Digital Cameras Offer Logical Choices

Digital cameras shine in applications that need high-resolution and high-speed images. Serial links make them easy to operate under PC control.

Jon Titus, Editorial Director -- Test & Measurement World, 10/15/1999

Why are people interested in expensive digital cameras for inspection and machine-vision applications? After all, inexpensive standard analog RS-170 (TV-format) cameras have been around for years, and using them couldn’t get much simpler. First, users can select analog cameras and frame grabbers from many suppliers. Second, the analog RS-170 video signals put out by these cameras travel over inexpensive coaxial cable and can run for hundreds of feet without difficulty. And finally, getting started with an analog camera in a machine-vision application is relatively inexpensive, with hardware and software package prices starting at a few thousand dollars.

Digital cameras don’t look much different from their analog relatives. But don’t let their simple looks fool you. These cameras increase their performance by placing signal-processing circuits right next to imaging sensors and away from noisy PCs. (Courtesy of Kodak.)

One of the biggest advantages of digital cameras over analog cameras is how they acquire an image. Analog cameras use alternating fields in two sequential scans to capture a complete image that a frame-grabber board in a PC must accurately digitize and reassemble into a complete image. The inherent time delay between scans can cause blurring in the image of a moving object.

In contrast, a digital camera employs a solid-state detector that captures an image all at once.1 A digital camera also comes with an electronic shutter that can operate as quickly as 1/32,000th of a second or less, enabling a digital camera to “photograph” a moving object without blurring the image. And unlike an analog camera that continuously captures 30 images/s, digital cameras provide controls that let a computer trigger an image capture at a specific time, perhaps when a product reaches a preset point on a production line.

Granted, the high prices of digital cameras have prevented some engineers from investigating these versatile devices. Today, though, costs are in the order of a few thousand dollars for a camera. Users can buy a complete package of camera, frame-grabber board, and software for less than $10,000. Regardless of cost, some machine-vision or inspection systems may demand the capabilities of digital cameras, such as large-image formats, high-resolution images, easy control, and simple interfaces to PCs.

Digital cameras offer a variety of image formats, from the standard 640x480-pixel format (300 kpixels) up to 2x2-kpixels (4 Mpixels). Thus, when an application requires either additional resolution to capture images of minute components or the capability to capture a larger image, a digital camera may offer the only solution.

Digital cameras also can provide increased resolution for each pixel. The resolution for an average analog camera can range from 6 to 8 bits, depending on the camera’s quality and the ability of the frame-grabber board to accurately convert the analog signal into digital information. Most digital cameras offer a minimum of 8 bits of resolution, and manufacturers sell cameras with 10-bit resolutions. Research-grade cameras offer even higher resolutions, but at a high cost. You won’t find these cameras in any but the most exacting machine-vision applications.

Convert at the Source
Digital cameras also improve on the performance of analog cameras by performing the analog-to-digital conversions right at the camera. The signal-processing circuits and the ADC reside close to the solid-state imaging element, thus reducing the effects of ambient electrical noise on the low-level video signal put out by the imaging device. The camera transmits only digital signals in parallel to a computer.

On the other hand, analog frame grabbers are complex. They must carefully synchronize analog-to-digital conversions to precisely digitize each pixel represented in the analog video waveform. That’s easier said than done because the analog signal contains no timing information for individual pixels. Connecting external timing signals to an analog frame grabber and using the frame grabber’s built-in phase-locked loops can help minimize timing errors, also called pixel jitter. Digital cameras, on the other hand, control the timing and digitization right at the camera, so the frame grabber does not have to synchronize analog video signals and digital clock signals. When an application requires very accurate and repeatable digitization of an image, a digital camera will perform better than an analog camera.

But unlike analog video signals than can travel for hundreds of feet, standard digital signals, such as those put out by TTL devices, can’t travel more than a foot or so. To overcome this limitation, camera manufacturers furnish differential RS-422-compatible outputs that can drive signals over about 5 m at 10 Mbps. But even under 5 m, RS-422 can’t handle rates of more than about 15 Mbps.

Some manufacturers now offer cameras that put out RS-644 signals. This new standard specifies low-voltage differential signals (LVDS) that use an RS-422-like protocol. And RS-644 signals operate reliably at high rates over longer distance with less susceptibility to EMI. For example, a 20-Mbps RS-644 signal can run up to 15 m, and a 50-Mbps signal can go as far as 5 m. Luckily, the RS-644 devices work well with RS-422 devices, such as frame grabbers, even though the standards specify different voltage levels for each: a w0.35 Vtyp swing for RS-644 and a w3.0 Vtyp swing for RS-422.

So, when an application demands a digital camera and a separation of more than a meter or so between the computer and the camera, a camera with RS-644 outputs may fill the need. You can also control digital cameras easily using—in most cases—a serial connection that controls shutter speed, triggering, and so on. (Some cameras also let you use external controls to set these parameters.) A standard RS-232C serial link can control a single camera, and an RS-485 serial link can control several cameras.

The RS-485 standard provides for multiple “stations,” or drops, along a communication line. In such a system, each receiver (a camera) has a set address, and it executes only the commands specifically addressed to it. RS-485 connections simplify a design because a PC can control several cameras using a single control line, not one line per camera. The frame-grabber boards that operate with digital cameras provide the necessary RS-232C or RS-485 interfaces that control cameras. You don’t have to tie up a PC’s other ports for control operations.

Check Board and Camera Compatibility
Unlike analog frame-grabber boards that work with almost any camera, you can’t interchange digital frame grabbers at will. Although camera and frame-grabber manufacturers agree on signal-level standards such as RS-232C and RS-644, they have no such standards for the connections between devices. As a result, you must examine the manufacturers’ information to determine which cameras and frame-grabber boards work together. The manufacturers do exchange product information, so you’ll find a good selection of compatible frame grabbers and cameras. And manufacturers supply cable sets that make it easy to connect the cameras to a computer system. T&MW

FOOTNOTE
1. Titus, Jon, “How Do CCDs Capture Images? ” Test & Measurement World, April 1999, pp. 19–23.