Laser chip hikes channel capacity
A laser chip capable of transmitting 110 channels comes from Ortel Corp. in Alhambra, CA. The 1310-nanometer distributed feedback, or DFB, laser chip offers a frequency range of 50 to 750 megahertz, with no more distortion than standard DFB laser chips that provide 80 channels in the frequency range of 50 to 500 MHz.
According to Larry Stark, Ortel`s vice president and business manager of broadband communications, this laser allows cable-TV operators to generate income from unused bandwidth. The frequency range from 550 to 750 MH¥is usually assigned for digital transmissions, but for a variety of reasons, digital technology for cable-TV systems is generally unavailable.
The Ortel laser would let cable-TV operators use currently unused digital bandwidth for additional analog services. Ortel is marketing the laser to generate income from the investment until digital services are implemented.
It is possible to modulate 110 channels onto existing DFB lasers, but this would result in additional distortion. "For the past four or five years," says Stark, "there has been steady progress in volume manufacturing of the DFB lasers for cable TV.
In the past, if you put more channels on, you got more distortion, but with the 110-channel laser distortion does not increase with more channels." The laser`s carrier-to-noise ratio is 51 decibels, composite second order is 63 dB, and composite triple beat is 65 dB.
Ortel has been producing lasers for the cable-TV industry since the introduction of its first DFB laser designed for this application in 1989. Early products provided 40 channels modulated onto approximately 5 milliwatts of optical output power. Today`s standard DFB lasers carry 80 channels on 16 milliwatts of power with less distortion and often cost less than the earlier lasers.
However, Ortel is not the only company offering a 110-channel laser product. Harmonic Lightwaves Inc. in Santa Clara, CA, has been offering a 110-channel transmitter to select customers since April 1995, according to John Dalquist, vice president of marketing. He says the company has sold the transmitters on a limited basis because not many customers have 110-channel converter boxes. Furthermore, it is not clear whether the increased number of channels will be attractive to industry.
According to Ortel`s Stark, "Cable operators are investing heavily in building 750-MH¥networks for digital services, and we understand their eagerness to begin generating revenue from their ramped-up networks even if the digital solution isn`t there yet." Dalquist says that his cable-TV provider recently had to drop some channels in order to offer others.
However, Richard Mack, vice president of Kessler Marketing Intelligence Corp. in Newport, RI, is not convinced of industry demand. He says, "It`s not clear why there is a demand for 110 channels. It might have to do with franchise renewal and licensing. The presence of additional channels could show that cable operators are keeping up with competition and offering channels for public access." Or, he surmises, it might be useful for pay-per-view and adult programming.
Paul Gemme, vice president for plant operations at Time Warner in Denver, CO, says, "This laser is significant if digital transmission and reception don`t arrive as soon as some people expect. Even if digital comes about sooner than later, the capability of 110 channels is useful. If you load the laser with 79 analog channels and some digital, the performance should be better than the 80-channel lasers now."
Because digital traffic adds distortion differently than does analog traffic, users would still receive lower distortion by using this laser rather than standard lasers. Gemme says that "no one wants to forge ahead and build something that turns out to be a nonstandard product."
Why not digital?
Analog transmission is the norm for cable TV because television sets accept analog signals. The industry has planned for the addition of digital transmission because it would allow two-way services to be rendered using well-developed technologies. Telephone and computer data transmission are examples of two-way services that require switched signals. Although analog switching is possible, the equipment for switching digital signals for telephone and computing applications has been developed over decades.
Two-way services also demand bandwidth for the customers to send signals to the system`s headend. "Because most cable-TV systems are segmented into nodes serving a small group of subscribers," explains Stark, "the bandwidth reserved for subscribers to talk `upstream` (from 5 to 40 MHz) allows transmission in this direction of at least 2 megabits per second per subscriber (assuming all 500 subscribers are transmitting at the same time)."
Many new networks have been designed to accommodate digital service, but the equipment to provide the service has not yet been brought to market. Dalquist says, "Digital set-top boxes were expected in 1993 and 1994, but they aren`t here yet."
Price may be a major factor when digital services become available. Dalquist comments, "Now that the euphoria has worn off, people wonder if a $500 to $600 addressable set-top box" for digital services will pay for itself. Compared to the cost of a digital set-top box, a box that receives 110 analog channels is only slightly more expensive than an 80-channel box.
Some cable-TV operators are not waiting for the digital boxes to offer switched services. ICTV in Cupertino, CA, has developed analog switched services with digital conversion at the headend. Also available are radio-frequency modems that modulate digital signals onto analog media such as coaxial cable. q
Yvonne Carts-Powell writes from Belmont, MA.