Analog video filtering technology of the hottest c

Analog video filtering technology of camera system

in most video systems, the video output of video encoder contains low-pass filter. These filters are used to suppress high-frequency noise and smooth the rising and falling edges of the video signal output from the video digital to analog converter (DAC). Traditional video systems usually use discrete passive filters. However, in the vast majority of video subsystems at present, the integrated filter amplifier is used to filter and amplify the video signal after the video DAC. This paper introduces in detail the various integrated video filter amplifiers provided by Maxim to adjust the balance weight to meet the needs of a variety of video applications

for video camera systems, the most common signals output by video DAC are video blanking, synchronization (CVBS) and luminance/chrominance (y/c) composite signals. Examples 1 to 8 introduce the configuration of eight Filter Amplifiers in detail, which have different DAC output signal DC level, signal amplitude and the combination of AC and DC coupled video signals. The common power supply voltage of integrated video filter is 5V or 3.3V. For applications requiring micro power consumption (examples 6 and 7), 1.8V or 2.5V power supply can be used to power the video filter amplifier. The filter amplifier (max9509) in these low-power applications uses Maxim's patented directdrivetm technology to obtain 2Vp-p video signals through internal fixed 8v/v gain

the following eight configurations have several common characteristics: all outputs are measured on a 75 Ω load. Therefore, when the output curve shows 1Vp-p, the output of the integrated filter amplifier is 2vp-p. In addition, all filters use 75% of TV NTSC color bar signals as signal sources

the reconstruction filter connects the video DAC and the video amplifier

in example 1, the output of the video DAC is connected to the max9502g video amplifier through the reconstruction filter. Bias the video signal output of the DAC to make the synchronization head close to the ground potential. Max9 each detail 502g filters and amplifies the signal to obtain the output signal of 2Vp-p, controls the measurement accuracy, and provides DC bias for the signal. The output of max9502g also provides bias, and its synchronization head is about 300mV higher than the ground potential. At the load, due to the 75 Ω voltage division structure at the output end, the level of the synchronization head becomes 150mV. Max9502g is a highly integrated solution, which takes up a small circuit board area and can effectively save space in most portable system designs

video DAC provides AC coupling signal to video amplifier

in example 2, video DAC provides AC coupling video signal to max9586 video filter amplifier. This is a very good solution for single power applications that require AC coupling signals and synchronous heads below ground potential. However, the output AC coupling signal does not place the black level at the ground potential; The black level changes with the content of the video signal. The max9586 can drive two DC coupled video loads or one AC coupled 150 Ω load

case 1 plus 0.5vp-p DC bias signal

case 3 is very similar to case 1, except that its DAC can only output 0.5vp-p DC bias signal. In this case, the max9502m with 12dB fixed gain is a more suitable solution. The video signal on the load is DC biased, and the synchronization head is about 150mV higher than the ground potential. In addition, the video signal output of the DAC must be above the ground potential. Max9502m can provide 2Vp-p video drive for a 150 Ω ground load

video DAC with only one CVBS or Y signal output

example 4 provides a very interesting configuration. In some applications, the DAC provides both y and C signals, while in example 4, there is only one output. This output can be selected between CVBS and Y signals, so that the CVBS signal can be generated using the summation (Synthesizer) circuit. It is difficult to provide these two kinds of signals at the same time on the same output, and it is also difficult to switch these two kinds of signals at the right time. Generally, a 2:1 multiplexer is used on the output line to realize this function. Fortunately, the max9524 video filter amplifier used in this example integrates two analog single pole single throw switches, which can be configured as a 2:1 multiplexer, so that the input can be properly selected and filtered and amplified by using one chip. Because the DC level of Y and C signals cannot be determined after summing; Therefore, the video signal should be AC coupled before filtering and amplification. The clamping circuit after the AC coupling capacitor establishes the bias level

designers should pay attention to the design of summing circuit for generating CVBS signal, and carefully consider the DC bias level of Y and C signals and the allowable voltage range of DAC. The CVBS signal generated by directly connecting y and C signals will exceed the DAC voltage range, which depends on the DC bias level of each signal

multi channel video output with y/c to CVBS synthesizer

max9512 in example 5 has four independent output channels, which can be ideal for multi-channel video output design. The device also has a synthesizer from y/c to CVBS, which generates composite video signals from y and C signals. Each output can drive two DC coupled video loads or one AC coupled 150 Ω load. The chip adopts Maxim's smartsleep Technology (not shown in the figure), which can detect the input signal or output load, and turn on or off different amplifiers accordingly, so as to reduce power consumption. This configuration is common in applications that provide one s terminal and two CVBS outputs

low power design with black level close to ground potential

example 6 the power consumption is greatly reduced due to the use of max9509. The device uses 1.8V single power supply, and the average power consumption is only 11.7mw. Another advantage of this configuration is that the black level is close to the ground potential, the output does not need a large coupling capacitor, and the video signal is between -300mv and +700mv, which is independent of the content of the video signal. Since the amplifier provides 8v/v fixed gain internally, the DAC output amplitude should be 0.25vp-p. For any type of DAC, this value can be obtained by changing the termination resistance of the output end

one way outputs y, C and CVBS signals

for some applications, DAC outputs only y and C signals, but the system must provide CVBS signals. In this application, the general solution is to use the summation circuit to generate the required output signal. This solution is similar to the summation circuit used in example 4, but since the required CVBS amplitude is only 0.25vp-p, an appropriate voltage can be generated. If the normal output of DAC is 1Vp-p, 0.25vp-p amplitude can be easily obtained by changing the termination resistance of DAC

Example 7 provides a filtering and amplification scheme for micro power applications. The designer reduces the output termination resistance of the DAC in proportion to obtain a suitable output amplitude (0.25vp-p). Since the DC bias level is uncertain (depending on the signal and summing circuit), the signal should be AC coupled to the max9509. The synchronous head clamping circuit offsets the input signal appropriately. The filter amplifier has DirectDrive function, and the output black level of the amplifier is close to the ground potential. In this way, large coupling capacitance is not required at the output. Therefore, the max9509 can drive the 2Vp-p video signal to a 150 Ω load

there is no DC bias in the 2016 general manager special award of Aluminum Corporation of China for two-way video output signals

for applications requiring two-way video output signals (for example, s terminal), the max9583 dual channel video filter amplifier provides a compact solution, as shown in example 8. Max9583 has 2v/v internal fixed gain, so it is suitable for DAC with 1Vp-p output. The output of the device can be AC coupled to one 150 Ω load or DC coupled to two video loads. The AC coupled video signal does not need any DC bias, and the black level changes with the change of video content

conclusion

this paper focuses on the common configurations in current video applications. CVBS and y/c are by far the most common output signals in such applications. In some high-end devices, there will be relatively rare YPbPr output signals. Video signals can be standard definition (SD) or high definition (hd) signals. Although these relatively rare applications are not discussed in this article, designers should understand that these designs also have corresponding integration solutions

U.S. patent 7061327

patent application in progress. (end)