Asynchronous Serial Interface, or ASI, is a physical (connector and electrical) definition for serial data over 75-ohm coaxial cable at rates at or less than 270 megabits per second. Electrically, the signal is typically around 1 volt.
ASI has one purpose only: the transmission of an MPEG Transport Stream (MPEG-TS). ASI is designed for that one purpose: the transport of the MPEG transport stream over coaxial cable, usually as part of its run within a transmission facility before conversion to fiber or wireless carriage.
The MPEG transport stream is the only standard protocol universally used for real-time transport of broadcast audio and video media today. Even when tunneled over IP, the MPEG Transport Stream is the universal lowest-common-denominator of all long-distance audio and video transport. In the US, it can be broadcast to homes as the ATSC Transport Stream; in Europe, it is broadcast to homes as the DVB-T Transport Stream. All broadcast satellite transmissions see it as the DVB-S Transport Stream.
It us usually made up of one or more television channels with accompanying audio, sometimes with additional audio-only or data transmission channels, When that composite data transmission path, asynchronous but formatted data, travels through space as RF, it is usually called DVB-S, DVB-T, or ATSC. But when carried on coaxial cable, unmodulated, it is called an ASI signal.
It is physically carried on 75-ohm coaxial cable, terminated with BNC male connectors on each end; the same data itself was or will probably be transported by wireless or fiber links, but then it will not be called ASI, even though it is precisely the same data.
ASI only refers to the data over coaxial cable. Specifically it refers to the electrical interface, a one-way transmission of serial data, and must be less than 270 megabits per second, the limit of the MPEG Transport Stream, and the rate of the DS4 telecommunications circuit which is typically used to transport the stream over commercial telephone/telecommunications digital circuits (Telco).
It is a one-way transmission, similar to RS-232 asynchronous data—a stream of raw but formatted zeros and ones—designed to primarily travel through coaxial cable at speeds that range from 6-200 megabits per second. Though 270 megabits per second is the rate of the underlying available bandwidth, Transport Streams, and therefore ASI transmissions, usually top out at around 200 megabits per second.
A Transport Stream, and thereby ASI when over coax, can carry one or multiple SD, HD or audio programs that are already compressed, as opposed to an uncompressed SD-SDI (270 Mbit/s) or HD-SDI (1.485 Gbit/s). An ASI signal can be at varying transmission speeds and is completely dependent on the user's engineering requirements. For example, an ATSC (US digital standard for broadcasting) has a specific bit rate of 19.392658 Mbit/s. Null characters, represented by the ASCII comma, are used to pad the transmission to that rate should the media itself not require the entire bitstream.
Generally, the ASI signal is the final product of video and audio compression for distant delivery, internal distribution, or broadcast to the public, as is today's digital television and cable.. Though it is codec agnostic and can carry any kind or data, It most often carries MPEG2 (H.262 video with MPEG-1 Layer II audio) or MPEG4 (H.264 video with MPEG-4 Part 14 audio), ready for transmission to a television or radio broadcast transmitter, microwave system or other device. Sometimes it is also converted to fiber, RF or the "SMPTE 310" format: (a synchronous version of ASI developed by Harris specifically for the 19+ megabit per second ATSC-transmitter input feed).
In all cases, ASI refers to Transport Stream transmission over coaxial cable, while that same stream, when traveling wireless, will have a different moniker.
There are two data transmission packet sizes commonly seen by the ASI interface and the cable carrying it: the 188 byte packet and the 204 byte packet, the fundamental building blocks of the MPEG Transport Stream.
The 188 byte format is by far the most common, packet size, used by the vast majority of transmissions. When optional Reed–Solomon error correction data are included, a format primarily developed by Cable Television industry, the packet grows an extra 16 bytes to 204 bytes total.
Time Warner Cable was the first company to implement the 204-byte standard in 1983.