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Wide-Bandwidth Video Switch Matrix/Drivers and Transmission Technology in Consumer Electronics

Thu, 09/27/2007 - 6:32am
A new transmission scheme for digital video signals paired with current transfer logic (CTL™) will help meet the market requirement for multiple display needs.

By Jeff Ju, Fairchild Semiconductor

With the strong demand ramping in the automotive accessory market, traditional car DVDs with a single CVBS and mono audio signal output to a single display will be unable to meet the market requirement for multiple display needs (drivers and passengers), and many A/V inputs such as GPS component inputs, backup/side camera

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Figure 1. Video Switch Matrix used in car DVD analog backend architecture.
inputs and many other auxiliary A/V inputs. This article will present a complete introduction to analog video applications for A/V signal conditioning. It will also explore new transmission schemes for digital video signals using a serializer/deserializer solution featuring current transfer logic (CTL™) technology and will illustrate the value of such technologies in real back-up cameral modular applications.

Car DVD Applications

As shown in the video output architecture in a car DVD design in Figure 1, the output video source from the car’s DVD to the TFT panel display is no longer a single DVD composite video output. It can be from a GPS system output with configurable video formats such as S-video, composite and component video. All these video outputs are fed into the video switch (up to 12 inputs or more) and all of the input to output routing paths are controlled by I2C interface through writing the value to the specific registers.

Through the adoption of a wide-bandwidth video switch matrix, any inputs can be either connected to any output or broadcasted to all outputs with minimal distortion, which allows the passengers in the car to watch either the same video content or a different one as they prefer. A ן db bandwidth of 100 MHz is ideal for almost all SD/PS/HD car DVD designs. The output buffer of the video switch matrix offers programmable gain to meet the requirements of a long cable application with gain needs of more than 6 db. If any further additional filtering need is required, the video filter with selectable bandwidth (8 MHz/16 MHz/32 MHz) can be placed after the switch matrix to act as reconstruction filter. In such a design, the cross-talk is another concern for both designers and consumers of this product. So in the design stage itself, optimization of both the trace inductance and mutual inductance are recommended. Of course, the audio signal path can also use a switch matrix similar to video path matrix with additional op-amplifier based active filters to enhanced SNR if needed.

Digital Video Data Transmission Technology

As the automotive intelligence market moves forward, the sensor-based intelligent video system has been widely adopted in the middle to high-end car market. As an example, the backup and side cameras are highly desirable by consumers. Instead of traditional analog video transmission technology, the digital interface, particularly differential signaling technology, has been widely adopted by designers due to its better noise immunity and the serializer/de-serializer architecture that features significant wires reduction between the camera module and the displays. Typical differential signaling

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Figure 2. CTL™ I/O technology used in car backup camera video data transmission.
technology used in such applications is Low Voltage Differential Signaling (LVDS) technology, which has about only 350 mV swing. The advantage of such technology is its constant current output on the driver output side. The constant current output allows it to have a constant edge rate with minimal overshoot and undershoots resulting in excellent signal integrity in addition to low EMI emission. The major limiting factor is at the receiver side. Usually the receiver architecture has a transconductance amplifier followed by an I-V conversion stage. This kind of voltage-sense based architecture relies heavily on the driver output current for higher throughput and is very sensitive to the capacitive load on the cable line. This is particularly true for longer cable data transmission at higher data rates in car backup camera applications. Typically, it requires a repeater at the receiving end for signal magnitude restoration.

With the newly emerging true current transfer technology (CTL), applications can have a desirable solution as is shown in Figure 2. The serializer basically serializes the parallel TTL input signals from camera module. The serialized high speed signal is then transmitted using CTL technology flying over the cables (only 4 wires). The serial video information is decoded back to parallel TTL signals at panel side by de-serializer to drive the display panel. Again, the advantage of this architecture is reduced cable/connector size, improved signal immunity with differential signaling technology and much less EMI emission (about 100 mV swing only) than traditional TTL and even LVDS interface (>10 db lower).

The major difference between CTL I/O technologies compared with LVDS technology is the receiver logic-state detection scheme. For CTL receivers, it detects the differential current instead of differential voltage for LVDS technology. So CTL interface is much less sensitive to the capacitive load compared than LVDS technology, which is great for car backup camera applications with longer cable requirement. The LVDS repeater that is usually required for high speed LVDS data path is no longer needed for CTL technology due to its true current detection scheme. In addition, CTL technology also saves more than 70% power than LVDS (1 mA per channel for CTL technology and 3.5 mA per channel for LVDS I/Os). In such high speed design, key attention needs to be paid for the high-speed serialized signal layout with equal length and equal load to the differential line to reduce the skews between plus and minus of the differential pair.

Summary

The continuous demand of more features in car cabinet music/display system continues to drive the fast adoption of the high performance analog products for either signal transmission/conditioning or analog front/back end routing. This market segment is becoming stronger in Asia, particularly China, with strong automotive market momentum. High driving capability, wide bandwidth video switch matrix/filter and low EMI differential signaling technology will become very popular in the market for the video output architecture with multiple display loads. About the Author

Jeff Ju is director of Asia Business Development for the Signal Path Analog Group, Fairchild Semiconductor. He has published numerous papers and patents in the high-speed signal design area. He works closely with portable and consumer customers to drive high-performance analog product definition/applications in Asia. You can reach him at jeff.ju@fairchildsemi.com.

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