Fiber-optics-based chip supports bidirectional data
DAVE WILSON
An application-specific integrated circuit developed by Acapella Ltd., based in Southampton, UK, lets designers build systems that use a single fiber-optic cable to transmit and receive both video and data.
The device, known as the ACS300, is a mixed-signal analog/digital controller transceiver integrated circuit that multiplexes real-time full-color video and full-duplex RS-232 serial data. It costs $50 to $60 and supports U.S. National Television Standard Carrier and German Phase Alternating Line color standards.
Simultaneous transmission and reception of data across the single link, with a choice of data rates to 64 kilobits per second, has been achieved with this video-multiplexing device. Video camera control and data feedback from the video source are supported by three full-duplex data channels or two low-frequency channels and one full-data rate channel.
The ACS300 uses an 880-nanometer light-emitting diode that transmits and receives video and data in a "ping-pong" manner. The system is based on the principle that if a light-emitting diode is forward-biased, it transmits light, but when reverse-biased, it acts as a receiver.
Typical applications include closed-circuit TV security and surveillance systems where remote control of pan/tilt functions are required, and applications where intelligent remote control is required in restricted or hazardous environments, such as underground or undersea.
According to Phil Tolcher, manager of computer-aided engineering at Acapella, the units can also be installed in existing twin fiber systems to double transmission capacity. These systems offer full-performance transmission to 19.2 kbits/sec at link lengths to 3000 meters.
Cost savings
Bryan O`Connell, a director of Acapella, says that the idea for the device originated from work the company did in developing a product for ICL, a British computer company. Because ICL preferred communications over a single, rather than dual, fiber to cut costs, Acapella developed the ACS100 device. The cost savings can be significant, because there can be tens of thousands of terminals, says O`Connell. Acapella eventually won a $1 million order to supply the ICL with 40,000 devices.
The company then turned its attention to developing a portfolio of other integrated circuits for the fiber optics market. O`Connell and his team of designers decided that there were similar communications issues--for instance, the need to simultaneously send video and data--that needed to be addressed in the surveillance market.
In this specialized market, video signals are normally transmitted from a surveillance camera to a monitor in a control room. Although it is important to send video in one direction, the motion of the camera must be controlled by transmitting directional information in the other direction. Such a problem implies that any design solution should make use of a full-duplex link.
One solution is to employ wavelength-division multiplexing, which the company rejected because of cost. Instead, Acapella`s designers decided to exploit the unused time between fields in the vertical interval of a video frame to transmit data. The chip that used this technique was dubbed the ACS300.
In an ACS300-based system, video is transmitted in one direction in the analog domain, while the data is time-compressed and sent as a high-frequency burst in the vertical interval between the video frames. At the beginning of the next field, video is sent in the opposite direction.
Bidirectional data and video
"Initially, we thought that the requirement would be for video in one direction and data in the other. But we realized that the device would be more marketable if we could support bidirectional data and video," says O`Connell.
According to Jon Lamb, a project manager at Acapella, the first in, first out filters and encoders on the device are similar to the traditional data transceiver functions that were built on previous devices. The rest of the device incorporates new functions that deal with video transmission, reception and amplification.
For reasons of timing, it was important to ensure that the data was synchronized with the video. "We needed to make sure that the data slotted into the time when there were slack transmission periods within the video signal," says Lamb.
Data is transmitted during the video field blanking period. Data is time-compressed by one ACS300 device, transmitted and then expanded by another ACS300 at the other end of the fiber. The video appears untouched even though three channels of data, in addition to the video, can be simultaneously transmitted.
The designers needed to employ time-compression and -expansion techniques to achieve their goals. "To the user, it is necessary to make it appear that both channels are transmitting and receiving simultaneously," says Lamb.
Most of the time, he says, one light-emitting diode is transmitting video from the camera. But at the start of a video field, the direction of transmission is reversed. Rather than a video signal going down the cable, data comes back. The diode receiving the signal then transmits light back to the receiver in a tight burst. Then they swap roles and the chip that was transmitting the video signal transmits a burst of data. All that occurs in the invisible part of the video picture at the start of the frame.
Time-expansion techniques are used to give the impression of simultaneous transmission. The data burst only lasts for a certain time, but the receiving device stretches it out to make it last the entire video field. In doing so, it appears that the data is transmitted at the same time as the video. Data and video thus appear to overlap, giving the impression of simultaneous transmission.
The other problem facing the designers is that a light-emitting diode is a poor receiver of optical signals. The typical current coming out of the diode might be only 100 microamperes peak-to-peak, which represents the complete dynamic range of the video picture, says Lamb.
For that reason, the chip amplifies the signal up to a normal 1-volt peak-to-peak composite video output to allow it to be displayed on the monitor. "You have to amplify the signal with a good bandwidth, too," says Lamb. "Not only do you have problems with amplifying a low signal, but you also have to get sufficient gain bandwidth to provide a good video bandwidth in the received signal," he explains.
The chip includes an automatic gain control block to cope with different fiber lengths as well as with the use of different light-emitting diodes. The gain control unit has a 100:1 range and is a pure-analog device. The ACS300 also has differential video output and a single-ended 75-ohm video output that can be used to drive the monitor directly. q
Dave Wilson writes from London.