Analog Frame Grabbers

Analog Frame Grabbers

Even with established standards such as RS-170, NTSC, CCIR, and PAL, great differences exist among analog frame grabbers. Differences appear through jitter, digization quality, and colour separation, all of which affect image quality. However, because it is difficult to compare frame grabbers from datasheet specifications, many OEMs benchmark several models before making a selection.

Some analog frame grabbers can handle multiple cameras either through multiple analog interfaces or multiplexing techniques, thus reducing the number of frame grabbers used in multi camera systems. When multiplexing video on a frame grabber, the resynchronization time required with each switching reduces the total frame rate. In the case of a multiple simultaneous input configuration, onboard memory will guarantee that images are transferred without loss of data.

Area-Array Cameras

Today’s frame grabbers commonly off neighbourhood operations such as convolution, off-loading the task from the host PC. This allows functions such as Bayer colour interpolation and dead-pixel management to be performed directly on the frame grabber. While some cameras offer Bayer interpolation internally, most frame grabbers can also handle the task. Several ways exist to interpolate camera data when a Bayer pattern is used. The most simple linear interpolation gives relatively poor image quality; an intermediate quality method is bilinear interpolation; the highest quality is obtained by nonlinear interpolation.

Similarly, dead-pixel management can be used to correct the images and reconstruct or interpret neighborhood pixel values. Each pixel is compared to its neighbors, and if there is a very high-or low-intensity pixel, a dead pixel may exist and a new value is computed using kernel-based algorithms. Although the host computer can perform these operations internally, it requires substantial processing power.

Frame grabbers that support line scan cameras are mostly used in applications requiring a high level of synchronization between the movement of objects on a conveyor and image acquisition. To interface to such conveyors, frame grabbers usually provide interfaces to TTL, RS-644, optocouplers, and pulse dividers (to adjust the encoder pulse-per-line ratio) and support for bidirectional encoders (where opposing movements are necessary). Linescan cameras can be difficult to integrate in cases where lines are generated constantly. Large quantities of data are generated and could create data-transfer issues-some frame grabbers may lose lines of image data.

Frame grabbers can also interface to trilinear line scan colour cameras. These cameras use a sensor comprising three lines, each covered with a colour filter-typically red, green, and blue. Here, the pixel information of each colour line does not represent the same physical location in space, so it is necessary to realign each colour. For example, if the camera has red, green, and blue filters, in that order, the green channel has to be delayed by one line and the blue by two to match the red channel. This spatial registration can be easily performed on a frame grabber, off-loading the PC of this function.

The ease of integrating a frame grabber into a vision system is determined by how simple it is to synchronize image acquisition with external events controlled using TTL, RS-644/LVDS, and optocouplers. The most general use of I/Os is for triggers: an external signal from a sensor or programmable logic device indicates that an image must be captured. In many applications, external lighting is required, and the frame grabber offers dedicated signal lines to synchronize strobe lighting with the camera additional digital I/Os that can be used for other types of acquisition or synchronization.