Asynchronous Serial Interface, or ASI, is a method of carrying an MPEG Transport Stream (MPEG-TS) over 75-ohm copper coaxial cable or multimode optical fiber. This kind of signal processing is quite common in the television industry as a means of delivering broadcast programs from the studio to the final transmission equipment before it reaches all the viewers being home. The ASI standard is maintained by CENELEC, the European Committee for Electro Technical Standardization, and is part of the collection of standards known as Digital Video Broadcast, or DVB.
ASI carries MPEG data as a continuous stream with a constant rate at 270 megabits per second or slightly less, depending on the application. It cannot run faster than this, which is the same rate as SDI (Serial Digital Interface) and the rate of a DS4 telecommunications circuit, too which is typically used to transport the stream over commercial telephone/telecommunications digital circuits. The MPEG data bits are encoded by using a technical method called 8B/10B which stands for 8-bit bytes mapped to 10-bit character codes. Encoding provides for and maintains DC balance and makes it possible for the receiving user to get and stay synchronized. When on 75-ohm coaxial cable, ASI is terminated with BNC male connectors on each end. Electrically, the coaxial standard specifies an output voltage of 800 millivolts peak-to-peak, while the receiver must be able to operate from a voltage anywhere from 200 mV to 880 mV. ASI is electrically identical to and has the same bit rate as standard definition SDI. When ASI is on optical fiber, it can run on both multimode fiber and singlemode fiber.
Serial Digital Interface (SDI) is a serial, digital interface, primarily for the transmission of uncompressed and unencrypted video data via coaxial cable or optical fiber. It is mainly used in the field of professional television studios and in the field of television broadcasters. The SDI interface is specified by the Society of Motion Picture and Television Engineers (SMPTE) and is a further development of analog video standards such as PAL and NTSC. The HD-SDI extension according to the SMPTE 292M standard is currently the predominant link for the transmission of uncompressed image signals for HD, digital intermediate and digital cinema in the studio and production environment. Image, sound and metadata can be transmitted over all HD-SDI connections. HD-SDI, like SDI in general, is found mainly in professional video equipment, and consumers have established lower-performance interfaces. HD-SDI devices are usually found only in studios, rental houses and cinemas. A relatively new field of application is the use in video surveillance systems, where over this standard images of special HD cameras over existing coaxial cable installations can be transmitted. Thus, HD-SDI in this sector enables high resolution images without complete retooling to IP video. The method is used to provide TV and video sets with an easy and efficient conversion to HD quality.
These examples of ASI to SDI conversion have been very popular units for Thor and have been used globally in countless applications for easy signal conversion and SDI distribution.