SECOND PART

Quads vs. Multiplexers

QUAD DISADVANTAGES: Recorded images are recorded in quad fashion. It is often difficult to view details from tape with a smaller image. Many Quads allow "zoom to full screen" on image playback, This expands the quad sized image to full screen for review. This image is usually considered inadequate due to lower resolution (the number of pixels, or, scan lines in the quad sized image is expanded to fill the entire monitor screen). There is little picture clarity remaining to view. With two page quads, only four cameras can be recorded at the same time. Sets of four must sequence to display and record.

MULTIPLEXERS: Multiplexers typically display the images of up to 16 cameras on a single monitor and record the cameras on a single VCR. Multiplexers usually incorporate several of the previously mentioned devices into a single package. Multiplexers are a convenient way for one operator to control systems with 16 cameras. A single monitor can display cameras individually, or, in multi-screen formats. All cameras are typically recorded to tape as full size images. This is accomplished by time sharing each image in sequence. Information is encoded in the recorded image which allows review of individual cameras from tape. Multiplexers have become widely used only during the past four years. Until recently, there were only a few manufacturers of Multiplexers.

MULTIPLEXER ADVANTAGES: Multiplexers combine numerous features into a single device (alarm inputs, video loss, camera titling, remote control capability, etc.). Because Multiplexers record cameras as full size images, tape playback is far superior to that of a Quad. Full size images are compressed in order to display several on the screen at the same time. Multiplexers typically allow you to view selected cameras while all cameras are still being recorded.

MULTIPLEXER DISADVANTAGES: The major disadvantage of a Multiplexer is that it is not "real time." The more cameras installed, the slower the individual images update on the screen and to tape. This eliminates the use of Multiplexers in applications which require "real time" video. In order to playback a multiplexed recording, you must decode the tape through another Multiplexer. Multi-screen viewing of nine, or 16 cameras may require a larger monitor.

So much for history. Did you notice that "real time" was not mentioned until we got to Multiplexers? That's because everything was real time prior to multiplexers. Explaining the difference between Quads and Multiplexers is a relatively recent phenomenon . So what is the difference and which one is better? The fast answer is that neither is better. Each is better for specific applications.

Quads are less expensive. They are well suited to an application with four, or, fewer cameras. It's great for a convenience store. You can see four cameras on one monitor and record them on one VCR. What happens when you have five cameras? Five, or, more cameras can put you into a second Quad, monitor and VCR. So a nine input multiplexer starts to look more attractive, price wise. You even have more inputs for future expansion. If the customer can deal with the less that "real time" updates, this is probably the best bet. So either a Quad, or a Multiplexer can be the most cost effective alternative.

Have you noticed that I have been placing "real time" in quotes? That's because it doesn't mean the same thing to everyone you ask. For most people it means fluid motion. It can be 30 frames per second (60 fields) to anyone who was raised by television. Unless you were raised anywhere is most of the rest of the world where it is 25 frames per second (50 fields). If you go to a movie theater, the film speed is 24 images per second. Some even say it is real time if it is individual snapshots which are updating "as it happens." All the world is visibly made up of a series of still images viewed closely together in time. Somewhere around 22 updates per second, our minds meld the still images into fluid motion.

You've probably heard buzz terms like, frame grabber, frame switcher and frame rate. The truth is that almost all the Quads, Multiplexers and Time Lapse Recorders used in the security industry are based on fields, not frames. There are exceptions, but, not many. The fields are typically "line doubled" to fill in the rest of the image. If you find yourself in a conversation on this subject with a potential customer, you're already in trouble. I use the term "image" whenever I can get away with it. Image can mean field, frame, or, picture. The audience subliminally fills in the term they are most endeared to.