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MAX7481 Scheda tecnica(PDF) 7 Page - Maxim Integrated Products |
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MAX7481 Scheda tecnica(HTML) 7 Page - Maxim Integrated Products |
7 / 13 page Bidirectional Video Filters/Buffers for Composite and S-Video Portable Media Devices _______________________________________________________________________________________ 7 device, such as a display in playback mode. In playback mode, internal switches S1, S2, S4, and S5 close the path from the video processor to the filter upon driving CVBSON and/or YCON high. Driving either YCON or CVBSON high enables the signal path from the video processor through the filter. Switches S6, S7, and S8 are open and not in the signal path in playback mode. Driving YCON high drives the Y signal to OUT1 and C signal to OUT2 while enabling CVBSON combines the Y and C signals to form composite video and output on OUT3. Both composite and Y/C video can be active in this mode. Pull P/R low for playback mode. Record Mode Record mode takes the video signal from an external video source and provides anti-aliasing for both com- posite and Y/C signals. Figure 5 shows the signal flow from the external video source to the video processor. Upon driving P/R high, switches S1, S2, S4, and S5 change states so that the signals utilize the same filters and now route to the video processor. Switches S6, S7, and S8 all close to ground so that the incoming signal sees a 75 Ω load termination to ground. The inputs IN3, IN4, and IN5 are AC-coupled with a 0.1µF capacitor that works with an internal clamping circuit. Enabling CVBSON drives the video signal from IN5 through the upper filter and outputs at OUT4. In this mode, OUT5 is disabled and pulled to GND. Enabling YCON drives the Y signal from IN3 through the upper filter to output at OUT4 and the C signal from IN4 through the lower filter to output at OUT5. Switch S3 connects OUT4 to either the Y channel if YCON is high, or the composite video channel if CVBSON is high. Driving both CVBSON and YCON high results in an invalid state, resulting in all output buffers shutting down and pulling to GND. Connection Sharing for Inputs and Outputs Outputs OUT1, OUT2, and OUT3 and inputs IN3, IN4, and IN5 can share one connector to save space. Figure 6 shows a possible interface at OUT1/OUT2/ OUT3 and IN3/IN4/IN5. In record mode, the switch closes, OUT1/OUT2/OUT3 go to GND, and the external 75 Ω resistor looks like a load-termination resistor. Figure 4 shows that in playback mode, IN3/IN4/IN5 are not used and the switches to GND are open. The exter- nal 75 Ω resistor serves as a back-termination resistor in playback mode and provides the cable termination in record mode. Bidirectional Video Filter and Buffer The MAX7481/MAX7482 output buffers are designed to drive both Y/C channels as well as composite video. The output buffers use the internally generated nega- tive supply to shift the blanking level of video outputs (composite and Y only) to ground so that the active video signal is above ground and the negative sync pulse is below ground. The C signal is shifted so that it is centered at GND. The output buffers are capable of driving a full 2VP-P video signal into a standard 150 Ω load DC-coupled to GND. The output buffers can drive an AC-coupled load provided that the load impedance is greater than 5k Ω. The MAX7481/MAX7482 utilize the same filter in both playback and record modes by using switches that route the signals through different paths in the two modes (see Figure 3). Switches S1–S8 change states depending on the mode of operation and control input settings. The use of these internal switches enables utilizing a single filter in both modes for a unique solution in space constrained and cost-sensitive applications. _______________________________________________________________________________________ 7 INPUT 500mV/div OUTPUT 500mV/div 2ms/div Figure 1. AC-Coupled Output INPUT 500mV/div OUTPUT 1V/div 2ms/div 0V 0V Figure 2. DirectDrive Output |
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