Media Server Technology: Karl Paulsen
Asynchronous Interfaces For Video Servers
The ability to ingest and output MPEG-2 streams via an asynchronous
serial interface, or ASI, is one of the emerging advances in media
storage. Media file servers offering an ASI-I/O provide more than
just another format; indeed, an ASI-I/O extends the functionality
from strictly a video/audio-bounded device to a transport stream-based
system that can store data in either a single program stream or
a set of multiple program streams. These can include private data,
control and other media information, formatted in an MPEG-compliant
structure.
Servers with data systems that simply store "files" have typically
included I/O architectures presenting signals in a number of standards:
NTSC or PAL analog video, ITU-R BT.601 coded digital video carried
in a SMPTE 259M transport, or any number of other forms including
SDTI and native DV-packets. These offerings are well suited for
"in-facility" record and playout, but as we continue to address
delivery of compressed digital streams on devices other than tape
exchanges, servers need an additional set of interfaces that meet
the growing interchange of MPEG-2 streams. Today, video server manufacturers
are offering next-generation products that include an ASI-I/O option,
usually as a single card installed in their more standard server
product lines. ASI-I/O configurations may include only an ASI-input
and ASI-output interface; or an ASI-only input and present either
an HD-SDI (SMPTE 292M) output, a downconverted HD-to-SD output;
or a series of standard definition-only outputs from which multiple
streams are extracted from the MPEG-2 transport stream, or streams,
and presented to the server outputs during playout. Stored data
may also be "forwarded" or archived as necessary, depending upon
the internal architecture of the server and the options installed.
Although certainly not new, from a technology perspective, ASI-or more formally, DVB-ASI-is frequently used in satellite transmission, interfacility links (i.e., STL or ENG feed), and telephony communications over both twisted-pair and coaxial cable physical links. ASI server applications include the delay of a contribution feed; the recording of compressed, higher-resolution signals; and the store-and-forwarding of multiple program streams contained in a single transport stream wrapper.
STRICTLY INTERFACE
ASI means asynchronous serial interface, but there is a widespread
misconception that ASI is a "video standards-based signal. ASI is
strictly an interface, the format for how the data is carried. We
hear this interface generally described as DVB-ASI, with the DVB
prefix -- a common term most prevalent in the European Digital Video
Broadcast standard. The formal document describing the professional
interfaces for DVB can be found in the ETSI TR 101 891 (Ver 1.1.1)
Technical Report and is available on the Web at www.etsi.org.
DVB-ASI is designed to transport MPEG-2 video streams, primarily
for television applications over coaxial cable, at up to 270 Mbps.
The DVB-ASI interface is a competing standard to that of SMPTE 305M
(SDTI)-a synchronous serial interface standard, which specifies
a datastream protocol used to transport packetized data, and whose
data packets and synchronizing signals are compatible with 10-bit
operation of SDI (ANSI/SMPTE 259M); and SMPTE 310M -- a standard
that describes the physical interface and modulation characteristics
for a synchronous serial point-to-point interface that carries,
in a low-noise environment, MPEG-2 transport bitstreams at rates
up to 40 Mbps.
The electrical implementation of DVB-ASI is similar to the familiar
serial digital interface (SDI) whereby a bitstream is carried serially
over 75-ohm coaxial cable. Unidirectional electrical signaling levels
(800 mV p-p) are the same as and have equalization properties characterized
in the same fashion as SDI for long cable runs (nominally up to
300 meters for Belden 1694A). DVB-ASI uses 8B10 encoding and is
compatible with varying bit-rate signals supporting single (SPTS)
and multiple (MPTS) program transport streams; with a link character
length of 10 bits made up of input octets sent as 10-bit link characters.
Many SDI routers can carry ASI, although isochronous/synchronous
switching is not possible.
DVB-ASI is carried at a 270 Mbps line rate consistent with the
SDI clock described in SMPTE 259M. The 270 Mbps bit-rate on the
link is derived from a 27 MHz byte clock multiplied by 10 bits.
The structure of the link automatically stuffs the pipe (up to the
payload limit) with null packets, which act as the heartbeat for
the signal, and allows DVB-ASI to remain compatible with varying
bit-rate (VBR) data signals. The ASI carrier transport can contain
a number of different bit-rate single (SPTS) or multiprogram transport
streams (MPTS); e.g., a 25 Mbps, 43 Mbps, two 15 Mbps, a couple
of 1.5 Mbps, etc., all carried at simultaneously up to the payload
capacity of the link. Other data, such as IP-encapsulated media
or control protocols, can also be formatted for MPEG and then multiplexed
into the same ASI stream (see Figs. 1-3).
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| Fig. 1 DVB-ASI interface packages multiple programs
framed onto a single 270 Mbps ASI carrier |
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| Fig. 2 ASI carrier comprised of four program streams,
multiplexed to a 35 Mbps MPTS (Multi-Program Transport Stream),
presented to an ASI interface and then carried on a 270 Mbps
link. |
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| Fig. 3 Signal flow of various delivery methods to video
servers over an ASI carrier. Optional ASI outputs are ASI-only
or HD/SD decoded internally on the ASI-I/O card provided by
the server manufacturer. |
The "A and S" in ASI stands for asynchronous, a form of data transmission in which information is sent one character at a time, and basically "free runs," meaning the signal is without regular time relationship. In general, asynchronous data may also be unexpected and unpredictable with respect to the execution of a program's instructions. Asynchronous transmission does not use a separate clock signal to send and receive units to separate characters by specific time periods. Instead, each transmitted character consists of a number of data bits (the character itself) preceded by a start bit (or "begin character" signal), and ending with an optional parity bit followed by one or more stop bits (or "end character" signals).
Be careful when designing and implementing the ASI transport in the conventional SDI environment. One must understand how peripheral MPEG encoders, decoders and muxes will be configured; and how or when to use certain active repeating devices (e.g., a switcher or serial digital distribution amplifier) or even other passive switching devices, any of which might alter or render the signal useless after passing it through that device. ASI is a polarity-sensitive signal structure, and requires that the proper "polarity" of the output spigots be observed. There can also be ASI interoperability problems, such as datastream unlocking, which might present a worthless collection of data that may not be realized until too late for recovery.
Servers equipped with ASI-I/O may be interlinked without concern for synchronization, making them ideal for moving SPTS or MPTS signals over interfacility links (STLs, satellite, etc.). Using ASI, an entire ATSC stream (e.g., PBS's four-program composite feed) can be captured and later replayed in full, or disassembled either internally to the server or externally through demux/decoder channels into individual program channels.
Furthermore, ASI can be mapped to IP or ATM for carriage over private
or public lines with suitable bandwidth between various sites, extending
the constricted point-to-point structure of ASI to a point-to-multipoint
configuration using external IT switch gear and allowing for long-haul
applications using standard telecommunications networking protocols.
Expect to see significantly more implementations for the ASI-I/O as high-definition programming distribution increases. For future proofing purposes, those looking at new video servers for "spot" or "program" playout should consider how their server I/Os can be migrated to an ASI-I/O architecture that permits shared and protected storage.
Karl Paulsen is VP of engineering at AZCAR (www.azcar.com).
Contact him via e-mail at: karl.paulsen@azcar.com
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