ETV Cookbook: Cookware: Transports

Once there was only one way to transport live media - in analog. Now, there are many ways and many are digital. The fundamental characteristics of each transport medium determines what services can be delivered.

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NTSC Analog Video

Analog Broadcasting
There are roughly 1700 licensed television stations in the United States. They operate according to NTSC standards required by the FCC, with provisions for stereo and secondary audio.

Analog Cable
Cable systems operate using the same NTSC standards as broadcasting for their basic service. Premium channels are sent with various proprietary forms of encryption applied to the analog signal, requiring a special decoding box for each television. The largest Multiple System Operators are shown below.

AT&T Comcast Corp - 21 Million subscribed households out of 30 million homes

AOL / Time Warner - 12.8 million households

Charter Communications (St. Louis) - 6.35 million households

Cox Communications - 6.2 million households

Adelphia - 5.72 million households

Cablevision (New York City) - 2.97 million households

Insight Communications (Illinois, Indiana, Kentucky, Ohio) - 1.41 million households

Digital Video

Digital Broadcasting
The FCC adopted standards for digital television that were developed by the ATSC. The FCC allocated a channel for digital broadcasting to every existing analog television licensee. The ATSC standards use 8VSB modulation, Dolby Digital audio compression and the MPEG video compression standard including capability for high definition formats. The ATSC also specified special Program and System Information Tables (PSIP) for channel naming and navigation. (See Recipes:DTV PSIP)

Digital Cable
Cable systems are beginning to employ digital compression in order to increase the number of program channels that can be delivered within the constraints of the cable bandwidth. They are also interested in the interactive capability of the digital processors in the set-top-box.

Most digital cable set-top-boxes cannot decode high-definition video formats. All cable systems use some form of QAM modulation, not the ATSC's 8VSB system. Cable systems transmit channel navigation information on a dedicated carrier, as opposed the ATSC's PSIP system. These three factors make digital cable and digital broadcasting receiving devices incompatible, even though they both use MPEG compression.

Although a few cable systems have made commitments to carry ATSC DTV broadcasts, most are remodulating the signal into QAM for delivery to cable set-top-boxes. Their standard-issue boxes cannot decode any HDTV broadcasts.

DBS - Direct Broadcast Satellite
From the beginning, Direct Broadcast Satellite utilized MPEG video compression in order to make the business viable. MPEG compression makes it possible to provide a large number of channels over the very expensive satellites. The main limitation of DBS is the expense of providing local services. This has lead to the development of DBS receivers that include both NTSC and DTV reception capability.

DirectTV - 9.8 Million Households

Dish Network - 5.7 Million Households

Data Broadcasting

NTSC Line 21 - Low Speed - 300 bps
An early form of data broadcasting was developed to provide closed captions for the hearing impaired. Very low speed data is modulated on line 21 of the analog vertical interval. The low speed makes the modulation very robust - allowing it to survive even low-speed VHS recordings. This data channel is also beginning to be used for triggers to Internet content for ATVEF enhancements. (See Recipes:ATVEF)

NTSC VBI - Medium Speed - 50,000 bps
Higher speed data modulation of other lines in the vertical interval was first deployed for Teletext applications in Europe. While not recordable by home VCRs, the NABTS standard is used commonly for closed-circuit applications, and can provide roughly 10 Kbps per line.

Dotcast - High Speed - 4,000,000 bps

Dotcast has announced plans to develop a system to provide 4.5 Mbps over existing NTSC transmitters. The system uses quadrature modulation of the video carrier and amplitude modulation of the aural carrier.

Dotcast

DTV - High Speed - 19,000,000 bps
Standards have been established by the ATSC for broadcast data delivery over DTV transmitters. Systems for data insertion and PC cards for data reception are available. Several nation-wide services have been proposed. Geocast had intended to provide a dedicated data receiver and content development services, but has closed.

Iblast

Wavo (Formerly WavePhore)

WaveExpress

Internet

Telephone Modem - Medium Speed - 50,000 bps
Although universally accessible, the speed of data access over telephone circuits is limited by the fixed data rate of the phone circuit itself. Modems have reached the limit of 56 Kbps maximum downstream, with 28 Kbps typical upstream. This speed is acceptable for low-grade audio, but is too low for video.

DSL - Broadband - 500,000 bps
Digital Subscriber Line technology carries high speed data on a regular phone line at super-audible frequencies. Although limited to locations close to the phone switching center, it provides high speeds at relatively low cost.

Cable Modem - Broadband - 500,000 bps
Cable modems provide high-speed data from the Internet using a data channel on the cable distribution system. However, some systems require a phone line for the data traveling from the consumer back to the Internet. Also, the data channel is shared, so the data rate may drop if many consumers use it simultaneously. To circumvent this problem, cable systems are rebuilding with dedicated data circuits via fiber to neighborhood groups of every 200 homes or so.