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Московский технический университет связи и информатики
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Основы телевидения
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Streams and files

17

A file is an ordered sequence of bytes explicitly having a start and an end, characterized by storage. A stream is characterized by realtime data transfer of unbounded duration on a unidirectional channel – that is, with no upstream channel for flow control, acknowledgement, or retransmission request. Table 17.1 provides a general summary of the characteristics of files and streams.

A file …

A stream …

 

 

… has predefined beginning and end

… has indeterminate beginning and end

… usually involves storage media

… usually involves an external data

 

interconnect

… permits “random access” to data

… involves sequential data access, typically

 

starting midstream

… has structure imposed at a high level;

… has structure imposed at a low level;

data is arranged arbitrarily

data is arranged to minimize buffering

… has no need for embedded delimiters

… contains embedded delimiters by which

 

essence elements can be identified “on the

 

fly”

… transfer usually occurs across a general-

… transfer usually occurs across a data

purpose network

interconnect

… transfer is usually free-running

… transfer is usually synchronized to a timing

 

reference

… transfer typically has variable data (bit) rate

… transfer typically has constant data (bit) rate

(VBR)

(CBR)

… transfer data integrity is guaranteed,

… transfer data rate is guaranteed,

but data transfer rate isn’t (best effort)

but data integrity isn’t (errors may intrude)

… transfer typically involves upstream

… transfer typically has no upstream

communication; transfers are generally

communication; transfers are generally

acknowledged

not acknowledged

Table 17.1 Files and streams are compared.

 

163

A stream is “live,” and suitable for realtime interface. A file is not intrinsically “live.” A file may be operated on or exchanged slower than realtime, in realtime, or faster than realtime. A portion of a stream can be recorded as a file, and a file can be streamed across an interface; however, generally, streams are structured for realtime use across interfaces and files are structured for nonrealtime use on storage media. Generally, video interfaces (and videotape recorders) are characterized as streams; video storage is characterized as files.

Historical overview

Video signals were historically conceived as streams. VTRs recorded continuous streams (traditionally omitting the vertical blanking interval, and in DVTRs, omitting the horizontal blanking intervals as well).

A stream interface conveys elements (analogous to historical analog video sync) that permit synchronization on the fly: A receiver can connect to a stream at any time, and begin operation within a fraction of

a second. Stream formats are designed to have a property of locality whereby essence elements to be presented simultaneously – typically, video and the associated audio – are located nearby in the stream, so as to bound the required buffer storage capacity, and to bound latency to access the essence required for presentation.

Historically, uncompressed digital video was streamed in the studio in realtime across SDI or HD-SDI interfaces. SDI and HD-SDI timing was designed such that analog video could be obtained simply by stripping off the stream synchronization and ancillary elements, performing digital-to-analog conversion, and inserting analog sync. Almost no buffer storage was required.

As digital recording and playback of compressed digital video became possible, the SDTI specialization of SDI was designed to “wrap” compressed video, then audio, for conveyance across SDI. Various compression schemes such as MPEG IMX and DV100 were accommodated. DV video can be conveyed in streaming mode across IEEE 1394 interfaces.

As computing and networking technology advanced, it became feasible to store compressed video, then uncompressed video, in files. It became common to

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DIGITAL VIDEO AND HD ALGORITHMS AND INTERFACES

The Apple iTunes model differs: Files are transferred for later playback.

exchange these files across TCP/IP Ethernet networks – first at 100 Mb/s, then 1 Gb/s, and soon, 10 Gb/s.

As commodity IT networking technology improved, the schemes that were used to package compressed video for transport across specialized interfaces such as SDTI were adapted to general-purpose networking. Compressed video in formats such as MPEG IMX and DV100 were stored in files as raw bytestreams. The MXF file format emerged as a mechanism to store video and audio essence in a more structured manner. Standards emerged to store compressed video in MXF files. An MXF file need not contain essence: It can refer to essence stored in separate files. Some formats for A/V storage in MXF files are “stream-friendly” in the sense that video and audio essence is stored in the MXF file in proximity to each other, suitable for playout with

a minimum of restructuring. Other formats have higherlevel structure more suitable to postproduction (for example, storing video and audio in separate files referred to by the main MXF file).

Today, file-based workflows are widely used in production and postproduction; however, stream-based techniques continue to dominate distribution of professionally produced content. Services such as YouTube, Hulu, and Netflix distribute video to consumers across what I call the big wooly internet – however, service and quality levels of these systems are lower than those associated with television broadcasting: The pictures aren’t at HD quality level. They stutter, and the audio loses sync. Internet-protocol television (IPTV) refers to adaptations of commodity TCP/IP-based networking to achieve the service and quality levels of broadcasting.

In professional video distribution, the file-based production/postproduction world and the stream-based distribution world meet at the playout server. The playout server includes a disk store with an associated file system. On the production side, the server is accessed asynchronously using IT networking. On the distribution side, a stream access mechanism reads files according to a timeline driven by house sync and timecode, and throttles playback accordingly. Realtime decompression of compressed video may be required. Dedicated stream interfaces then launch the content into the distribution network.

CHAPTER 17

STREAMS AND FILES

165

 

Physical layer

 

Serial digital video SDI and HD-SDI interfaces are based

 

upon 10-bit words that are serialized, “scrambled,” then

 

conveyed unidirectionally as a bitstream onto a single

 

wire. The scrambling technique permits payload data

 

transfer rate to equal the bit rate on the wire; however,

 

signalling sync requires certain data values to be

 

prohibited from appearing in video.

 

Commodity computer interfaces are based upon

 

8-bit bytes. Historically, data was serialized onto

 

a single conductor (e.g., Ethernet); however, a number

 

of computer-oriented interfaces (e.g., PCIe and Thun-

 

derbolt) serialize data onto multiple “lanes.” In some

 

physical interfaces, data is typically mapped – for

 

example, using the 8b/10b scheme – so that all 8-bit

 

byte values can be conveyed across the interface, while

 

allowing the receiver to recover the clock and the data

 

framing for arbitrary data. The bit rate in the channel of

 

such encodings is somewhat higher than the payload

 

rate – in the case of 8b/10b mapping, 1.25 times higher.

 

Other interfaces (e.g., DVI and HDMI) use a dedicated

 

clock wire to establish timing; arbitrary data can then

 

be serialized and transferred without data value restric-

 

tions. Some interfaces reverse the direction of data

 

transfer across each conductor (or pair); others have

 

dedicated wires (or pairs) in each direction.

 

Stream interfaces

SDI, HD-SDI

SDI was designed for uncompressed 4:2:2 SD; it has

 

a data rate of 270 Mb/s. (SDI was adapted to SDTI for

 

compressed video; however, SDTI is now largely obso-

 

lete.) HD-SDI was designed for uncompressed 4:2:2

 

HD; it has a data rate of about 1.5 Gb/s. Recently,

 

a 3 Mb/s adaptation of HD-SDI has been standardized

 

and commercialized. Details are found in SDI and HD-

 

SDI interfaces, on page 429.

DVI, HDMI, and DisplayPort

DVI, HDMI, and DisplayPort are digital interfaces

 

designed for connection of computer graphics

 

subsystems to displays, across cables at lengths up to

 

3 m. Apart from a very low-rate reverse channel –

 

display data channel, DDC – that communicates display

 

characteristics upstream to the graphics subsystem, DVI

 

and HDMI are unidirectional.

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DIGITAL VIDEO AND HD ALGORITHMS AND INTERFACES
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