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Another issue related to video signals is the amount of bandwidth (BW) required to transmit the video signal. Figure 7-3 depicts a standard composite NTSC video signal that has not been compressed by any compression scheme. The video signal bandwidth is 5.75 MHz with an additional 0.25 MHz guard band on the upper sideband for a total bandwidth of 6 MHz. To transmit just one frame of this signal requires 192 MHz. The Nyquist rate specifies 2 x signal BW (or 2 x 6 MHz) x 16 bits for digital signals and 256 color resolution of bandwidth. 192 MHz is a whopping amount of bandwidth. An OC-3 port speed at 155 MHz and $10,000 per month access charge is not sufficient to transmit standard, full motion, NTSC video images with only 256 colors. To transmit 65K colors, a 384 MHz BW is required. Well, fortunately, there are compression schemes that allow the video signal to be reduced in BW when it is digitally encoded. The current digital compression scheme giving the most compression for the buck is Motion Picture Equipment Group-2, or MPEG-2. MPEG-2 compresses the video signal down to 1.2 Mbps to 7.5 Mbps, depending upon the quality of signal preferred with the lower bit rate yielding the poorest quality picture and typically used for applications that do not require high quality video, such as teleconferencing. 7.5 Mbps full motion color video has a bandwidth that is a good fit with ATM and ADSL technologies. A 5xDS-1 port costs about $4,000 per month and provides 7.5 MHz bandwidth. A metropolitan area the size of Dallas has approximately 450,000 CATV subscribers who pay approximately $40 per month for subscriber basic rate access. With a port charge of $4,000 per month, it does not take too many of those $40 per month CATV subscribers to pay the expense of shipping video over ATM (and ADSL) to the home. Sure, the expense of an ATM-based video network headend will be about the same as a CATV headend. However, the largest expense in the CATV industry is installing and maintaining the coaxial delivery system. The ATM video network eliminates the coaxial delivery system burden and there is essentially no replacement burden. If ATM does not put the CATV industry out of business within five years, I will eat my hat.

Multimedia

At the beginning of 1996, it was estimated there were 34 million PCs in American households with an additional 16 million in households around the rest of the world. Some estimates of the total number of PCs in the world equal 100 million in the year 2000. It is anticipated that the growth in PCs will occur primarily in Asia and Europe. These 100 million households are the more affluent households that are likely to subscribe to data access services for pleasure and work at home.

Computer games and other forms of digital entertainment are washing over the gunwales of our cultural ark. While the entertaining virtual worlds can certainly claim to be multimedia products with their video images and audio bytes, it is really the Internet that provides the greatest motivation for developing multimedia applications.

Internet-based electronic commerce and educational applications will absorb an amazing quantity of audio and video bytes over the next five years.

We are familiar with the interminable wait while images slowly flicker to life when accessing web pages. Modem companies delivered 56 Kbps modems in short order and if you are lucky enough to have one in your computer, assuming your Internet Service Provider (ISP) has installed 56 Kbps modems in its facility, then the "web wait" portion of the "www" is not as wearing on the patience as 9.6 Kbps access.

When the web first came into existence, everything available on the web was text based and 9.6 Kbps seemed adequate for viewing the information. Then, folks started getting creative and graphics, images, video, and sound were soon added to web sites in a grab bag of multimedia potpourri. As the graphics and images become more detailed and varied in color and tone, more bits are required to display the images. Table 7-1 lists the number of bits required to display some typical monitor images. The figures given in the table are "raw" numbers that represent images without compression. Compression techniques for video reduce the number of bits significantly while also reducing the quality of the video image somewhat. A 65K color, full motion video image can be compressed as much as 100 fold, giving a 1.5 Mbps (157 Mbps/100) transmit speed.

Monitor Display 640x480 still images (bits) 640x480 full motion video (bits/second) audio stereo quality (x8 sampling) (bits) Total video+sound (bits)
text (80x25) 16,000 N/A N/A 16,000 (text only)
black & white image 2,457,600 78,643,200 160,000 78,803,200
16-color image 2,457,600 78,643,200 160,000 78,803,200
256-color image 2,457,600 78,643,200 160,000 78,803,200
65,536-color image 4,915,200 157,286,400 160,000 157,446,400

Table 7-1. Computer monitor display bandwidth

The Real Audio and Video World

Audio and video signals come in two basic flavors—Real Time (rt) and Not Real Time (nrt). A real-time audio and/or video signal is a signal that has a fixed relationship to the clock as the seconds are ticking by. An example of a real-time application is the transmission of live sports events or the transmission of a video signal over a coaxial cable from the local CATV service provider. Not real-time video applications include those video movies rented from certain locations on Friday night and played in the VCR. Real-time applications require the audio and/or video signal to be transmitted and displayed virtually simultaneously in real time. Not real-time audio and video applications do not require the signal to be transmitted and displayed virtually simultaneously.


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