NI VideoGenerator Memory and Image Resolution ConsiderationsPrimary Software: Other NI Software>>NI VideoMASTERPrimary Software Version: 1.0.3 Primary Software Fixed Version: N/A Secondary Software: N/A Hardware: Modular Instruments>>High-Speed Digitizers (Scopes)>>NI VideoMASTER for Analog Video Analysis-PXI
Problem: How much memory will I need on my NI 5421 to generate a given test pattern? Also, if I am going to create my own image to generate, how should I determine the size of my image? Solution: Creating an Image When determining how much memory is going to be allocated when generating a given test signal, it is important to first understand how the image resolution is determined for a particular video format. The generation of an analog test pattern is based on converting a bitmap image, line by line, to a pattern of analog signals. When actually calculating the analog signal based on a bitmap signal there is a caveat that needs to be taken into account. A normal bitmap image does not actually have enough pixels to map, pixel-to-sample, to create an analog signal of the appropriate size. For example, to generate a 640x480 pixel image, as with M-NTSC, it is not sufficient to simply generate 640 samples per line to accurately represent the color information in a given line of 640 pixels. There is a best fit for the number of samples used to represent a given line of a test signal for each video format. This number is 1272 (1044 active) samples per line for NTSC and M-PAL, and 1280 (1040 active) for all other formats. The following two tables show the timing information of the NI 5421 for per line generation and the active image specifications for CVBS formats.   When generating a test image from a bitmap that is 640 pixels wide, the VideoGenerator software interpolates the image data using a linear interpolation algorithm. While the interpolation does not produce any visible effect on the image, it is still a measureable difference. Therefore, if you are creating a custom bitmap image to generate for precision measurements, using M-NTSC, make sure that the resolution of your bitmap image is 1044x480, so that no interpolation is necessary. Memory Calculation When generating a signal, one line of that signal gets downloaded to the onboard memory and is regenerated to produce the output image. So in order to calculate the amount of memory needed for a particular image generation, it is important to note some of the information listed in the above tables. The best way to show the memory calculations is through an example. The following example is for generating a single test pattern using the PAL video format. For PAL we use 1280 samples per line and there are 625 lines per frame. We also need 4 frames before the color carrier repeats itself in phase, so we get a whole number of periods of the color carrier. The 4 frames will be one cycle, which is repeated by the generator. Therefore, the calculation for a PAL signal is as follows: 1280 samples/line x 625 lines/frame x 4 frames x 2 bytes/sample= 6.4MBytes of memory. For the NTSC signal the numbers are: 1272 samples/line x 525 lines x 2 frames per cycle x 2 bytes/sample = 2.6MBytes of memory. Related Links: Knowledgebase 4WJ817FS: What Video Standards Does NI VideoMASTER Support? Knowledgebase 4W0BM30O: What is the NI BNC-2178 VideoMUX and How Do I Install It? Attachments:
Report Date: 04/30/2009 Last Updated: 09/06/2011 Document ID: 4WTD350O |
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