NTT Research and Development 2008 Review of Activities

The digitization of television broadcasting is continuing. As for broadcasters, the digitization of video equipment and materials used for business purposes is taking priority, and uncompressed digital video signals (such as those covered by the HD-SDI^*1 specification) are being heavily used. Long-haul transmission of uncompressed digital video signals via optical fiber is carried out between broadcasting stations and between event sites and broadcasting stations, however, as the coverage areas for video-material transmission and live relays are expanded, it is being necessary to further increase the range of long-haul transmission. In accordance with that requirement, it is becoming necessary to amplify and relay signals that are attenuated during transmission on optical fiber.

Be that as it may, in the test signal, called a "check-field signal" (i.e., a pathological signal), in an uncompressed digital video signal, "bursts" of consecutive identical digits (i.e., binary "0"s or "1"s)—lasting 26µs for the HD-DSI specification or 53µs for the SD-SDI^*2 specification—are included. Meanwhile, a conventional optical-fiber amplifying repeater is a device that amplifies the signal, relays it, and transmits it over long distances. However, if the burst length in the signal exceeds 10µs, degradation of the signal waveform is generated by a phenomenon called "gain transient response". For that reason, relaying such an uncompressed digital video signal and transmitting it over long distances has been difficult up till now by means of an optical-fiber amplifying repeater. In response to that difficulty, we have developed an optical burst-mode amplifying repeater. By simultaneously amplifying (i.e., "coamplifying") the signal light with a continuous light (called a "gain-clamp light") in the same wavelength range as the signal light, this repeater can suppress the gain transient response and thus extend the range of long-haul optical-amplification relay and transmission of uncompressed digital video signals.

The optical burst-mode amplifying repeater also applies technology that is currently under investigation for extending the transmission range of fiber-to-the-home signals (which have high "burstiness") to uncompressed digital video signals. From now onwards, in addition to developing the repeater for uncompressed digital video signals, we will continue to develop a repeater for FTTH use.

• *1 HD-SDI: High Definition Serial Digital Interface
• *2 SD-SDI: Standard Definition Serial Digital Interface