RealVideo

masonchung

RealVideo is a proprietary video format developed by RealNetworks. It was first released in 1997 and as of 2008[update] is at version 11. RealVideo is supported on many platforms, including Windows, Mac, Linux, Solaris, and several mobile phones.

4 L4 z7 o, Z& a" t3 I8 y* qRealVideo is usually paired with RealAudio and packaged in a RealMedia (.rm) container. RealMedia is suitable for use as a streaming media format, that is one which is viewed while it is being sent over the network. Streaming video can be used to watch live television, since it does not require downloading the video in advance.
% ^1 j/ I3 Q) @4 Y; H6 a" @
; b8 ^  K+ X/ _# P4 lTechnology
The first version of RealVideo was announced in 1997 and was based on the H.263 codec. At the time, RealNetworks issued a press release saying they had licensed Iterated Systems' ClearVideo technology and were including it as the RealVideo Fractal Codec[1]. However, support for ClearVideo quietly disappeared in the next version of RealVideo.
0 x/ t% T: V; P  U
RealVideo continued to use H.263 until RealVideo 8, when the company switched to a proprietary video codec. RealVideo codecs are identified by four character codes. RV10 and RV20 are the H.263-based codecs. RV30 and RV40 are RealNetworks' proprietary formats. These identifiers have been the source of some confusion, as people may assume that RV10 is RealVideo version 10, when it is actually the first version of RealVideo. RealVideo 10 uses RV40.
- l4 \7 m6 m& A* L
RealVideo can be played from a RealMedia file or streamed over the network using the Real Time Streaming Protocol (RTSP), a standard protocol for streaming media developed by the IETF. However, RealNetworks uses RTSP only to set up and manage the connection. The actual video data is sent with their own proprietary RDT protocol. This tactic has drawn criticism because it made it difficult to use RealVideo with other player and server software. However, the open source MPlayer project has now developed software capable of playing the RDT streams.
- ~$ \0 F" Z' b' G# D5 o
2 \, [0 d' y& R2 {7 bTo facilitate real-time streaming, RealVideo (and RealAudio) normally uses constant bit rate encoding, so that the same amount of data is sent over the network each second. Recently, RealNetworks has introduced a variable bit rate form called RealMedia Variable Bitrate (rmvb). This allows for better video quality, however this format is less suited for streaming because it is difficult to predict how much network capacity a certain video stream will need. Video with fast motion or rapidly changing scenes will require a higher bit rate. If the bit rate of a video stream increases significantly, it may exceed the speed at which data can be transmitted over the network, leading to an interruption in the video.


" N  X4 O6 ~8 r5 nRealVideo Players
% L) N+ _/ p( M4 H# g6 h# }* BThe official player for RealVideo is RealNetworks RealPlayer, currently at version 11, and is available for various platforms including Windows, Macintosh, and Linux. RealPlayer has been quite controversial due to RealNetworks' reputation for invasive advertising and hard-sell tactics.[1] Some people refuse to use RealPlayer and have sought alternative means of playing RealVideo content.
8 T- B; t+ h+ J9 ?' R- u
+ G- ?0 @9 O3 kSeveral other players exist, including MPlayer and Real Alternative. Many of these rely on the dynamically linked libraries (DLLs) from the official RealPlayer to play the video, and thus require RealPlayer to be installed (or at least its DLLs, if not the actual player). However, the open source ffmpeg library (and its DirectShow counterpart ffdshow) can play the older H.263 RealVideo and does not require RealPlayer or any parts thereof.

/ {- t& Z* }8 j- ^RealNetworks has also developed the open source Helix player, however support for RealVideo in the Helix Project is limited because RealNetworks is still keeping the codecs proprietary.
) ?5 l, V$ L$ I  {& B% g
% ^# q* L7 J5 l* tRealPlayer does not record RealVideo streams, and RealNetworks has advertised this feature to content owners such as broadcasters, film studios, and music labels, as a means of discouraging users from illegally copying content. However, other software exists which can save the streams to files for later viewing. Such copying, known as time-shifting, is usually considered legal.

# F" B3 n4 Y2 R& o6 I
  m* O/ a5 D# f  _) \' M6 QCodecs
RealVideo files are compressed using several different video codecs. Each codec is identified by a four character code. Below is a list of the codecs and the version in which each was introduced:

rv10, rv13: RealVideo 1.0, based on h.263 (included with RealPlayer 5)
6 b" j# \* j! l2 ?5 A3 B- |  crv20: RealVideo G2 and RealVideo G2+SVT, also based on h.263 (included with RealPlayer 6)
. \: n3 `! Z9 q: |3 x9 ]2 I& D+ b3 }rv30: RealVideo 8, suspected to based largely on either H.263 or an early draft of H.264 (included with RealPlayer 8)
rv40: RealVideo 9, suspected to be based on H.264 (included with RealPlayer 9)
# Q! X/ d8 E+ l8 {( hrv40: RealVideo 10, aka RV9 EHQ (included with RealPlayer 10). It is an encoder side improvement and therefore is fully backwards compatible with older players which has the RV9 decoder. As a result it uses the same 4CC.
While the newest version of RealPlayer should be able to play any RealVideo file, other programs may not support all codecs.