Digital Cameras

Digital cameras use three types of signals—data lines, a pixel clock, and enable lines. Data lines are parallel wires that carry digital signals corresponding to pixel values. Monochrome digital cameras typically represent pixels with 8, 10, 12, or 14 bits. Color digital cameras typically use up to 8 or 10 bits per color for each pixel. Depending on your camera, you may have as many as 30 data lines, or wires, representing one pixel. The number of data lines per pixel determines bit depth.

The pixel clock is a high-frequency pulse train that indicates when the data lines contain valid data. On the active edge of the pixel clock—which can be either the rising edge or the falling edge, depending on the camera—the value of the digital lines is input into your image acquisition device. The pixel clock frequency determines the rate that pixels are acquired.

Enable lines indicate when data lines contain valid data. The Line Valid signal in digital cameras provides the same type of information about where each line is located as the HSYNC signal provides for analog cameras. In digital cameras, the Line Valid signal is usually active for the entire duration of the line, rather than indicating only the start of the line. At the end of that row, the Line Valid signal goes inactive until the next row of pixels begins. The Frame Valid signal in digital cameras provides the same type information about where each line is located as the VSYNC signal provides for analog cameras. In digital cameras, the Frame Valid signal is active during the acquisition of an entire frame. At the end of that frame, the Frame Valid signal goes inactive until the beginning of the next frame.

Digital line scan cameras consist of a single row of pixel elements and require only a Line Valid timing signal. Area scan cameras provide both the Line Valid and Frame Valid signals.

Taps

Increasing the speed of the digital camera pixel clock or acquiring more than one pixel at a time can greatly improve camera acquisition speed. A tap is a group of data lines that together carry one pixel. A camera that latches only one pixel during the active edge of the pixel clock is known as a single tap camera. Other cameras have multiple pixels on separate data lines that are all available during the active edge of the pixel clock. These multi-tap cameras are available with as many as 10 taps. Cameras with multiple taps require more data lines but provide faster data transfer.

Camera Files

Because digital cameras vary in specifications such as speed, image size, pixel depth, number of taps, and modes, NI-IMAQ requires a camera file specific to your camera to define all of these values in order to use that camera with your image acquisition device. You can find a list of camera files that have been tested and approved by National Instruments online at the National Instruments Industrial Camera Advisor at ni.com/zone. You also can create your own camera files using the NI Camera File Generator, which you can download from the National Instruments Machine Vision Web site at ni.com/vision.

Types of Digital Cameras

There are three main types of digital cameras: parallel, Camera Link, and IEEE 1394.

Parallel

Many older or specialized digital cameras use a parallel interface that provides a wide range of acquisition speeds, image sizes, and pixel depths. Parallel cameras often require users to customize their cables and connectors to suit their image acquisition device.

Camera Link

The Camera Link standard was developed to ease the challenges of the custom cable interface between parallel digital cameras and image acquisition devices. National Instruments, as part of the Camera Link Standards Committee consisting of camera and frame grabber manufacturers, developed this standard to offer speed and triggering functionality with the ease of standardized cabling. This standard allows for high-speed image capture.

IEEE 1394

The IEEE 1394 standard offers a simple daisy chain cabling system with a standard interface. IEEE 1394, however, lacks the data throughput capabilities of the parallel and Camera Link interfaces. IEEE 1394 also lacks trigger synchronization capabilities.

The following table describes the advantages and disadvantages of the three main digital interface standards.

Interface	Advantages	Disadvantages
Parallel standard	High Speed	Bulky cabling No physical or protocol standards for interfacing with image acquisition devices
IEEE 1394 standard	Simple cabling that allows daisy chaining and use of hubs and repeaters Low cost No camera files required	Slower data transfer rate
Camera Link standard	High speed Uniform cables	More costly than IEEE 1394 10 m cable length limit