Sunday, May 24, 2009

Who cried when analog died?

If you’re something of a newcomer to machine vision – say in the last five years or so – then the idea that cameras were once analog probably seems rather quaint. Digital interfaces like Firewire and CameraLink have become so pervasive, not to mention easy, that it’s hard to imagine anything different. But it wasn’t always that way.

Not so long ago every vision system used analog cameras. In fact if you’re at a factory with a long history in machine vision it’s quite likely that you have a few such systems quietly working away. Have you thought what you’re going to do when those cameras finally die?

Well I’m going to save that discussion for another day, because for this post I want to summarize a major difference between older and newer cameras, and that’s progressive scan versus interlaced.

The first thing to note is that while digital doesn’t automatically mean progressive scan, it does tend to be that way. This means that your old analog cameras are probably interlaced while a new Firewire camera is almost certainly progressive. Let’s take a few moments to cover the difference.

Interlacing is the way TV’s worked. Because broadcast bandwidth was limited, rather than transmit the whole frame at the required rate, the decision was made to transmit half the frame. This means that for frame ‘n’ only the odd numbered lines in the image are updated, while for frame ‘n+1’ only the even lines are updated.

This was enough to fool the human eye into believing it was seeing moving pictures, even if the image quality, on a frame-by-frame basis, wasn’t that great.

Then along came progressive scan for our High Def TV’s. In this system the whole image is repainted, line by line, for every frame. The result is a sharper image, especially when the subject is in motion.

So if you need to replace an interlaced camera it may well be a progressive scan camera that you hook up. Why don’t you check back tomorrow for a discussion of what that might mean?

1 comment:

David Dechow said...

As one who integrated plenty of analog cameras (and yes even videcon tube cameras), I felt I had to make a comment about the analog camera technology mentioned in the post and in the AVT/Imaging Technology article.
In general, I think the article, while technically correct, perhaps did not do a good job of explaining the day to day implementation of interlaced cameras and the use of progressive scan. (Note that "progressive scanning" or frame integration, was available even in analog cameras as early as the late '80s.) Overall, the article highlighted some very unique special cases, and in doing so made it seem that these specialty uses of the interlaced image were the norm. It wasn't clear enough that the product is targeting OEM custom applications where proprietary software was making use of the benefits of the interlaced camera. I feel it should have been more obvious that the discussion doesn't apply to general purpose machine vision, and that people aren't going to be able to just go out and replace their analog cameras with the "interlaced digital" camera.

In reality, for the vast majority of machine vision applications, the analog interlaced camera was just a means to an end: it was the standard inexpensive camera, the RS170 frame grabber worked nicely with an interlaced signal, and it simply converted the signal into contiguous image memory just like you would use now with a GigE camera. Of course, the slight temporal shift during field integration was a problem if there was any motion whatsoever. For those apps you would have to either use just a single field (at half resolution), or get a camera that would integrate a full frame at once (like a progressive scan).

This discussion could take up a whole machine vision technology/history article, but my point here is that the replacement of an analog camera for many, many applications does not necessarily involve getting an "interlaced" type of digital camera - the plain Firewire, GigE, or CameraLink should do just fine most of the time with appropriate support components, hardware and of course new software.