Analog Cameras Still Play a Role - They are Widely Available and Cost Less than Digital Cameras

Several months back I wrote a bit on the survival of analog cameras and their place in the industrial imaging industry, "Analog isn't dead yet, Sentech's STC-400 and its counterparts still have a place in industrial imaging."

Analog cameras still play a role

Analog cameras are widely available and cost less than digital cameras.

By Ann R. Thryft, Contributing Technical Editor -- Test & Measurement World, 10/1/2009

“Designers of machine-vision systems have continued to use analog TV cameras wherever they could because of their low cost and high availability,” said Steve Kinney, director of technical pre-sales and support for JAI. “These have been the highest performing cameras for the lowest dollar cost for a long time, because of their standard formats. Eight to 10 years ago, three quarters of the cameras used in machine vision, by unit volumes, were analog cameras.”

The analog cameras used in electronics inspection output signals in either NTSC or PAL TV formats for color, and either the RS-170 or the CCIR 601 standard for monochrome, said Kinney. Most cameras used in machine vision are progressive scan, rather than interlaced, since the progressive-scan image-transfer method doesn't suffer from the problems interlaced cameras have with creating sharp images of moving objects, he said.

Analog cameras can be more cost-effective than digital cameras if all that's needed is a live view, said Joe Cook, VP of sales for Toshiba Teli's eastern territory. “Due to the economy, people may choose to install analog cameras for cost-savings reasons, even in a new system,” he said. “But if you need to connect analog cameras to a PC, by the time you add a frame grabber and software, the cost can be more than that of a digital camera. Competitive digital technology is bringing down the price of digital cameras, as more vendors have moved into this market.”

The move to digital cameras occurred for several reasons, including higher-bandwidth standards, greater connectivity, and easier interfacing to a PC, said Kinney. “In the early days of digital cameras, as the decline of TV-standard cameras started, most machine-vision cameras were still analog even if they weren't TV-format analog,” he said. “The proportion of machine-vision camera unit shipments represented by analog cameras is declining rapidly; today it is 35 to 40%.”

Applications for analog

In electronics inspection, Toshiba Teli sees analog cameras still being deployed in areas such as pattern recognition and pass/fail inspection for quality control, said Hisa Ishigami, the company's VP of engineering. Typically, engineers use them at both the beginning and the final stages of semiconductor manufacturing. “At the beginning of the line, [analog cameras] may detect surface defects,” he said. “At the end, they may be used to check whether the chip package is labeled correctly and in the right location and whether wires are bonded properly from the chip to the package.”

“We see analog cameras being used in legacy applications in electronics inspection, such as older wire-bonding, pick-and-place, and AOI [automated optical inspection] equipment as well as legacy slice-and-dice and wafer-inspection equipment,” said Andrew Buttress, Sony's product manager for visual imaging products. “Most new capital equipment designs are using strictly digital interfaces. But in the current economic downturn, a lot of the electronic and semiconductor tool OEMs have put other refinements into their tools to increase productivity without necessarily changing the camera, so analog cameras are still being used in upgrades to existing designs.”

Another reason for analog's popularity in the semiconductor and electronics inspection space has been the distance its signals can travel on coax cables, said Kinney. “Before the GigE Vision standard, if you wanted to run cameras 300 to 400 feet without connecting them to a PC, only analog could go that distance, although with some tradeoffs such as signal degradation and loss of amplitude,” he said.

Analog limitations

Lower performance, in terms of both resolution and frame rate, have been two of the drivers in the conversion from analog to digital technology. “The main disadvantage of analog today is the lower resolution compared to what is being offered in digital,” said Cook. “Compared to analog cameras, digital cameras are available in higher resolutions, so you can process more data in less time with more detail.”

In general, progressive-scan analog cameras have higher resolutions than typical analog cameras, such as interlaced cameras, said Buttress. “But you don't see the much higher megapixel-level resolutions in analog technology.” The highest end of analog resolution for machine vision is 1280x960 pixels, and the maximum speed likely in older analog cameras was 30 fps. “With the newer progressive-scan analog cameras, you can get up to 200 fps using partial scan—that is, without using the full resolution. In other words, you have to reduce the field of view,” he said.

Buttress also commented on the costs associated with analog cameras. He acknowledged that compared to digital cameras, most older analog technology is relatively inexpensive, but he added, “With analog, however, the image data is represented as a voltage level, and that data has to be digitized to make an image. So, you have to add a frame grabber for digitizing the image data, and that adds cost.”

Another major problem with analog is noise. Coax cable carries a small signal, so a 10-mV or 20-mV noise spike shows up in the signal, Kinney explained. “In general, a digital signal is immune to noise until it reaches a very high threshold,” he said. “A little noise doesn't matter to digital cameras, but it has very visible effects in analog camera images.”

The need to improve productivity is pushing customers to migrate away from analog cameras, said Buttress. Customers must now inspect more parts per minute, which means they must take more images per second, so not only is resolution increasing, but also frame rate. Depending on the application, however, these two don't always increase at the same rates. “For example, when inspecting a populated circuit board with an AOI system that moves cameras around the viewing area, you may want a 1- to 2-megapixel camera and need to run it at 60 or 90 fps, perhaps using Camera Link to get that speed,” he said. “But in wafer inspection, you may need to view feature sizes as small as 2 microns and will require 16 megapixels to do so, but the speed may be only 3 fps.”

The move from analog to digital technology in machine vision, however, is neither easy nor simple: There's a lot to learn, said Ishigami. “With digital cameras, you have the camera; the camera interface, such as Camera Link, FireWire or GigE, and an input device such as a frame grabber, FireWire card, or GigE port supporting jumbo frames; the software; and the computer,” he said. “The first time you work with FireWire or hook up GigE or Camera Link, you're using different cables, and you are also using digital files, so there's a huge learning curve involved over analog.” To install digital cameras on a production line, engineers need specialized computer-related knowledge, such as how to connect IEEE 1394 FireWire and a familiarity with Windows. “There's a lot of industry knowledge about how to use analog cameras, and it's easy to hook up analog cameras to existing equipment in a wide variety of applications,” Ishigami said.

Today, analog cameras are used mostly in situations with legacy equipment, staff that are used to analog, or both, said Kinney. “Since analog cameras use mature technology, with limited performance that requires fewer parts than newer digital models, manufacturers have been able to create some very small, lightweight models tailored to the needs of specific applications,” he said. “These may be certain types of pick-and-place equipment and other equipment with repetitive, high-G motion, where the small size and weight of analog cameras is an advantage, as is the ability to connect them via flexible coax cables.” Although the reasons for using analog cameras are diminishing, they may remain in some of these applications for quite a while.