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`Choosing SureStream or Single Rate Encoding
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`When you encode a clip with RealSystem Producer, the most important decision you make is whether to create
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`a single-rate clip or use SureStream to generate a multi—rate clip. This choice is simple:
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`0 To stream a clip with RealServer, choose SureStream
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`0 To host a clip on a Web server, choose the single rate option.
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`However, to use RealSystem Producer eflectively, you should understand what SureStream is. This will help you
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`later ifyou decide to change the RealSystem Producer encoding defaults.
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`What is SureStream?
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`Introduced with RealSystem G2, SureStream technology gives the best p0ss1ble playback experience for
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`difierent types ofnetwork connections. With SureStream, you can encode your source file for optimum
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`streaming over, for example, 28.8 Kbps modems, 56 Kbps modems, and 112 Kbps dual ISDN connections, as
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`illustrated in Figge 1. Users with 28.8 Kbps modems receive the stream at the lowest bit rate, while users with
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`fister connections get a stream with better quality because ofthe extra bandwidth. RealSystem Producer Basic
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`encodes two speeds per clip, RealSystem Producer Plus up to eight speeds.
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`Figure 1: SureStream Clip Encoded for Several Bandwidths
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`RealF'lay-ers “Q U.
`. "
`23.3 Kbps
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`I
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`SureSlream
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`cup @ 4..
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`. O
`. . . ,. . . . .
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`‘0.
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`“Ia
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`‘1' a
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`55 Kbps
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`Q.*'l
`.
`a Q a
`112 Kbps
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`A SureStream clip can also "downshift" when a connection gets bogged down. Suppose a viewer starts out
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`receiving a higher bit rate stream through a 56 Kbps modem Ifthe connection starts to sputter as the network
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`gets crowded, RealServer automatically downshifts to a lower bit rate stream The audio or video quath drops
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`with the lower rate stream, but the clip keeps playing. When the interference clears, RealServer upshifis to the
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`higher rate stream RealPlayer doesn't need to rebufier during this shifting.
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`Why SureStream won't Work with Web Servers
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`SureStream works only with clips streamed with RealServer. Because SureStream wraps several streams into a
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`single clip, a server has to know how to extract just one stream from the clip. RealServer can do this, but Web
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`servers, which don't know the details of SureStream, send down all the encoded data for a SureStream clip
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`rather than just one stream
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`Duress Streams
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`Although you can encode a clip for just one connection speed with SureStream, you should never do this when
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`delivering a clip with a Web server. Even when encoded for just one speed, a SureStream clip contains "duress"
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`streams that let it downshifi during network turbulence. For example, a SureStream music clip encoded only for
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`28.8 Kbps has duress streams at 16 and 11 Kbps. A Web server downloads all these streams, wasting
`bandwidth.
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`Multiple Bandwidth Choices Through SMIL
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`When using a Web server, you can provide different bandwidth choices by encoding your source file several
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`times for diiferent connection speeds. You then provide multiple links on your Web page (one for each clip), or
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`use SMIL's <switch> tag so each RealPlayer chooses the right clip automatically. SMIL isn't as flex1ble as
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`SureStream, and you won't get downshifiing, but using SMIL makes it easier for your viewers to get the best clip
`for them
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`See the SMIL chapter inRealSystem Production Guide for instructions on using a < swi t ch> tag.
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`SureStream File Size
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`Because each SureStream stream increases the clip's file size, an encoded clip may be larger than its source file,
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`even though the clip is compressed. Ifthe clip's file size is an issue, limit the SureStream streams to just the
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`essential speeds. Table 1 gives ballpark figures for file space required for each minute of a RealAudio or
`RealVideo stream
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`Table 1:Approximate Size for RealAudio and RealVideo Clips
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`Target Audience
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`28.8 Kbps modem
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`56 Kbps modem
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`64 Kbps single ISDN
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`112 Kbps dual ISDN
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`Corporate LAN
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`256 Kbps DSL/cable modem
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`384 Kbps DSL/cable modem
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`512 Kbps DSL/cable modem
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`
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`
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`RealVideo
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`0.15 Megabytes/minute
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`0.25 Megabytes/minute
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`0.33 Megabytes/minute
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`0.59 Megabytes/minute
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`1.1 Megabytes/minute
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`1.65 Megabytes/minute
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`2.56 Megabytes/minute
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`3.30 Megabytes/minute
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`
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`When using SureStream to encode RealAudio or RealVideo, add the figures from Table 1 for each connection
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`speed you encode, then multiply by the length in minutes the source file plays. Table 1 shows that ifyou encode a
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`RealAudio clip for 28.8 Kbps and 56 Kbps modems, for example, the clip takes about 0.39 Megabytes of disk
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`space per minute (0.15 + 0.24). Ifthe audio is five minutes long, the clip willbe about 1.95 Megabytes in size
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`(0.39 x 5).
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`The figure you get is an approxirmtion based on using the RealSystem Producer default settings. The actual file
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`size varies depending on the combination of streams you've chosen, the type of audio, and Whether you‘ve
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`changed the defaults. Ifyou encode for just one connection speed, the file will likely be larger than you calculated
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`because ofthe duress streams. When you encode ReaIVideo for multiple connections, the file size may be lower
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`than you calculated because different RealVideo streams may use the same RealAudio stream for a soundtrack.
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`Copyright © 2001 RealNetworks
`For information on RealNetworks' technical support, click here.
`Comments on this document? Click here.
`This file last updated on 04/10/01 at 16:17:57.
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