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Abstract: CXA2075M is a video decoding chip produced by SONY. The chip converts analog RGB signals into a combined video signal, making it ideal for image processing conversion and video game console applications between personal computers and high definition televisions. The article analyzes the structural principle and main features of CAX2075M, and gives the typical application circuit of CAX2075M.
Keywords: decoder; RGB; video signal; subcarrier
1 Overview
CXA2075M is a video decoding chip produced by SONY. It converts analog RGB signals into combined video signals. The chip has different pulse generators to meet the coding needs. It can encode the input composite sync signal, subcarrier and analog RGB signal to output the combined video and S terminal Y/C output signals. Compared with CXA1645M? CXA2075M has the following excellent performance?
(1) The number of external components is reduced to five, and only components such as clamp capacitance, adjustable capacitance, and filter resistance can be used;
(2) There is a trap inside, so the trap can be used outside the device;
(3) The RGB output band is higher than the CXA1645M.
The CXA2075M can be widely used in VGA interfaces for HDTVs and digital TVs, interface conversion circuits for personal computers and TV video converters, and video games.
figure 1
   The CXA2075M chip uses a bipolar silicon monolithic integrated circuit process. The main electrical parameters (maximum ratings) are as follows:
â—Power supply voltage VCC: 12V;
◠Working temperature Topr: -20 ~ +75 ° C;
◠Storage temperature Tstg: -65 ~ +150 ° C;
â— Allowable power consumption PD: 780mW.
2 pin description
The internal structure and pinout of the CXA2075M are shown in Figure 1. The function of each pin is as follows:
1 pin (GND1): Ground of all circuits except RGB, including the ground of video and Y/C output circuits. When wiring, the wiring of GND1 should be as short and wide as possible.
2 to 4 feet (RIN, GIN, BIN): Analog RGB signal input.
5, 9, 11, 13, 14, 18 feet (NC): empty feet.
6-pin (SCIN): Subcarrier input. A sine wave or pulse of 0.4 to 5.0 Vp-p can be input.
7 pin (NPIN): NTSC and PAL system conversion end. It is NTSC when connected to VCC and PAL when connected to GND.
figure 2
8 feet (BFOUT): BF pulse monitoring output. Can drive a 75Ω load.
SYNCIN: The composite sync signal input is TTL level, should be less than 0.8V when low, and high (greater than 2.0V) during SYNC.
12-pin (Vcc1): Power supply for all circuits except RGB.
Pin 15 (COUT): Color saturation signal output. It can drive a 75Ω load for the C signal output of the S terminal.
16-pin (YOUT): Y signal output. It can drive a 75Ω load for the Y signal output of the S terminal.
17-leg (YTRAP): An external notch can be used to reduce the color mixing caused by the subcarrier frequency component of the Y signal. When using the COUT pin, connect a capacitor between YTRAP and GND or connect the capacitor in series with the inductor. The capacity and inductance can be determined according to the requirements during design, and should not affect the YOUT pin.
19 feet (VCC2): RGB power supply terminal. Used to power video and Y/C output circuits. A large 10μF capacitor on this pin can be used as a current source.
20 feet (CVOUT): Video signal output. It can drive a 75Ω load for video signal output from a TV.
21, 22, 23 feet (BOUT, GOUT, ROUT): Analog RGB signal output that can drive a 75Ω load for extended connection to VGA display output.
Pin 24 (GND2): Ground of the RGB signal. When wiring, the connection line of GND2 should be as short and wide as possible.
3 Operating instructions
3.1 Y (brightness) signal generation
The CXA2075M's analog RGB signals are input from pins 2, 3, and 4, respectively, and clamped in the clamp circuit, and output from pins 23, 22, and 21, respectively. The dot matrix circuit performs encoding according to the input signal to generate a luminance signal Y and color difference signals R-Y and B-Y. The Y signal input hysteresis limit together with the color saturation signal C determines the lag time. Then, when the additional CSYNC composite sync signal is input from pin 10, the Y (brightness) signal can be output from pin 16.
3.2 Generation of C (chrominance) signals
The subcarrier is input from pin 6 and sent to the phase converter to change its phase by 90°. The subcarriers are then sent to the modulator and modulated by the R-Y signal and the B-Y signal. The modulated subcarriers are mixed, and the higher harmonics are removed by a bandpass filter, and finally output as a C (chrominance) signal from pin 15. At the same time, the Y and C signals are mixed and output as a composite video signal from pin 20.
image 3
   3.3 Synchronous pulse signal generation
The line and field sync pulse signals required by the CXA2075M are generated by the separation of the composite sync signal through the 10-pin SYNCIN input. The waveform is shown in Figure 2.
4 Precautions for use
When using the CXA2075M? Note the following:
(1) Make sure that the analog RGB signal input has a maximum value of 1.0Vp-p and that the impedance is low enough. High impedance affects color saturation and chromaticity. When the input RGB signal exceeds 1.3Vp-p, the clamp level will be out of control.
(2) The SC input (Pin 6) can input either a sine wave or a pulse, and the level range should be between 0.4 and 5.0 Vp-p. However, when a pulse is input, its phase changes by a few degrees compared to the input sine wave. In the chip, the SC input is biased towards 1/2 of VCC. Therefore, when a 5.0Vp-p pulse is input, especially when the idleness deviates from 50%, the high/low level pulse voltage will exceed the VCC and GND voltages in the chip, causing subcarrier distortion. In this case, the duty cycle must be kept at 50%.
(3) When designing a printed circuit board, pay attention to the wiring of VCC and GND. In order to match the VCC pin, é’½, porcelain or other capacitors with good frequency characteristics should be used. And the designed lines should be as short and wide as possible.
(4) Adding a resistor and capacitor to the subcarrier (SC) input and sync (SYNC) input pulses at the input front end eliminates high frequency components in SC and SYNC. But be careful not to enter pulses with high frequency components. Otherwise, high-frequency components will enter important components such as VCC and GND, causing system failure.
   (5) An external resistor on the 75Ω driver output pin prevents oscillations caused by tens of picofarads of capacitance between each pin.
(6) For YTRAP pin? Pin 17? The following three methods can be used to reduce the color mixing caused by the subcarrier frequency component in the Y signal. The first is to install a 30-68pF between YTRAP and GND. Capacitance. The second is to connect a capacitor and inductor in series between YARAP and GND. The third is to use the trap filter inside the chip.
5 application circuit
Since China's color TV adopts PAL system, this paper only lists the typical application circuit of CXA2075M adopting PAL inter-nal TRAP mode. The specific circuit connection is shown in Figure 3.
Figure 4 shows a VGA/video converter designed with the CXA2075M. In the figure, the VGA 15-pin connector on the left side can be connected to the VGA port of the graphics card of the computer. The output part can be connected to the TV through the video port or S terminal. The converter can also interface with a computer monitor for simultaneous display.
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