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SDRAM-accessing strategy for video processing

29 Jul 2015  | Yu-Chieh Chen, Tai-Shan Liao

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Many video devices, such as the Analog Devices ADV7179 DAC, have analogue-base band-TV interfaces for phase-alternating-line (PAL) and National Television System Committee (NTSC) video signals. Unfortunately, these kinds of DACs accept video in interlaced-image format only, but you may need progressive-scan video instead. Furthermore, many of the progressive-scan images vary in size, which makes it more difficult to convert a progressive-scan image to an interlaced image. Therefore, you need a universal and efficient image buffer, such as SDRAM or DRAM, as a strategy for separating the image field.


Figure 1: The pixel-clock signal puts image data into FIFO memory, which later synchronizes and goes into system RAM.


Figure 1 shows the timing for a typical progressive-image data format. The upper four signals include the progressive-image source, including a frame-synchronisation signal, a line-synchronisation signal, a signal, and a pixel clock with pixel-image data. The lower two signals are the frame-synchronisation signal, which contains many line-synchronisation signals when the frame-synchronisation signal is high, and the line-synchronisation signal.

The pixel clock writes the progressive-image data into FIFO (first-in/first-out) memory. A higher-rate data clock can then write the data into RAM when each line-synchronisation signal is low. This procedure ensures that the progressive-image data will correctly write into SDRAM regardless of how the pixel clock changes because of the various progressive-image data sizes. When the RAM write-enable signal or RAM read-enable signal is high, the system writes data into or reads data from SDRAM.


Figure 2: The frame-synchronization signal of progressive-image data alternates between reading odd and even rows of data in memory.


Figure 2 shows the frame-synchronisation signal of progressive-image data and the frame-synchronisation signal of interlaced-image data. The write-new-data and read-old-data enable signal executes at every line-synchronisation signal of the progressive-image data when at a low level and at every frame-synchronisation signal when at a high level. You can execute the read-old-data enable signal only when the frame-synchronisation signal is low, however. This scenario occurs when there are no valid image data in this period. Figure 3 shows the data flow of the SDRAM-accessing procedure. A frame may, for example, contain 15 rows, in which you define the row data to count from 00 to 0e. Image data for odd rows are one, three, five, seven, nine, 11, 13, and 15, and image data for even rows are two, four, six, eight, 10, 12, and 14.


Figure 3: The state of the frame-synchronization signal determines whether the system performs a read or a write operation.


By using this SDRAM-accessing strategy, you can generate the interlace data and synchronise it with the frame-synchronisation signal of the original progressive data. Thus, you need not worry about image size. Moreover, it can easily tune the interlaced-image data timing, changing the number of blank rows, without changing the write-into- or read-from-SDRAM sequences. You need to decide only which line-synchronisation signal in low-level periods reads the old image data from the SDRAM.


About the authors
Yu-Chieh Chen and Tai-Shan Liao are with National Applied Research Laboratories in Hsinchu, Taiwan.


This article is a Design Idea selected for re-publication by the editors. It was first published on March 18, 2010 in EDN.com.




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