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January 2001 Issue

Digital Signal Carriage: Pitfalls, Concerns, Considerations, and Suggestions - Part 3
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In the last installment of our three-part series on digital data, we examine digital carriage concerns, such as how "must carry" rules affect the over-the-air digital and existing analog channels and how to transfer digital data from one device to another without converting to analog.

The Society of Cable Telecommunications Engineers (SCTE) standard known as Digital Video Subcommittee (DVS)-031 specifies that U.S. cable companies use the technique of 64-QAM (quadrature amplitude modulation) or 256-QAM, but doesn’t preclude experimenting with 1024-QAM in the future.

Over-the-air broadcasters in the United States have decided to use 8-VSB (vestigial sideband). Because over-the-air has to be more robust than cable, which is supposed to be an enclosed environment, it suffers a little in throughput compared to 64- and 256-QAM. In Europe, off-air broadcasters use coded orthogonal frequency division multiplexing (COFDM) and digital video broadcasting (DVB)-C, -S or -T standards.

The customer must have a box or digital TV (DTV) set that demodulates the different modulation schemes, not to mention the 18 different display formats associated with the Advanced Television Systems Committee (ATSC) standard for standard definition television (SDTV) and high definition television (HDTV). These ATSC formats define an aspect ratio of 4 by 3 or 16 by 9; different lines of resolution; and the scanning technique, such as progressive or interlaced.The cable companies may include even more display formats than the 18 that ATSC already covers.

If broadcasters transmit with 8-VSB modulation, cable providers may:

1. Carry it on the cable system unaltered;
2. Carry it unaltered but at a different frequency; or
3. Use 8-VSB to 64-QAM or 256-QAM converters.

Luckily, the consumer electronics consortium persuaded the television industry to include a 64/256-QAM chip in the newer DTV sets to make them cable-ready. Buying a $2,000 TV set may not require purchasing a digital set-top box, but perhaps only a point-of-deployment (POD) card from the local cable company.

Choosing an option

The first option is to carry the signal on the cable system unaltered. The problem with this choice is there are different frequency allocations associated with cable television and over-the-air, which is 2 MHz offset from the EIA-S542 channelization plan except for channels 2 through 13.

If we use an over-the-air digital broadcast at its assigned frequency, 2 MHz may be wasted and the analog composite triple beat (CTB) would fall in the "haystack" (see Figures 1 and 2), assuming those frequencies aren’t already populated with signals.

The second option is to carry it unaltered, but at a different frequency. According to the ATSC standard, 8-VSB is the modulation format of choice for over-the-air digital broadcasts. This modulation uses a 0.31 VSB, not 0.75 VSB as regular analog channels use. QAM and 8-VSB have a suppressed carrier unlike analog channels, and QAM specifies a center frequency. Measurements may be made by setting a center frequency and performing an integrated average power measurement, even though the actual modulator’s carrier frequency could be different.

A third option is to convert the 8-VSB to 64-QAM or 256-QAM, allowing more channels per 6 MHz bandwidth.

An 8-VSB channel has a payload of about 19 megabits per second (Mbps), while 64-QAM has 27 Mbps, and 256-QAM has about 38 Mbps. These numbers reflect throughput after the overhead of forward error correction (FEC) steals some speed away. A higher cost is associated with this third option of converting because of the demodulation and reprocessing.

Another concern is whether broadcasters will decide to go with 16-VSB or orthogonal frequency division multiplexing (OFDM) after the cable operator has installed 8-VSB equipment.

If you go with this option, you may need a modulator or transcoder that is able to convert from one modulation scheme to another, and also be ‘frequency agile.’ Look at cable channel 65, for example. The analog visual carrier would be located at 469.25, or at 471.25 for over-the-air channel 14. If retransmitting the 8-VSB signal in the first or second option, you should place the signal at 470.8 MHz or 468.8 MHz to allow for roll-off for the filter skirts and accommodate the 0.31 VSB. With the third option, the QAM signal could be located at 471.00 or 473.00.

Note that the Federal Communications Commission Web site at www.fcc.gov/oet/dtv/start/dtv2-69.txt updates digital transmissions in certain U.S. cities, channel allocations and launch dates. Over-the-air digital transmission is a potential ingress source similar to two-way radios and pagers. Placing critical services at these frequencies could result in potential interference from off-air digital ingress.

Must carry rules

Digital transmission requirements not only affect cable companies, but also broadcasters. Transmission power requirements will increase because broadcasters transmit at much higher frequencies, necessitating more power for the same coverage area.

If the cable TV industry must carry both the 6 MHz analog and digital channel, more redundant space must be allocated. Cable companies already squeeze as many channels as possible in their line-ups. Los Angeles and New York City, for example, have 11 local broadcasters each. What happens with this new channel? Must I carry the whole 6 MHz digital channel or only the broadcaster’s primary video content? The TV and digital set-top industry appear to be the biggest winners in this transition.

What to carry?

If the whole 6 MHz is carried, the broadcaster may decide to use the cable network for interactive services. Six MHz is a lot of bandwidth in comparison to Moving Pictures Experts Group (MPEG-2) compression and high orders of modulation. What about SDTV and HDTV? Must the high definition be carried, or just standard definition? Will it change depending on the circumstances and time of day? Only two HDTV channels will fit in the 6 MHz channel depending on the action in the movie and modulation scheme used.

HDTV more than likely will use an aspect ratio of 16:9 compared to 4:3 for analog TV. Some HDTV proposals include: 1,920 pixels by 1,080 lines, 1,280 by 720 lines, or standard definition with line tripling 704 by 480 lines to give the perception of more resolution. SDTV will use the typical aspect ratio of 4:3, such as 704 by 480 lines or 680 by 480 lines. This will be done with interlacing or progressive scans. Progressive scans require more bandwidth to transmit an analog signal than interlaced scanning.

Digital channels also must be multiplexed together in the time domain, and packet identifiers (PIDs) and conditional access. Statistical multiplexing also will need to be done, allowing more efficient carriage of these channels, and keeping them separated and secure at the other end. It is more efficient to combine slow-speed channels with high-speed channels. Remember, the channel speed requirements will be changing dynamically from one second to the next.

What about electronic program guides (EPGs)? Broadcasters’ EPGs now must synchronize with each cable company’s EPG.

Open access

The FCC won’t touch it—yet. Local governments are different and say "unbundle it." If AOL and others want high speed, they can build their own networks or buy one. Another option is to make their sites free as do other Internet service providers (ISPs). The Canadian regulatory authorities have ordered cable operators to lease access to their systems to traditional ISPs. The operators are required to submit cost data to justify rates they plan to charge other ISPs. AT&T, Comcast, and others say they will revisit the issue after their contracts expire in a few years. Now that AOL is acquiring Time Warner, the whole open access debate takes on a takes on a new perspective.

Digital transfer

What will happen with the DTVs that have no digital interface? The first requirement for the new digital devices is to transmit an analog output for legacy equipment, such as analog TVs or VCRs. Newer equipment, such as printers, DTVs, cameras/camcorders and DVDs, should interface with each other directly without having to convert to analog first. This will be done with either a universal serial bus (USB), which has a throughput of about 12 Mbps, 1394/FireWire or some other type of interface, such as Bluetooth specification for wireless transmissions.

SCTE standard DVS-194 is linked to the set-top box specification known as OpenCable. This specification for digital cable television signals will use MPEG-2 formatting and 64-QAM and 256-QAM. The Institute of Electrical and Electronics Engineers (IEEE) standard 1394-1995 specifies the link between set-top boxes and digital devices known as "Firewire."

Firewire interface

Firewire consists of a four- or six-wire, shielded, twisted-pair bundle with data transfer rates up to 400 Mbps. It provides a way to send digital information securely over three or more feet of cable. It allows peer-to-peer communication unlike USB, which requires a host. It also is able to supply up to 60 watts of power allowing some devices to operate without a power cord. IEEE-1394 is not just a cable, but a protocol. Sony has this same type of interface, which it calls I-link (see Figure 3).

Copyrights

What about security and copyright protection? A standard known as 5C is the most accepted when it comes to set-top security and copyright protection. It is a copyright-protection encryption method started by five companies: Hitachi, Intel, Matsushita, Sony and Toshiba. The movie industry, which formed the Copy Protection Technical Working Group (CPTWG), the cable industry and set-top manufacturers also support this standard. TV manufacturers don’t want to see limitations placed on regular TV broadcasts and are either against this totally or are not sure. The computer industry feels the same way about Internet limitations. The POD card interface also will need a copy protection standard, which is currently under development. It allows someone to buy that $2,000 digital TV and obtain a POD from the local cable company without having to use a set-top box.

What’s it all mean?

Time will tell what happens with open access, must carry and other government mandated requirements. The future is coming fast. Great strides have been made in computer technology, laser performance, storage capability and service offerings. When more services are accepted into the everyday life of customers, it may be time to realize that our networks are severely limited in regards to return path bandwidth.

Five MHz to 15 MHz is nearly useless because of the inherent low frequency noise located in that region. Soon we will have to expand the return bandwidth instead of increasingly making the services more complex and subsequently less robust. The only thing holding us back is the EIA-S542, incrementally related carriers (IRC) and harmonically related carriers (HRC) frequency allocation tables. As soon as the over-the-air broadcasters go digital and relinquish their original analog frequencies, the cable/telecommunications network may be treated like the local area network (LAN) it really is.

We know networks are transforming and equipment requirements are changing. It’s well worth the time to find a common platform and upgradability to protect investments going forward into the future. Many devices now are using software upgrades to take equipment to the next level. It makes upgrades easier by downloading software from a CD or the Web instead of shipping equipment to a vendor, helping eliminate downtime from upgrade turn-around time. Designing flexibility, scalability and redundancy is a must for today’s highly complex networks.

John J. Downey is broadband network engineer with Cisco Systems. He may be reached at .

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