Two-way CATV HFC applications should be grouped into two major markets, in part because such applications are driving different (not necessarily divergent in principle) paths in the technology upgrading and standardization:
• Digital video over cable TV networks (although in the transition from analog to digital broadcasting, cable systems will also deliver analog channels)
• Data services over cable TV networks.
In fact the two markets will converge, since multimedia applications relate to both. The road to such a confluence is being paved now through the standardization process, and the success achieved by each of the parts will depend on both cost and technological accomplishments. In either case digital video or data application services are to be delivered both in broadcast and in interactive mode.
Digital Video over Cable TV Networks. First it is interesting to point out differences and key technological and performance aspects of analog versus digital video. Unlike analog video, which degrades in quality, digital video is virtually identical to the parent. There is a major difference between the way computers and television handle video. When a television set displays its analog video signal, it displays the odd lines (the odd field) first and the even lines (the even field) next. Each pair forms a frame, and there are 60 of these fields displayed every second (30 frames every second). This is referred to as interlaced video. Instead, the computer displays each line in sequence, from top to bottom. This entire frame is displayed 30 times every second. This is often referred to as noninterlaced video.
Analog formats include NTSC (National Technical Standards Committee), used in the United States, Mexico, Japan and Canada. Phase alternation line (PAL) is used in western Europe, and Sequential Couleur a Memoire (SECAM) is used in France, Russia, and Eastern Europe (Hewlett Packard, p. 4).
Digital formats include Joint Photographic Experts Group (JPEG), Motion JPeG (MJPEG), MPEG, Advanced Video Interleave (AVI) from Microsoft, Indeo from Intel, and CellB for use on Sun SPARCstations. Of these digital formats, only MPEG is an internationally recognized compression standard.
Benefits of digital video are in random access to stored information, the ability to compress the video, and the low cost of reproduction without loss of quality. From a transmission point of view, a number of digital channels can be sent where only one analog channel was allocated. VCR video is in analog format running at 30 frames/s. To achieve the same quality on the desktop, without compromising on frame rate, resolution, color resolution, or image quality, one needs hundreds of gigabytes of disk space. Twenty-five centimeters (10 inches) of videotape is required to record one second of video. Compression is the solution, and there are two types:
• Intraframe compression compresses each individual video frame. JPEG and MJPEG use intraframe for compression ratios of 20 : 1 to 40 : 1. The larger the ratio, the poorer the quality.
• Interframe compression only looks at the video data that have changed. This also reduces the bandwidth necessary to carry the video stream. MPEG uses interframe compression and can achieve up to 200 : 1 compression rates. Roughly, 9 Mbyte of hard-disk space is required for every minute of MPEG compressed video. CATV and DBS use MPEG-2.
TV broadcasting and video conferencing, which allows a group of users to communicate in real time through the use of live streaming audio and video, are the main applications of digital video.
Data Services over Cable TV Networks. Data services can be divided into narrowband and broadband applications according to the amount of bandwidth required. Telephony is narrowband, and multimedia contents are broadband.
Telephony. We may include voice services within data interactive applications. Telephony can be offered by cable by using IP to deliver voice through the use of a gateway located at the cable headend. Telephony networks can either be private or allow access across the Internet or to and from the PSTN. Videotelephony is a natural successor to telephony, although more complex. There is a tradeoff between quality, bandwidth, and real-time.
PUSH Applications. PUSH technology is the delivery of information from server to client over IP. Webcasting and multicasting (transmission of files or streaming audio and video to preselected multiple users) are PUSH applications.
Virtual Private Networks. Since a cable system acts as a broadband network, operators are capable of establishing virtual private networks (VPNs) as completely closed environments (intranets) allowing users to access the Internet.
HomeMonitoringandSecurity. Cable networks can be used to monitor homes for fires, break-ins, and medical emergencies. The available bandwidth of a cable network, combined with the system’s being always on, enables emergency information to be quickly transmitted.
Telemedicine. Medical doctors, patients, hospitals, clinics, and mobile units can use the CATV broadband technology for quick transfer of huge files to provide diagnosis, treatment, consulting, and education.
Immersive Environments. An immersive environment is a virtual social environment, managed by a computer program, where the user is represented within the program as an animated character (avatar) visible to all of the users. This is clearly a broadband multimedia application, where sound, data, and video come together, requiring a great amount of bandwidth along with critical constraints on latency.
One goal for a media immersion environment (MIE) is for people to interact, communicate, collaborate, and entertain themselves naturally in a shared virtual space while they reside in distant physical locations. The MIE has applications in many domains where physical presence is expensive (e. g., distance learning); impossible (e. g., space exploration), unsafe (e. g., nuclear studies), or inconvenient (e. g., entertainment), or where more than one person must be involved (e. g., remote medicine). The principal function of MIEs is to synthesize multimodal perceptions that do not exist in the current physical environment, thus immersing users in a seamless blend of visual, aural, and (potentially) haptic information.