Glossary

A

artifact: A visual imperfection in an encoded video clip. Too many artifacts can make the video look blocky.

B

bandwidth: The upper limit on the amount of data, typically expressed as Kilobits per second (Kbps), that can pass through a network connection.
binary tag: A SMIL tag that comprises opening and closing tags, such as <ref> and </ref>. Many unary tags can become binary tags when necessary to enclose other tags.
bit: The smallest unit of measure of data in a computer. A bit has a binary value, either 0 or 1.
bit rate: A measure of bandwidth, expressed as the number of bits transmitted per second. A 28.8 Kbps modem, for example, can transmit or receive around 29,000 bits per second.
blank time: A period during a presentation in which RealPlayer is not paused, but no activity occurs onscreen. You typically insert blank time with the SMIL begin attribute.
broadcast: To deliver a presentation, whether live or prerecorded, in which all viewers join the presentation in progress. Contrast to on-demand.
buffering: The receiving and storing of data before it is played back. A clip's initial buffering is called preroll. After this preroll, excessive buffering may stall the presentation.
byte: A common measurement of data. One byte consists of 8 bits.

C

cable modems: Devices that allow rapid transmission and reception of data over television cable. They are digital devices, unlike dial-up modems, which transmit analog data.
camel case: A capitalization convention in which words in a phrase are joined, and each word after the first begins with a capital letter. SMIL 2.0 attributes and values generally use camel case, as in soundLevel or whenNotActive.
CBR: Constant Bit Rate. A type of RealVideo encoding in which all parts of the video play back at the same bit rate. Contrast to VBR.
CHTTP: A version of HTTP supported by RealPlayer. Files designated with chttp:// are downloaded through HTTP and stored in RealPlayer's cache.
client: A software application that receives data from a server. A Web browser is a client of a Web server. RealPlayer is a client of Helix Server.
clip: A media file within a presentation. Clips typically have an internal timeline, as with RealAudio and RealVideo.
codec: Coder/decoder. Codecs convert data between uncompressed and compressed formats, reducing the bandwidth a clip consumes.

D

download: To send a file over a network with a nonstreaming protocol such as HTTP. Contrast to stream.
DSL: Digital Subscriber Line. A technology for transmitting digital data over a regular telephone line much faster than through dial-up modems.
duress stream: A low-bandwidth SureStream audio or video stream that Helix Server uses if a connection's available bandwidth drops greatly.

E

encoding: Converting a file into a compressed, streaming format. For example, you can encode WAV files as RealAudio clips.

F

Flash: A software application and an animation format created by Macromedia. RealPlayer can play Flash animations and stream them in parallel with other clips, such as RealAudio clips.
Flash Player file: A compressed Flash file format (file extension .swf) suitable for streaming. To stream Flash, you export the Flash Player file and tune it so that it plays well in RealPlayer.
fps: Frames Per Second. The number of video frames that displays each second in a streaming video clip.
frequency response: A measure of audio clip quality. The higher a clip's frequency response, the more frequencies it can faithfully reproduce.

H

Helix Server: RealNetworks server software used to stream multimedia presentations to RealPlayer.
Helix Server administrator: The person in charge of setting up and running Helix Server.
HTTP: Hypertext Transport Protocol. The protocol used by Web servers to communicate with Web browsers. In contrast, Helix Server streams clips to RealPlayer with RTSP. See also CHTTP.

I

inline switching: Switching between alternative clips without using a SMIL <switch> tag.
ISDN: Integrated Services Digital Network. Technology that makes digital data connections at 64 or 112 Kbps possible over telephone lines.
ISP: Internet Service Provider. A company that provides access to the Internet. Many ISPs have Helix Server available to stream media clips.

K

kilobit (Kb): A common unit of data measurement equal to 1024 bits. A kilobit is usually referred to in the context of bit rate per unit of time, such as Kilobits per second (Kbps).
kilobyte (KB): A common unit of data measurement equal to 1024 bytes or 8 Kilobits.

L

LAN: Local Area Network. A computer network confined to a local area, such as a single building. LANs vary in speed, with bandwidth shared among all networked devices.
lossy: A compression scheme that lowers clip size by discarding nonessential data from the source file. Both RealAudio and RealVideo are lossy.

M

metafile: Another name for a Ram file.
mouseover: The action of moving a computer screen pointer over an interactive area. An animated button may change appearance on a mouseover, for example.

N

namespace: An XML declaration that identifies the features used in a SMIL presentation. For SMIL 2.0 and higher, the <smil> tag must declare a namespace.

O

on-demand: A type of streaming in which a clip plays from start to finish when a user clicks a link. Most clips are streamed this way. Contrast to broadcast.

P

PNA: A proprietary protocol Helix Server uses for backward compatibility with RealPlayer 3 through 5. URLs using PNA start with pnm://.
port: A connection to a server, designated by a number such as 8080. Helix Server uses different ports for the RTSP, HTTP, and PNA protocols.
prefetch: To stream clip data to RealPlayer before the clip plays back. A clip's preroll can be prefetched minutes before the clip plays, for example, masking the preroll from the viewer.
preroll: Buffering that occurs just before a clip plays back. Preroll should be no more than 15 seconds.
presentation: A group of clips coordinated through SMIL and streamed from Helix Server to RealPlayer.

R

Ram file: A text file that uses the file extension .ram or .rpm. It launches RealPlayer and gives it the URL to a streaming clip or presentation.
RDT: RealNetworks Data Transport. The proprietary data package Helix Server uses (along with RTSP) when communicating with RealPlayer. Contrast to RTP.
RealAudio: A clip type for streaming audio over a network. RealAudio clips use the .rm extension.
RealOne Player: The successor to RealPlayer 8, RealOne Player combines streaming and digital download technologies. It supports the SMIL 2.0 and 1.0 standards.
RealPix: A clip type (file extension .rp) for streaming still images over a network. RealPix uses a markup language for creating special effects such as fades and zooms.
RealPlayer 10: The latest version of the RealPlayer media client. It supports all of the media formats and streaming features described in this guide.
RealPlayer G2: The RealNetworks client software that introduced plug-ins and the ability to update itself. It, along with the later RealPlayer 7 and RealPlayer 8, supports the SMIL 1.0 standard.
RealProducer: The primary RealNetworks tool for encoding RealAudio and RealVideo clips.
RealText: A clip type (file extension .rt) for streaming text over a network. It uses a markup language for formatting text.
real-time: Delivered as it occurs. For example, a live event is streamed across a network in a real-time broadcast.
RealVideo: A clip type for streaming video over a network. RealVideo clips use the extension .rm.
rebuffering: An undesirable state in which RealPlayer must pause a presentation to wait for streaming data to arrive. Rebuffering can result from network conditions, or a poorly produced presentation.
RTP: Real-Time Transport Protocol. The open, standards-based data package Helix Server uses (along with RTSP) to communicate with RTP-based clients. Contrast to RDT.
RTSP: Real-Time Streaming Protocol. An open, standards-based control protocol that Helix Server uses to stream clips to RealPlayer or any RTP-based client. Contrast to HTTP.

S

server: 1. A software application, such as a Web server or Helix Server, that sends requested data over a network.; 2. A computer that runs server software.
Shockwave Flash: See Flash Player file.
SMIL: Synchronized Multimedia Integration Language. A markup language for specifying how and when each clip plays within a presentation. SMIL files use the extension .smil.
stream: 1. To send a media clip over a network so that it begins playing back as quickly as possible.; 2. A flow of a single type of data, measured in Kilobits per second (Kbps). A RealVideo clip's soundtrack is one stream, for example.
SureStream: A RealNetworks technology that enables a RealAudio or RealVideo clip to stream at multiple bit rates.

U

unary tag: A SMIL tag that includes a closing slash, as in <ref/>. Many unary tags can become binary tags when necessary to enclose other tags.
URL: Uniform Resource Locator. A location description that enables a Web browser or RealPlayer to receive a clip stored on a Web server or Helix Server.

V

VBR: Variable Bit Rate. A type of RealVideo encoding that enables RealPlayer to play different parts of the video at different bit rates. Contrast to CBR.

X

XML: Extensible Markup Language. The parent language for SMIL. XML allows one to develop flexible, standardized languages for any purpose.