Video Transmission & Compression

During the past 18 years, traffic and freeway management agencies have been integrating the use of CCTV cameras into their operational programs. The heavy use of this technology has created a need to deploy very high bandwidth communication networks. The transmission of video is not very different from voice or data. Video is transmitted in either an analog or digital format. Video transmitted in an analog format must travel over coaxial cable or fiber optic cable. The bandwidth requirements cannot be easily handled by twisted pair configurations.

Video can be transmitted in a digital format via twisted pair. It can be transmitted in a broadband arrangement as full quality and full motion, or as a compressed signal offering lower image or motion qualities. Via twisted pair, video is either transmitted in a compressed format, or sent frame-by-frame. The frame-by-frame process is usually called "slow-scan video".

Full color broadcast analog video requires a substantial amount of bandwidth that far exceeds the capacity of the typical twisted pair analog voice communication circuit of 4 KHz. Early commercial television networks were connected via Coaxial cable systems provided by AT&T Long Distance. These networks were very costly to operate and maintain, and had a limited capability.

Transmission of analog video requires large amounts of bandwidth, and power. The most common use of analog video (outside of commercial broadcast TV) is for closed circuit surveillance systems. The cameras used in these systems use less bandwidth than traditional broadcast quality cameras, and are only required to send a signal for several hundred feet. For transmission distances (of analog video) of more than 500 feet, the system designer must resort to the use of triaxial cable, or fiber optics. Depending upon other requirements, the system designer can convert the video to another signal format. The video can be converted to a radio (or light) frequency, digitized, or compressed.

Cable companies have traditionally converted television broadcast signals to a radio frequency. With this technique, they can provide from 8 to 40 analog channels in a cable system using coaxial cable (more about multiplexing later in this chapter). Cable company operators wanting to provide hundreds of program channels will convert the video to a radio frequency, and then digitize. The cable company is able to take advantage of using both fiber and coaxial cable. These are called HFC (hybrid fiber coax) systems. Fiber is used to get the signal from the cable company main broadcast center to a group of houses. The existing coaxial cable is used to supply the signal to individual houses.

Early freeway management systems used analog video converted to RF and transmitted over coaxial cable. Later systems used fiber optic cable with either RF signal conversion, or frequency division multiplexing (see Multiplexing in this chapter).

With the introduction high bandwidth microprocessors and efficient video compression algorithms, there has been a shift from analog video transmission systems to digital systems. New processes such as Video over IP (Internet Protocol) and streaming video allow for the broadcast of video incident images to many user agencies via low (relatively) cost communication networks. Before looking at the systems, let's take a look at the various types of video compression schemes.

Video Compression

Compressed Video – Since the mid-1990s, FMS system designers have turned to digital compression of video to maximize resources, and reduce overall communication systems costs. The digital compression of video allows system operators to move video between operation centers using standard communication networks technologies.

Video compression systems can be divided into two categories – hardware compression and software compression. All video compression systems use a Codec. The term Codec is an abbreviation for coder/decoder. A codec can be either a software application or a piece of hardware that processes video through complex algorithms, which compress the file and then decompress it for playback. Unlike other kinds of file-compression packages that require you to compress/decompress a file before viewing, video codecs decompress the video on the fly, allowing immediate viewing. This discussion will focus on hardware compression technologies.