment pulse produces real jagged vertical flyback in a series of  cruise back. Serration also occur at intervals of half a line. Thus, a complete vertical alignment pulse width is three lines. Following the vertical alignment is a another group of six pulse equation and a series of horizontal pulses.

During the vertical blanking period as a whole, there is no information on the resulting image, because the signal level is black or blacker than black so that the repetition of vertical traces can be left empty.

In a signal at the summit, the first pulse is a full line of credit beyond the previous horizontal alignment; in signals below for the next field, the first pulse is as far as half a line. The difference this time and a half lines between the even fields and odd continues through all subsequent pulses, so that the pulses of the vertical alignment for successive fields MRV interwoven set time for the odd lines.

Decommissioning & Cruise back V and V (V Blanking and V Scanning)

 

Serrated vertical sync pulses that force the vertical deflection circuit to start the flyback. However, the flyback generally will not begin with the start of vertical alignment because the alignment must build a toll-charge in a capacitor in order to trigger circuits & cruise back. Jika kita asumsikan bahwa flyback vertikal dimulai dengan pinggiran leading dari gerigi ketiga, maka waktu dari satu garis berlalu selama penyelarasan vertikal sebelum flyback dimulai. If we assume a vertical flyback starts by leading edge serration of the three, then the elapsed time of one line for the vertical alignment before the flyback starts. Juga enam pulsa untuk menyamakan yang sama dengan tiga garis terjadi sebelum penyelarasan vertikal. Also six pulses to equalize the same with the three lines before the vertical alignment. So 3 + 1 = 4 lines left blank at the bottom of the image, right before the vertical loop trail begins.

How much time is required for the flyback depend on cruise back, but the repetition time of a typical vertical traces are 5 lines. Once the loop trail cruise back file from the bottom to the top of raster, produced five complete horizontal lines. Repetition vertical trail can be completed with ease during vertical blanking time.

With 4 lines left blank at the base before the flyback and 5 lines emptied during flyback, 12 garis tersisa dari total 21 selama selama pengosongan vertikal. 12 remaining lines of the total 21 during during vertical blanking. The 12 blank lines are located on top of raster on the surface tracking down a vertical direction.

In summary, 4 lines left blank at the bottom and 12 on the top line in each field. Cruise back lines generated during vertical tracking, but that made black by the vertical blanking, forming black rods at the top and the bottom of the image.

High image is slightly reduced by the discharge, compared with a raster that is not emptied. However, height can be fixed easily by enlarging the amplitude of the sawtooth waveform for vertical & Cruise back.

Experimental Procedure:

1. 1.  Set-up equipment as shown above, connect the video out VCR / VCD with CRO input.

2. 2. ON the instrument.

3. 3. Set the appropriate CRO to be easily observed (use MODE switch on the TV-H position and / or TV-V, in    accordance with the observed images.)

When seeing a wave of horizontal synchronization MODE switch put on the TV-H position, while to see a wave of vertical sync put the MODE switch on the TV-V position.

4. 4. Observe and picture synchronization pulses and horizontal blanking, vertical blanking pulse, the front porch and rear, and image information.

5. 5. Image of the wave forms and determine the voltage.

Question:

1. 1. What is the frequency of horizontal sync and vertical sync?

2. 2. What system is used in the video?

 EXPERIMENT RESULTS:

Ø              Horizontal

·         Synchronization  :  3 volt

·         Blanking               :  2,2 volt

·         V / Div                  :  1 V

·         T / Div                  :  5 µs 

     

          Vertikal

·         Synchronization  :  3 volt

·         Blanking               :  2,2 volt

·         V / Div                  :  1 V

·         T / Div                  :  5 µs

DATA ANALYSIS

From the picture above we can see the contrast difference from the same side of the horizontal and vertical. And also on evacuation signal is also the same size. And the result of data obtained during alignment 3 volts and also at the time of discharge voltage value 2, which both serve targeted value obtained from TV-horizontal and TV-vertical.
For the front porch value magnitude is 0.2 volts, to the back porch voltage values measured at 3.5 volts while the voltage value information signal measured is 4.3 volts.

CONCLUSION

• Comprises a composite video signal consisting of variations of image information, pulse blanking (blanking), and pulse synchronization, each based on a function of time.