TV video signal waveforms, standards and descriptions

-- This paper mainly uses ET521-F1 to generate signals and uses et 521a to measure waveforms.

I. Brief description of color TV signals (CVBS)

1. Video Signal

Video signals include image signals (main processes) and hidden signals (inverse processes ). The color TV Video Signal in China adopts the PAL standard. The video signal is transmitted by scanning. The signal is scanned at 25 frames per second (a complete image), with 625 lines per frame, split into odd rows and even rows to scan each other-that is, 50 rows per second and 312.5 rows per game. The image signal is in the scanning process. The Scanning direction is from left to right and from top to bottom. The first line of an odd row starts at the top left of the screen, and the last line ends at the middle of the screen. The first line of an even row starts at the center of the screen, the last line ends at the bottom right of the screen. The two scan rows are interspersed to form a complete frame of image. That is to say, the start and end phases of two adjacent signals are different. The field Scan Frequency is 50Hz, the cycle is 1/50 S = 20 ms, the row Scan Frequency is 25 × 625 (50 × 312.5) = 15625Hz, and the cycle is 1/15625 S = 64 μs. The nominal video band width of a video signal is 6 MHz (the maximum frequency of brightness details is 6 MHz ).

Return of the Scan-the reverse direction is opposite to that of the normal process, forming the line and field signals.

 

2. Image Signal

The image signal includes the Brightness Signal Y and the color signal C. The color signal C is decoded to the V and U signals. The black and white images are obtained separately from Y signals. The Y and u and v signals are transformed into red (R), Green (G), and blue (B) signals by matrix transformation, then the mixed color is displayed as a color image.

Color Difference signal (r-y, B-Y) is the difference between the base color and the brightness signal. V signal is compressed red signal, V = 0.877 (R-Y); U signal is compressed blue signal, u = 0.493 (B-Y ). Sometimes the V and U signals are also expressed by r-y and B-Y respectively. Due to the relationship between Y = 0.3r + 0.59G + 0.11b (0.299r + 0.587G + 0.114b), when transmitting y, r-y, B-Y signal, the G signal is contained in the middle of the Y signal.

Brightness signals are amplitude-adjusted in high-frequency television signals. Different average dc levels in video signals indicate different brightness levels. Generally, an eight-level incremental brightness video signal is used to reflect the eight-level brightness levels of black and white images. See figure 1.

The color signal is modulated by a 4.43 (4.43361875) MHz color sub-carrier. The V and U signals are modulated on the sub-carrier with a 90 ° phase difference orthogonal balance. The V and U modulated signal FV and Fu mixed color signal C, the FV is a row-by-row inverted phase. Color signals are both amplitude modulation and phase modulation. In the AM, the average DC level of the signal reflects the brightness, and the AC amplitude reflects the color saturation (when the color saturation is 0, the sub-carrier amplitude is 0; the color saturation increases, and the sub-carrier amplitude increases ). In the phase adjustment, the phase of the sub-carrier reflects the color tones (different colors ). The waveform of the color signal is shown in figure 2.

We can use a common oscilloscope to observe the frequency, amplitude, and waveform of video signals, but the phase is not easy to observe. To facilitate the observation of the color signal waveform, we usually use the eight-color video signal. The waveform is shown in figure 3.

The eight-step DC level of the Eight-color bar signal represents the eight-level brightness from white to black. White, yellow, green, green, purple, red, blue, and black bars appear in a specific position of the red, green, and blue bars: green in 1, 2, 3, 4, red in 1, 2, 5, 6, blue in 1, 3, 5, 7. Due to the adjacent coincidence, the unique waveform is formed: one pulse in each green line, two pulses in each red line, and four pulses in each blue line. The green, red, and Blue signals (color decoding output) waveforms are shown in figure 4, Figure 5, and figure 6.

Color signal C by the comb filter separation of FV, Fu and then by Synchronous Detection of the color difference signal R-y, B-Y, also has a specific waveform, see Figure 7, fig 8.

When detecting a color TV, enter an eight-color signal to check whether the color on the screen matches the standard; you can also use an oscilloscope to view the specific waveforms of r-y, B-Y and G, R, and B signals, you can determine whether the video or color decoding circuit of the TV is normal (you do not need to check the phase of the color sub-carrier ). The decoding process of the color video signal is shown in Figure 9.

Recently, the TV set uses a large-scale integrated circuit. Some circuits in Figure 9 are integrated into the integrated circuit. Multi-standard, a variety of input, painting, Frequency Doubling digital processing circuit, making the signal flow more complex, but generally can find y, C, V (R-Y), u (B-Y), R, G, B, and other basic signals and waveforms.

To facilitate the observation of the signal waveform, the signal waveforms marked on various TV maintenance drawings are mostly measured in various parts when the eight-color signal is input.

3. Hidden Signal

In addition to transmitting image signals, Video Signals also transmit hidden signals. The line-back Transmission Line Blanking signal, and the line-back Transmission Line Blanking signal. Set the white power of the main image signal to 0.7 V, the black level to 0 V, and the hidden signal level to below 0 V (0--0.3v). Therefore, the hidden signal is in the black area, it is not displayed on the screen (text and television and other information transmitted in the reverse phase must be converted before it can be displayed in the main phase ).

In order to synchronize the display at the receiving end and the signal at the sending end, both the field synchronization and the row synchronization pulse are added to the on-site and row elimination signals. In order to correctly link the scanning of odd and even fields, the frontend and backend balanced pulses are added to the synchronous pulse respectively. For color decoding, a color synchronous pulse is added to the end of the row synchronous pulse.

 

Ii. Standard and qualitative and quantitative analysis of Video Signals

1. Video Signal standards

The image signal amplitude is 0.7 V, the row-and field-synchronized pulse amplitude is 0.3 V (opposite to the image signal), and the peak-peak VP-P = 1.0 V

In actual application, VP-P is between 0.9-1.2 V (75 Ω input load. VP-P is too small, the image is dim, and the synchronization is unstable. VP-P is too large, and the image distortion and brightness levels are deteriorated.

A common oscilloscope cannot display the details of a frame or a video signal waveform (containing hundreds of rows of information ). The waveform of a single line of signal is usually used to represent the waveform of the video signal. The waveforms of each line of the Eight-Color Strip signal are basically the same in the normal process of a scanning (the line-by-line inversion of the color v signal cannot be reflected in the waveform ), therefore, the eight-color bar signal of a line is often used as the waveform of the video signal (see figure 3 ).

The standard video signal is composed of a DC signal. The starting point of the synchronous signal is the same as that of the waveform. However, many video devices (such as DVD players and digital TV set-top boxes) use DC-separated capacitors in the video output, and the DC components in the signal have changed, this will cause image synchronization instability and gray distortion on some TV receiving devices. Generally, televisions have DC level recovery circuits for signals. Different televisions have different recovery capacities and different requirements for video signals.

2. Field invisibility Signal

Generally, the field signals of the observation TV only show the field invisibility, and the total width of the field blanking pulse is 1612 μs (25 cycles + 12 μs ). The field blanking includes the first five pulses, the field synchronization pulse (slotted), the last five pulses, and the row synchronization pulse in the back-balanced phase. The first and second balanced pulse and field synchronous pulse waveforms are shown in figure 10.

During television maintenance, the field signal only needs to observe the synchronized pulse of the field. The field synchronous pulse amplitude is 0.3 V, the total pulse width is 160 (5 × 27.3 + 5 × 4.7) μs, and the repetition period is 20 ms. To ensure the row synchronization during the on-site blanking, five slots are opened in the on-site synchronization signal, with a width of 4.7 μs, the field synchronization signal is divided into five synchronous pulses with a width of 27.3 μs and five synchronous pulses with a width of 4.7 μs. In the video mode of et 521a, the waveform of the field synchronization signal is displayed, as shown in Figure 11.

If the field synchronization signal amplitude is too small or the relative position of the start point and the black level is unstable (DC level deviation), the venue will not be synchronized or the field jitter will occur. In the video signal output from some digital TV set-top boxes, the screen conversion causes a large DC deviation from the field synchronization signal. Some of the connected TV venues do not beat synchronously. The waveform is shown in Figure 12.

3. Line Frequency Signal

The scanning cycle of a row is 64 μs, and the standard image signal amplitude is 0.7v ± 20mV. Video Signals are usually observed using a line cycle.

If you want to view the brightness level of an image, you can use an eight-step black/white signal (see Figure 1) to adjust the white balance, secondary contrast, and secondary brightness. White Balance Adjustment requires that the brightness of the eight steps displayed on the screen be black, gray, white, and not partial. Adjust the secondary contrast and secondary brightness. On the screen, we can see that the brightness levels are clearly defined for a sufficiently large contrast; in the signal waveform, the signal amplitude is large enough, and the layers of each step are still separated accurately.

If you want to see the color of the image, you can use an eight-color signal (see figure 3) or a monochrome signal. The waveform of the monochrome signal is shown in Figure 13. The phases of the color sub-carrier of the red, green, and blue monochrome signals are different, but the waveforms cannot be separated.

4. Line Blanking Signal

The line blanking signal includes the line synchronous and color synchronous signals, and the total width of the Line Blanking pulse is 12 ± 0. 3 μs. The starting point of the synchronous pulse is black, the standard amplitude is 0.3v ± 9mV, the direction is black level opposite to the image signal, and the pulse width is 4.7 ± 0. 2 μs, the recurrence of the row synchronization signal is 64 μs. The color synchronous pulse is the shoulder behind the synchronous pulse and is at the frontend of the synchronous pulse of 5.6 μs ± 0. 1 μs, symmetric above and below the black level, the standard amplitude is 0.3v ± 9mV; the color synchronization signal contains 10 ± 1 color sub-carrier pulse with a frequency of 4.43mhz, the duration is 2.25 ± 0. 23 μs. See Figure 14.

The palth inverted phase recognition pulse is hidden and restored during color decoding. The frequency of the inverted phase recognition pulse is 1/2 (7.8) kHz, and the pulse width is 8 ± 0. 2 μs.

5. Brief introduction of NTSC Signal

The field frequency of NTSC signal is 60Hz, the cycle is about 16.7 ms, the row frequency is 15.750 kHz (30 frames, 525 rows), and the cycle is about 63.49 μs. The color sub-carrier frequency is 3.58 (3.579545) MHz. balanced orthogonal modulation without line-by-line inversion is adopted. The color synchronization signal is 8-11 color sub-carrier pulses with a frequency of 3.58mhz.

See Figure 15.

The field synchronization signal waveform of NTSC is shown in Figure 16.

The waveform of the NTSC field synchronization signal is different from that of the pal field. The PAL field synchronization signal is divided into five pulses, and the NTSC is divided into six.

 

Iii. Functions of Signal Analysis for TV maintenance

1. Video Signal Analysis

After high-frequency modulation, the video signal and sound signal are transmitted to the user's TV through wireless transmission or wired transmission. After high-frequency reception and intermediate frequency processing, the TV signal restores the audio signal and the image video signal. TV maintenance generally starts to use an oscilloscope to observe sound and image signals after the intermediate frequency.

Familiar with the standard video signal waveform of TV, which helps to detect and analyze the measured waveform in TV. In recent years, the control and Signal Trend of video signals have become more complicated due to the Multi-standard TV sets, in-picture, AV, TV switching, frequency doubling, and digital processing. In the repair without images or colors, in addition to analyzing the signal control and switching status, it also checks whether the signal is normal along the Signal Direction (the signal type is different, the standard is different, and the direction is different ). It is very important to use an oscilloscope to measure the video signal and the signal waveform in the decoding process. The waveform of a common TV signal varies with the image, and it is not easy to see whether it is colored. If an eight-color signal is input, a relatively stable and easily identifiable waveform can be obtained, it is conducive to qualitative and quantitative analysis.

Sometimes, to watch or adjust the black/white balance of the TV, you need to input the black/white video signal (Brightness Signal), commonly used eight-step black/white signal (see figure 1 ). The brightness signal from the signal source, that is, the black and white video signal, carries a synchronous signal. However, some of the brightness signals measured in the TV do not contain synchronous signals because of processing, the waveform is shown in figure 17.

If the image suffers an interference fault, you can enter a white or black field signal. The waveforms are shown in Figure 18 and Figure 19. The waveform of the signal is simple. The R-y, B-y, R, G, and B are in straight lines. The signals are detected by the oscilloscope, which is easy to find out where the interference pattern is generated.

You can enter a black/white signal to check if the TV sets have a color rejection fault. If the color is still partial, it is a black and white imbalance, a picture tube drive or a picture tube fault. If the black and white are normal, it is a color decoding fault or one of the two color signals has a fault, you can use the oscilloscope to check.

In the actual television video circuit, the signal amplitude and polarity (waveform up and down reverse) will change in different detection positions. Therefore, the waveform and measurement standard of signals are marked in different measurement positions on the maintenance drawings of many televisions (generally, the eight-color signal shall prevail ).

When the contrast of the TV is adjusted and increased, the video signal amplitude increases accordingly; when the brightness is increased, the DC level of the signal increases; when the color saturation increases, the range of the color sub-carrier in the signal will increase. The old-fashioned TV sets are adjusted based on the DC level. The current TV sets are adjusted based on the data bus.

In addition to understanding the switching status and signal trend, we also need to know some special switches. For example, the control of the blue screen: When the received signal is poor, after the CPU detection, turn off the external signal and convert it to the internal blue screen signal (or identify the wrong action caused by the circuit failure ). Some ICS also have the function to cut off the signal. For example, when the 8th-pin power of ta8889 is raised, the R, G, and B signals are cut off to form a black screen; when the voltage of one of the 32, 33, and 34 feet of cxa1587 is lowered, the output of B, G, and R is cut off and the screen is black .......

When the protection circuit of TV sets, such as ABL and IK, also cut off the image signal or reduce the image signal amplitude.

That is to say, measuring the video signal and the signal waveform in the decoding process is the basic method to overhaul the video circuit when you understand the working status and switching status of the TV set.

2. Analog-digital signal conversion and transition

The digital processing of TV signals changes the waveform of traditional analog signals.

The image processing in the picture changes the waveform of the image signal, and the frequency (period) of the image signal changes by means of Frequency Doubling and line-by-line scanning .......

Signal Simulation-after digital conversion, the signal waveforms become completely different. Generally, a digital image signal without compression can be transmitted using a set of 0-1-Level Digital pulses of eight lines. For example, Y0-Y7, C0-C7, R0-R7, G0-G7 ....... There are also more complex digital modulation signals, which are encoded in a specific way after a 0-1 Level Digital pulse string. Some digital pulses can be viewed using an oscilloscope, but the signal analysis is not as intuitive as the waveform analysis of analog signals. In recent years, circuit boards of television and audio/video devices have some special digital interfaces that can be connected to computers for signal data analysis and processing.

Digital TVs can be completely digitalized from post-production to modulation transmission, from receiving and demodulation to display drivers, and all digital signals can be obtained. However, there are still a large number of analog TVs, and the conversion from analog TV to digital TV requires a transitional period. A set-top box is an important tool for converting digital signals into analog signals. Ordinary analog TVs will play a role for a long time after the arrival of Digital TVs.

Video signals can only transmit image signals of less than 400 lines. HD signals of more than 720 lines must be transmitted using R, G, B, HD, and VD analog methods, what's better is digital transmission via DVI or HDMI.

3. synchronous signal analysis

Synchronous signals are used for scanning and deflection synchronization, color decoding, picture-in-picture, graphic TV, multiplier, line-by-line and other digital processing, character generation, CPU protection (some TVs are started for a period of time, if the computer master Integrated Block CPU fails to receive the synchronous signal returned by the field oscillation, it will protect the operation and cut off the power supply ).

The trend of synchronous signals is more complex. Different Processing Circuits and different standards will switch to different directions. To maintain the Synchronization Circuit, you must check the switching status and the waveform measurement with the oscilloscope along the synchronization signal direction. Note that the synchronous signal (Sync) is usually a video signal or Brightness Signal. The synchronous signal of the field and the row is obtained only after the integration of the synchronous separation and field synchronization and the differential circuit of the row synchronization. These circuits may be integrated into large-scale integrated circuits. Synchronous signals can be observed in the video mode, field synchronization or row synchronization of the oscilloscope; or the synchronous signals of the video state can be observed in the conventional mode.

The synchronous signal from the TV signal is compared with the oscillating signal generated in the machine, and the driving signal HD and VD of the line and field are generated. The waveform and amplitude of HD and VD signals vary with television sets. For the waveform of HD and VD, see Figure 20 and Figure 21.

In the early stage, the line and field Oscillator in the TV were produced by a voltage controlled oscillator circuit, which is usually produced by the frequency division of the crystal oscillator (500-503khz or 4.43 MHz. Some TV sets have a line, field, and 3.58mhz oscillator generated by dividing the frequency by 4.43mhz.

In order to make the phase of the line scan accurate and stable, the generation of HD also requires feedback from the line scan output a line back pulse-sand castle pulse to compare with the line synchronous signal in the TV signal, generates Error Correction voltage to control the phase of HD, so that the line scan output is synchronized with the scanning of received TV signals. Generally, the sand pulse is taken from the branch winding (such as filament voltage) of the row output or the capacitor partial pressure of the primary back-peak pressure, and the positive pulse is obtained. This saradb pulse (Line Backward pulse) is often used as a line synchronous signal (H. Sync) sent to the TV's CPU for character generation or line oscillating stop protection detection. The Sandburg pulse is also sent to the color decoding circuit, which serves as the synchronization signal for color decoding of the PAL Color TV signal. When the TV receives the PAL signal without color or color instability, if the color of the NTSC signal is normal, except for Crystal Oscillator faults, it is likely that the sand Fort pulse is not delivered correctly (the signal is missing or the amplitude is insufficient ). The waveform and amplitude of the Sandburg pulse at different measurement points of different TV sets and television sets are different. For the waveform of the saradb pulse, see Figure 22.

In the maintenance of TV sets, the observation of color synchronization signals is also very important. When the Middle-end feature of the TV is not properly adjusted (mid-cycle partial adjustment) or the video-over circuit is faulty (the frequency feature is deteriorated), image distortion, blur, and abnormal color may occur, in the signal waveform, the line-synchronous shoulder deformation (up and up) of the hidden signal is displayed, and the color synchronization signal is abnormal or disappears.

This article focuses on the analysis of the TV signal itself, the actual measurement of the video signal in the TV, the measurement of the line, field synchronization and scanning circuit waveform, need to be discussed in another article

 

Du yunsui 2009-3

 

Note 1: ET521-F1 without black and white video signal (Brightness Signal y) output, can be from the ET521-F1 circuit board at the bottom of the chip IC 11 feet (1 foot close to 4.43mhz Crystal Oscillator, concatenates a 100 μcate capacitor and a 62 Ω resistor.

NOTE 2: ET521-F1 output without DC components (strings with capacitors), the measured signal waveform DC level will deviate.

 

Figure 1: Eight-level ladder Brightness Signal Waveform

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Figure 2: color signal waveform (included in the eight-color signal)

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Figure 3: 8-color signal waveform

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Figure 4 green output signal waveform after 8-color signal Decoding

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Figure 5: Red output signal waveform After decoding of the Eight-color signal

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Figure 6: Blue output signal waveform After decoding of the Eight-color signal

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Figure 7 R-Y signal waveform in 8-color signal Decoding

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Figure 8 B-Y signal waveform in 8-color bar signal Decoding

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Figure 9: color video signal Decoding

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Figure 10 balanced pulse and synchronized field pulse waveform

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Figure 11: Field synchronous signal waveform

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Figure 12: waveform of the field synchronization signal with DC Deviation

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Figure 13. monochrome Signal Waveform

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Figure 14: Synchronized pulse and synchronized color pulse waveform

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Figure 15: NTSC Signal Waveform

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Figure 16: NTSC field synchronous signal waveform

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Figure 17: white field signal waveform

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Figure 18: Black-Field Signal Waveform

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Figure 19: Brightness Signal Waveform without synchronous pulse

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Figure 20: line drive signal HD waveform (56 feet of an5192)

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Figure 21: VD waveform of the field drive signal (58 feet of an5192)

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Figure 22: line back-flow sand Fort pulse waveform (taken from filament voltage)

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