Advanced Camera Analog Triggering and Synchronization Techniques


Timing an image acquisition is crucial in some applications. Several methods are available to trigger an image acquisition for different levels of control over the timing of the acquisition. Acquisition timing control among multiple cameras can be accomplished by synchronizing cameras to each other or to an image acquisition device. This document discusses different triggering and synchronization techniques using a single camera and multiple cameras. For more information about the nature of a video signal generated by a camera, refer to Anatomy of a Video Signal.

Triggering and Synchronization with a Single Camera

Non-asynchronous Reset: (IMAQ PCI/PXI 1407, IMAQ PCI/PXI 1408, IMAQ PCI/PXI 1409, IMAQ PCI/PXI 1411)
Triggering only the acquisition device while using the camera in free-running mode creates a variable delay between the time the acquisition device receives the trigger and the time the image is acquired. The delay occurs because the acquisition device must finish acquiring the current frame before acquiring the triggered frame. To see a visual comparison of non-asynchronous and asynchonous modes, refer to Using the JAI CV-A1 Camera in Asynchronous Reset Mode With the IMAQ 1409. Cameras that do not support asynchronous reset mode must be triggered using non-asynchronous reset mode.

Asynchronous Reset: (IMAQ PCI/PXI 1408, IMAQ PCI/PXI 1409)
Asynchronous reset allows the camera to output a frame immediately upon receiving a trigger. Without asynchronous reset capability, the image acquisition device waits for the camera to finish outputting the current frame before acquiring the next full frame. There are two modes of acquisition available depending on the type of the data coming from the camera. Standard analog cameras will output the following video formats: RS-170 or CCIR for monochrome video and NTSC or PAL for color video. These cameras can only be used with IMAQ PCI/PXI 1409 in External HSYNC/VSYNC (HLOCK only) Mode, which means the board will only lock onto the horizontal synchronization signals generated by the camera. Nonstandard cameras often use specialized video timing and larger image sizes to improve performance for a specific application and can be used with an image acquisition device in External Lock Mode (supported by both IMAQ PCI/PXI 1408 and IMAQ PCI/PXI 1409). These cameras provide pixel clock (PCLK), horizontal sync (HSYNC), and vertical sync (VSYNC) signals to the image acquisition device for applications that require asynchronous reset. For a single camera, the camera will start outputting the frame to the image acquisition device as soon as the trigger is detected .

For more information on asynchonous reset, refer to Asynchronous Reset with the IMAQ PCI/PXI-1408 and Using the JAI CV-M10 Camera in Asynchronous Reset Mode with the IMAQ 1409

Triggering Modes of a Line Scan Camera: (IMAQ PCI/PXI 1409)
A line scan camera can also be triggered to acquire a line or variable number of lines asynchronously. Three types of acquisition modes are used with line scan cameras:

  • Trigger Each Line: This mode is often used in line scan applications when you want to trigger your line scan lines from a motor's encoder signals or some other signal of varying frequency. This triggering works quite differently from normal triggering where you continuously receive Line Valids. When triggering each line, a Line Valid is only received on each trigger, and you must configure IMAQ Trigger for "trigger each line" mode.

  • Variable Height Acquisition (VHA): This mode is often used in line scan applications where you do not know the exact size of the object you are acquiring. In VHA mode, you can wire an external sensor that indicates whether an object is within the field of view, and you can trigger your image acquisition from this object's present/not present signal. The advantage of this mode is that the acquired image is "variable height", and your object is guaranteed to fit in one image, as opposed to area scan cameras that may acquire only part of an object due to the set image size. For more information, refer to Variable Height Acquisition.

  • Trigger Each Line and VHA: This mode is often used with a conveyor belt application where the line rate of the camera has to be in sync with the speed of the conveyor belt. When the conveyor belt has variable speed, varying acceleration/deceleration, or intermittently stops, you can use VHA with Trigger Each Line at the same time. In this mode, you can route the encoder signal to trigger each line and use VHA to accurately acquire an image of an object with varying height and speed.

Triggering and Synchronization with Multiple Cameras

Non-Genlocked (Free-run): (IMAQ PCI/PXI 1408, IMAQ PCI/PXI 1409)
Use this mode to connect multiple cameras to one image acquisition device without synchronizing them to each other (without genlocking or asynchronous resetting). When the trigger is received for a particular channel, the board will acquire the next full frame from the desired channel. The image acquisition device will have to resynchronize to the next camera signal, which will add an additional amount of time delay in order to synchronize to that camera's timing signals. If the camera is in the middle of a frame, then the image acquisition device will wait for the next frame before acquiring the image. This is a good solution for capturing images that are not time sensitive. If the object is moving or the event is time-critical, this method is not deterministic, and the event might be expired.

Genlocked: (IMAQ PCI/PXI 1408, IMAQ PCI/PXI 1409)
Use this mode to connect multiple cameras to one image acquisition device using a method called Genlocking, which ensures that the cameras start sending their frames simultaneously. Genlocking is the term used for sharing the timing information from one camera or source, usually referred to as the master, with one or more additional cameras, usually referred to as slaves. The image acquisition device can lock on to the timing signal of the master camera, and then digitize any of the slave camera signals using the same timing information. This method removes the delay caused by the board reacquiring and resynchronizing when another camera signal is acquired. Sharing the timing information between multiple cameras insures that when the multiplexer switches camera signals to the ADC, which occurs at the beginning of a new frame, the next camera will start sending the new frame. Therefore, there is no time delay associated with the wait for the next frame. Not all cameras have the ability to be Genlocked, and custom cabling is required. Since the cameras are Genlocked, but there is only one ADC, the highest frame rate for each channel is the Full Frame Rate of the Video Signal divided by the number of Video Signals. For example, 30 fps max / 3 cameras = 10 fps / camera. This mode can be combined with Asynchronous Reset mode for more precise timing.

Each Camera on a Separate Image Acquisition Device: (IMAQ PCI/PXI 1407, IMAQ PCI/PXI 1408, IMAQ PCI/PXI 1409, IMAQ PCI/PXI 1411)
Use this mode to connect multiple cameras to multiple image acquisition devices without asynchronous triggering. Connecting each camera signal to its own image acquisition device will insure a dedicated ADC, allowing each frame to be acquired at its full frame rate. Since there is only one camera signal for each ADC, resynchronization of the video signal is not required. The maximum delay between the trigger and acquiring a frame is a maximum of one frame period delay (30 fps has a maximum of 33.3 ms delay), because the image acquisition device waits for the next full frame. If one frame period delay is too long of a delay, then the event might be expired. This mode can be combined with Asynchronous Reset mode for more precise timing.