NI PXI/PCI-5431
Video Signal Generation
The NI PXI/PCI-5431 generates an analog video signal by converting a data array from digital to analog. This digital data array is downloaded to the NI 5431 onboard memory. The resolution of the data is 16-bit, even though only 12 bits are used to represent the analog signal (12-bit DAC).
The update rate of the DAC depends on the video format, as shown in the following table.
NI 5431 Timing Information
| Video Format | Data Rate (MHz) | Total Samples/Line | Active Image Samples/Line | Total Line Duration | Active Line Duration |
|---|---|---|---|---|---|
| M-NTSC/ M-PAL | 20.013986 | 1,272 | 1,044 | 63.556 µs | 52.16 µs |
| All others | 20.000000 | 1,280 | 1,040 | 64.000 µs | 52.00 µs |
When a bitmap image is converted to a video signal, the image pixels are extracted line-by-line, and the composite video signal is calculated based on the image pixel values. However, a normal bitmap image with an aspect ratio of 4:3 includes fewer pixels than needed to map, one-by-one, the calculated samples of a video line. For example, in M-NTSC, a typical bitmap image size is 640 × 480, which is 480 lines of 640 pixels (640 = 480 * 4 / 3), but the number of active image samples per line in the final video signal is 1,044.
To calculate 1,044 active video samples based on 640 image pixels, the software interpolates the image data using a linear interpolation algorithm. While this interpolation has very little visible effect on the resulting composite video image, it is still measurable. Therefore, if you use the active images to perform precision measurements, you must have an input bitmap image of exactly 1,044 x 480. The following table shows the different image size values to use depending on the actual video format.
Active Image Specifications for the Different Video Formats
| Video Format | Total Active Lines | Square Pixels per Line | Active Video Samples/Line | Interpolation Ratio | Active Line Duration |
|---|---|---|---|---|---|
| M-NTSC/ M-PAL | 480/486 | 640 | 1,044 | 1.63125 | 52.16 µs |
| All other formats | 576 | 768 | 1,040 | 1.35417 | 52.00 µs |
In the BMPs directory, you can find full-precision bitmap images in the 1,044×480 folder (use with M-NTSC or M-PAL formats) or the 1,040×576 folder (use with all other formats).
Make the same considerations for insertion test signals (ITS). To keep the full accuracy (avoiding automatic interpolation) of a test signal inserted in the image, such as the vertical blanking interval shown in Gray Scale Image and Extracted Line Profile, you must make sure that the used test line data arrays have the correct length, which is 1,044 samples for M-NTSC and M-PAL, and 1,040 samples for all other formats.