RealNetworks introduced RealVideo with RealSystem 4.0, making streaming video available over the Internet. This chapter covers RealVideo production and explains other video formats that RealSystem G2 can stream. It also provides tips for capturing high-quality video and working with digitized video source files.
The following steps summarize how to create a RealVideo clip. The goal throughout this process is to get the best video quality with the smallest streaming file size. At each step you can do several things to achieve this goal.
Unless you start with a digitized video source file, you shoot a video or use existing recorded content on tape, satellite, laserdisc, or so on.
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Additional Information |
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| "Recording Video" provides guidelines for capturing video. |
You next digitize the video to a standard file format, such as AVI or QuickTime. With your preferred video editing software, you can then edit the video as necessary. If you are broadcasting live, however, you encode the streaming video directly from the source.
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Additional Information |
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| See "Digitizing Video" for tips on video editing. |
With your digitized file optimized or your live broadcast ready to go, you encode your source file as RealVideo. When you do this, you target a network bandwidth and choose a codec or set of codecs for your audio track.
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Additional Information |
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| See "Producing RealVideo" for more on RealVideo production. "Choosing RealAudio Codecs" explains RealAudio codecs. |
With your presentation ready to go, you make your RealVideo clip or broadcast available through your Website. If you are combining video with another streaming clip, such as RealText, you write a SMIL file that assembles the pieces.
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Additional Information |
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| Chapter 7 explains how to create a SMIL file. See Chapter 10 for instructions on linking your Web page to a RealVideo clip or a SMIL file. For more on live broadcasting, read Chapter 11. |
Observe the following points if you intend to shoot a new video rather than use existing video content. Because video loses image quality when compressed for streaming, always start with the best video source available.
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Additional Information |
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| For pointers on producing audio, see "Recording Audio". |
Whether you shoot a video yourself or digitize existing material, it's important to start with a high-quality video format. The following are common videotape formats in order of descending quality:
It's important to consider the video's final size before you shoot the first frame. To reach Web users with 28.8 Kbps modems, you should produce a video approximately 176x132 pixels. With this small size, you need to frame important visual elements well. If you show an instructor pointing to a chart, for example, zoom in to make the chart material visible.
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Additional Information |
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| For more on window size, see "Digitizing Video". |
The less that changes from frame to frame, the more the video file will compress. RealVideo compression technology reuses existing data when frames are similar, so a video with relatively stationary subjects ("talking heads") will compress more than a music video with rapid scene changes and a lot of movement. You can do the following to cut down on unnecessary movement:
Of course, you don't want to eliminate all dynamic elements! When you do include rapid movement, give enough time for objects to resolve. Because of low frame rates and high compression, objects coming to rest may appear blurry at first. If you have a dialog box popping up on a computer screen, for example, show that box stationary for a few seconds so that the image resolves.
Bright lighting at a constant exposure keeps the foreground detail crisp and cuts down the compressed file size. Use uniform, light colors in backgrounds and clothing. Dark colors and complex textures such as paislies and stripes add to the file size. They can also degrade the video with unwanted visual effects.
If you are not broadcasting RealVideo live, you export the video from your source (tape, disc, satellite feed, or so on) and digitize it on your computer or video editing station. You can then edit the file with your preferred video editing software before encoding it as RealVideo. The following sections provide tips for capturing video.
It is better to work with uncompressed formats. Otherwise, you compress the source once when you digitize it and again when you encode it as RealVideo. This double compression can decrease the image quality. Use a compressed source format only if your RealVideo encoding tool supports the file as input. You can use compressed AVI as long as the RealVideo encoding machine has the same Video for Windows (VFW) driver used to compress the AVI.
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Additional Information |
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| "Producing RealVideo". |
Unless you are short on disk space or your video capture card recommends a different window size, capture video in a window 320 pixels wide by 240 pixels high. With your video editing software, you can then reduce the window size before encoding the RealVideo clip.
The following table, which you should use as a general guideline only, shows three common RealVideo clip sizes that maintain the 4:3 aspect ratio used in television. You can produce RealVideo at any width-to-height ratio, however.
| Width x Height Pixel Size | RealVideo Bandwidth | Image Quality |
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| 176x132 (show size) |
20-500 Kbps | Good to High |
| 240 x 180 (show size) |
100-500 Kbps | High |
| 320 x 240 (show size) |
200-500 Kbps | High |
For each window size, the table lists bandwidth guidelines that assume a desired frame rate of 10 to 15 fps. The table shows that for 28.8 Kbp modems (20 Kbps usable bandwidth), you should use a window no larger than 176x132. Although its overall quality will be good, the image may encode at less than 10 fps. You'll get better quality when encoding for higher bandwidths.
Keep in mind that you can create a 240x180 window for bandwidths lower than 100 Kbps, or a 320x240 video for bandwidths lower than 200Kbps. But you're not likely to get high image resolution and 10-15 fps. The video might encode at 5-7 fps, for example, which may be acceptable for your audience. Experiment with different sizes, playing the encoded clip back in RealPlayer G2 to test its quality.
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| With your RealVideo encoding tool, you can choose to encode a large video for low bandwidths as a "slide show," which streams approximately one frame per second. |
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Additional Information |
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| See Chapter 3 for details on targeting bandwidth. |
Although RealVideo can encode up to 30 fps, the maximum recommended frame rate for streaming video over a network is 15 fps. Higher frame rates unnecessarily increase file size and may cause playback problems on slower machines. RealVideo encoding tools use a frame rate that varies with the bandwidth target and the emphasis on smooth motion or image quality. With video source at 15 fps, your encoding tool can vary the RealVideo clip frame rate between 1 and 15 fps as needed.
Always use 24-bit color. Lower color resolution produces poor quality videos. Higher color resolution greatly increases file size without adding significant image quality.
Use this formula to calculate the approximate size in Megabytes of a digitized video file:
(pixel width) x (pixel height) x (color bit depth) x (fps) x (duration in seconds)8000000
Suppose you want to capture a three minute video at 15 frames per second with 24-bit color in a window that is 320x240 pixels. Your digitized source file would be approximately 622 Megabytes:
(320) x (240) x (24) x (15) x (180) / 8000000 = 622 Megabytes
If necessary, you can conserve disk space by decreasing the window size or lowering the frame rate.
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Tip |
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| Some 32-bit operating systems limit a single video file to 2 Gigabytes (2048 Megabytes). At a 320x240 window size and 15 fps, this translates to about 9.5 minutes of video. To work around this, create separate source files, encode them as separate RealVideo clips, and merge the RealVideo clips with RealVideo editing tools available from http://www.real.com. |
Video capture places a large burden on a computer's CPU and hard drive. Be sure to have a fast machine. To avoid dropping frames during video capture, use a hard drive specially made for audio and video work. On Windows machines, you can use any video capture card that supports Video for Windows.
Video playback devices commonly have two output types, S-video and composite. Use S-video, which generally produces better results.
RealVideo's high compression rate makes it well-suited for streaming video over the Internet or intranets. A RealVideo clip uses the file extension .rm and typically includes an embedded soundtrack encoded as RealAudio. You start with a video file in a digitized format, then encode a RealVideo clip from this source file using a RealVideo encoding tool. Your encoding tool should be able to accept at least one of these input formats:
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Additional Information |
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| See "RealNetworks Encoding Tools" for more on RealVideo encoding tools available from RealNetworks at http://www.real.com. |
Like RealAudio, RealVideo uses a "lossy" compression scheme that discards parts of the source file during encoding. The following table provides a reference for RealVideo codecs. An "X" in the G2, 5, or 4 column indicates that a clip encoded with this codec can be played by RealPlayer G2, 5.0, or 4.0. Earlier versions of RealPlayer do not play RealVideo.
When you encode RealVideo, you choose an overall bandwidth target or set of targets, then set parameters such as audio type (voice or music), compatibility with earlier versions of RealPlayer, and an emphasis on smooth motion or sharp images. After you make your choices, the encoding tool choses the audio codec or codecs to use and encodes the clip using a variable frame rate. The following sections give tips on using RealSystem encoding tools.
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Additional Information |
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| See the tool's manual or online help for step-by-step instructions on encoding RealVideo, as well as more information about encoding tool options. RealVideo encoding tools are available for purchase or free download at http://www.real.com/products/tools/. |
Always keep a copy of the original video source file. To edit the RealVideo clip, modify the source file as necessary, then encode the file again as RealVideo. You cannot convert RealVideo clips to other video formats.
When you encode a RealVideo clip, consider whether it will play in parallel with another clip. If so, you can set an option in your encoding tool so that the RealVideo clip does not consume all available bandwidth. For a 28.8 Kbps modem connection, for example, the encoder standardly encodes the clip to use 20 Kbps. You can alter this default value so that the video uses 12 Kbps, for example, leaving bandwidth free for another clip.
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Additional Information |
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| Chapter 3 explains more about target bandwidths. |
You can create a single RealVideo clip encoded for up to four bandwidths with the SureStream codecs introduced in RealSystem G2. You can also specify backwards compatibility with earlier versions of RealPlayer. The encoding tool then encodes the clip for your selected bandwidths with the SureStream codecs. It also includes in the clip an encoding that uses an older codec and targets the lowest bandwidth choice.
For example, you can encode a single clip for the target connections shown in the table below. Based on its connection speed, RealPlayer G2 receives the G2 SureStream 20, 32, 45, or 80 Kbps stream. Earlier versions of RealPlayer receive the 20 Kbps backwards-compatible stream regardless of their connection speeds. Note that the RealAudio codec choices shown below are examples only. The encoding tool may choose other codecs depending on a voice or music soundtrack, and which features of the video you want to emphasize.
| Target Connection | Clip Bit Rate | RealAudio Codec | RealVideo Bandwidth |
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| 28.8 Kbps modem (backwards compatible) |
20 Kbps | 8 Kbps Music | 12 Kbps |
| 28.8 Kbps modem | 20 Kbps | 8 Kbps Music-G2 Mono | 12 Kbps |
| 56.0 Kbps modem | 32 Kbps | 11 Kbps Music-G2 Mono | 21 Kbps |
| 56.0 Kbps ISDN | 45 Kbps | 20 Kbps Music-G2 Stereo | 25 Kbps |
| 112 Kbps dual ISDN | 80 Kbps | 44 Kbps Music-G2 Stereo | 36 Kbps |
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Additional Information |
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| For information on RealAudio codecs, see "Choosing RealAudio Codecs". |
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| To support multiple bandwidths with non-SureStream codecs, you must encode a separate clip with each codec. You then use a SMIL file to specify bandwidth choices. For more on bandwidth selection through SMIL, see "Setting Bandwidth Choices". |
When you encode with the new RealVideo standard codec, the encoding tool crops the video window to multiples of 4, so a 176x132 video stays its original size. With the older standard codec, the tool crops to multiples of 16 pixels, however, so a 176x132 video encodes at 176x128 pixels. Encoding tools also let you manually crop the image window to leave areas of the source video out of the RealVideo clip.
RealVideo clips encoded for 200 Kbs or higher bandwidths may have image quality and frame rates that overburden processors slower than 120 Mhz. You should therefore use high-bandwidth RealVideo only for fast machines with fast connections.
RealSystem can stream several video formats in addition to RealVideo. The following table lists the streamable formats and shows whether RealPlayer G2 for a certain operating system (Windows 95 or NT, Macintosh, or UNIX) can play back the video. RealSystem supports all Vivo codecs. For other video formats, however, it typically does not stream codec-compressed files. When codec compression other than Vivo is supported, codecs are not included with RealPlayer G2 and must reside on the playback machine already.
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| RealSystem plug-ins may exist for other video formats. Check http://www.real.com for information about additional video formats you can stream. |
Observe the following points when streaming video formats other than RealVideo:
