Camera Selection – Digital or Analog?

Digital Camera
All cameras used in industrial application are normally digital. However, when one discuss digital or analog, the industry is referring to the video output from the camera – digital or analog video output.

Advantages of Digital Solution
Digital is the preferred solution for applications that require higher resolution images, deeper gray scale, faster frame rates and better signal quality than can be found with a standard analog camera.

Resolution
The resolution of most analog cameras is effectively limited by their historical link to television standards and the corresponding analog signal used to transmit the image to the receiver. On the one hand, this compatibility has allowed machine vision designers to take advantage of the tremendous variety of equipment and technology available and the enormous economy of scale from the television industry. On the other hand, it has often limited their solutions to less than 13 MHz data rates and hence to deliver images on the order of 720 x 485pixels.While non-standard analog cameras are available that produce higher resolution images, costs increase once the decision has been made to move away from the familiar territory of standard video, whether the camera is analog or digital. And as we will see in the discussion that follows, digital cameras bring so many additional advantages that non-standard analog cameras tend to offer only short-term solutions. Since digital cameras are not limited by compatibility with television standards, camera resolutions can soar to the limits of technology. Digital area scan cameras with 2K x 2K pixels are commonplace and digital line scan cameras are readily available with resolutions greater than 4K pixels.

Gray Scale
While enhanced spatial resolution can often allow you to simplify your image analysis by capturing finer object details, higher spatial resolution may not be the answer if changes in brightness across the object of interest are subtle. Improving the imaging system’s ability to discern finer changes in brightness — to deliver more bits of gray scale — can allow image analysis software to pick out details that would otherwise be over-looked, even when additional spatial information is available. This is an area where digital cameras really excel and where analog cameras quickly falter.

The very nature of the analog transmission medium, the ubiquitous RS170A used in virtually every analog camera, means that images delivered from the camera to image capture hardware rarely if ever possess even 8bits of valid information once digitized. Frequently 6 bits or less of valid data are actually delivered into your computer’s system memory. Noise induced by nearby electrical equipment and signal reflections from the cables are just two sources of noise that is added to the image data before it is digitized by the frame grabber. This noise once digitized is difficult to remove and can make the image-processing task much more difficult. Electrically induced noise is still present in the signals delivered to the frame grabber from a digital camera; so what is the difference? First, as you would suspect from the name, digital cameras digitize — i.e., convert the analog images to digital information in the camera within a quiet, shielded electrical environment, before the influences of the outside world have a chance to affect the sensitive analog information. This means that for the first time it is possible for a camera to send images with not just 8 bits of valid information but with10 bits, 12 bits or more. All this means that digital cameras deliver very fine gray scale detail to the frame grabber and hence into system memory. These better images can mean better results for you.

Frame Rates
As mentioned above, RS170 or any of the other international television standards are limited to 25 to 30 fps. If you need to continuously capture information faster, there are two alternatives: a non-standard analog camera or a digital camera. From the discussion above you see that if you are going to spend the extra money for a non-standard analog camera, why not take the extra step and go digital. With a wide variety of digital cameras available spanning frame rates from less than 20 Hz to more than 1000 Hz, plus the better image integrity available from a digital camera, digital becomes the logical choice.

Advantages of an Analog Solution
While signal quality, speed and image flexibility all play into the decision to incorporate digital cameras and frame grabbers into new designs, these advantages do not come without some drawbacks. The Lack of Standards Television standards developed in both the U.S. and internationally have created a plug-and-play market for analog cameras and frame grabbers where components from many manufacturers will operate together; however, the lack of digital standards has often made mating digital cameras and frame grabbers a frustrating task. Digital cameras come with a variety of pixel resolutions and gray scales, multiple data channels and data formats, and unique cabling requirements. This makes getting a digital solution up and running a much more daunting task. If one camera and frame grabber does operate together, then it may well be the case that changing cameras or grabbers will yield a system that does not work without custom assistance from either the camera or frame grabber manufacturer.

Cost and Cabling
The tremendously high volume of television equipment produced each year has also meant that analog cameras are relatively inexpensive. Not so for the much lower volume digital cameras in use today which command significantly higher prices (but deliver higher performance).And while analog cameras frequently connect to the grabber with a single thin coax cable that can span long distances (but with the corresponding noise added along the way), digital cameras often have heavy, cumber-some cables with 50 or more wires that can span at the most a few meters. In addition, each digital camera has its own unique cable requirements, adding to the complexity of the solution.

Analog or Digital?
Decision is application specific. It is recommended to discuss your requirement with a vision integrator or imaging consultant. The knowledge that these engineering experts can provide to you will help you to save time and costs in your integration and implementation of vision in your machine.

Reference
The Essential Guide to Digital Video Capture
(Author: Dr Richard Dehoff)

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