What Constitutes a "Standard" Analog Video Signal and When Do I Need a PCI-1410 to Acquire Non-Standard Analog Video Signals?Hardware: Image Acquisition (IMAQ)>>Analog Image Devices>>PCI-1405, Image Acquisition (IMAQ)>>Analog Image Devices>>PCI-1407, Image Acquisition (IMAQ)>>Analog Image Devices>>PCI-1408, Image Acquisition (IMAQ)>>Analog Image Devices>>PCI-1409, Image Acquisition (IMAQ)>>Analog Image Devices>>PCI-1410, Image Acquisition (IMAQ)>>Analog Image Devices>>PCI-1411, Image Acquisition (IMAQ)>>Analog Image Devices>>PXI-1405, Image Acquisition (IMAQ)>>Analog Image Devices>>PXI-1407, Image Acquisition (IMAQ)>>Analog Image Devices>>PXI-1408, Image Acquisition (IMAQ)>>Analog Image Devices>>PXI-1409, Image Acquisition (IMAQ)>>Analog Image Devices>>PXI-1411
Problem: I have a camera that has a specification of 525 lines per frame. It states that it is a standard analog video camera and uses RS-170 format. I thought that a standard analog signal consisted of 640 rows and 480 lines (i.e.: 640 x 480). Solution: A "standard" camera is one that uses a common, well defined signal format, with a defined pixel clock rate, line rate, and number of lines per frame. This basically means either RS170/NTSC or CCIR/PAL, which have 525 lines and 625 lines per frame respectively. This refers to the actual number of hsync pulses during one frame period and not the sensor resolution. Typically about 30 lines used for blanking, closed caption, and other communication / synchronization signals. Normally, 480 lines are acquired for RS-170 only because it is a standard image size (640 x 480). Almost all analog cameras will output a few extra valid video lines and a user could acquire these lines if desired. The situation is slightly different in the horizontal direction, because the camera does not explicitly control the number of pixels per line sampled by the frame grabber. In the horizontal direction, the number of pixels on the camera sensor is sent as an analog signal and is then resampled as 640 square pixels by the frame grabber. This gives the image the correct aspect ratio for processing or for display on devices which have square pixels (the IMAQ Image control displays square pixels). A standard analog camera with non-square pixels, such as one specifying 768 pixels in the horizontal direction, will still send a standard analog signal which will be sampled as 640 pixels by the frame grabber to maintain aspect ratio. If so desired, a frame grabber can be forced to oversample or extract more than 640 pixels from an analog signal sent from a standard camera, however, the resulting image will not have the correct aspect ratio. It will look "stretched" in the horizontal direction. The PCI-1405, PCI-1407, and PCI-1411 are frame grabbers that require a standard video signal. The PLL (Phase Lock Loop) on these boards is looking for the hsync pulses at a specific rate (~15.7kHz for RS-170, ~15.6kHz for CCIR) and will generate a pixel clock at rates that correspond to these standard formats. Remember, the analog signal from the camera does not carry a pixel clock, so the board must recreate the clock by looking at the hsync pulses. The clock must be in phase with the hsync signal and at the right rate. The PCI-1409 can handle these same standard formats, plus you can configure the board to accept the hsync, vsync, and pixel clock on separate externally connected lines for cameras that do not use a standard video format. If the signal is not "standard" the board must be told explicitly where to start each frame and line, and where to sample each pixel. Incidentally, this is exactly how digital cameras work, all hsync, vsync, and pixel clock signals are sent over with the image data. Unfortunately, most nonstandard analog cameras do not transmit a pixel clock, which can make it difficult to use the PCI-1409. The PCI-1410 was developed to accommodate these nonstandard analog cameras. The PCI-1410 has a new PLL that is more flexible. It can accept almost any hsync line rate (>8kHz). The PCI-1410 only needs to know many clock cycles per line to generate and can automatically generate the right pixel clock. For RS-170 video, there are 780 clock cycles per line, but for non-standard cameras this can be almost any value, and it should be listed in the camera documentation. Usually it's about 10-30% more than the actual number of pixels acquired per line. Related Links: NI Vision Hardware: Analog Video Acquisition Boards KnowledgeBase 3OHEMS6W: Configuring the PCI-1410 With a Non-Standard Camera or Video Source KnowledgeBase 3i9FKE6W: Differences Between the PCI-1410 and the PCI-1409 Attachments:
Report Date: 10/07/2005 Last Updated: 07/31/2008 Document ID: 3Q6FPGMQ |
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