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Acquisition

Acquisition technology has seen major changes in the last few years, particularly with the adoption of new bus technologies. Traditional frame grabber technology has been joined by a range of generic interfaces such as FireWire and USB (2.0 and 3.0) along with Gigabit Ethernet.

Further frame grabber based acquisition interfaces such as CameraLink HS and CoaXPress have also been released to provide solutions for the ever increasing data rates of new, high-end cameras. However, existing interfaces, especially CameraLink, are still commonly specified in new applications. As acquisition relates closely with camera technology this section should be read in conjunction with the camera technology section.

Acquisition considerations

All acquisition signal types transmit image data from the camera to the host system. Each technology has its own strengths and weaknesses and not all mechanisms are suitable for every application. Ensuring data integrity, especially in industrial applications, is important, and some of the newer interface standards include this, ensuring that if anything does go wrong with an acquisition, the application is notified and appropriate measures can be taken without system crashes. This section discusses various mechanisms used in acquisition systems and can help when choosing the right interface technology and the correct product and supplier.

Direct transfer

The simplest method of transferring image data from a camera into a PC is to use a direct transfer method using one of the generic interfaces found on most computers. The most common types of direct transfer are USB, IEEE 1394 (FireWire) and Gigabit Ethernet. The simplest direct transfer method is to send the data directly to PC's memory. The chipsets used for direct transfer have small memory FIFOs (First In, First Out memory buffer) to buffer the data in case of latency on the PC bus.

These FIFOs are often limited to a few image lines and therefore, if bus or processor contention occurs, data can be lost. To ensure this does not happen, some implementations have detection mechanisms, using checksum and packet counting. Direct transfer implies that the technology used provides a guaranteed bandwidth so that there is no need for either side to buffer data. The arrival of PCI Express has made PC bus contention much less of an issue since it provides a point-to-point connection. For systems with higher data throughput rates, direct transfer can be problematic and a robust image acquisition framework should be used.

Most direct transfer technologies deploy DMA (Direct Memory Access) engines which empty the FIFOs as they fill. This means that each DMA burst has a limited size before the next bus arbitration is required and the physical data locations are requested. This can lead to FIFO overflow as data rates increase and can lead to data loss.