Chapter 4: Video Production

RealNetworks introduced RealVideo with RealPlayer 4, making streaming video available over the Internet. This chapter covers RealVideo production techniques, providing tips for capturing high- quality video, working with digitized video source files, and using RealProducer to encode your clips.

For More Information: For specific instructions about encoding RealVideo clips using RealProducer, refer to RealProducer 10 User's Guide, available at http://service.real.com/help/library/encoders.html. You can download RealProducer from http://www.realnetworks.com/products/producer10/inde x.html.

Understanding RealVideo

A video consists of two parts: the visual track and the soundtrack. In a RealVideo clip, the soundtrack is encoded with RealAudio codecs, and the visual track is encoded with a RealVideo codec. Both tracks are packaged in a RealVideo clip that, like a RealAudio clip, uses the file extension .rm. This section explains how RealVideo encodes a source video for streaming. This information will help you to produce high-quality streaming clips.

Tip: Keep in mind that everything discussed about RealAudio clips in Chapter 3 also applies to the soundtracks in RealVideo clips.

For More Information: RealVideo clips may also use the file extension .rmvb, which denotes variable bit rate (VBR) clips. For more on VBR, refer to RealProducer 10 User's Guide.

RealVideo Bandwidth Characteristics

Because RealVideo uses RealAudio to encode a video's soundtrack, a chunk of the clip's bandwidth first goes toward the audio. The visual track is then squeezed into the bandwidth that's left. For 56 Kbps modems, for example, RealVideo clips stream at 34 Kbps, leaving 22 Kbps of modem bandwidth for overhead. How much bandwidth the visual track gets depends on how the audio is encoded. With an 8 Kbps RealAudio voice codec for the soundtrack, the visual track gets 26 Kbps. With a 16 Kbps music codec, though, the visual track gets just 18 Kbps.

Possible Audio and Visual Tracks in a 56 Kbps RealVideo Clip

At low bandwidths, how you encode the soundtrack can affect how the visual track looks. RealAudio music codecs typically consume more bandwidth than do voice codecs. Music's greater frequency range requires more data than does speech, so a music soundtrack consumes more bandwidth than a spoken one. A video with an audio narration might therefore look better than one accompanied by music, as there would be more bandwidth available for the visual track.

At higher streaming speeds, the soundtrack uses proportionally less of the clip's bandwidth, so differences in soundtrack encoding affect visual quality less. At speeds above 100 Kbps, you get high-quality sound that uses no more than a quarter of the clip's streaming bandwidth. The following table lists the standard target audiences for RealVideo streams, giving the clip streaming speeds and the RealAudio codecs used for the soundtracks, broken out by audio type.

Note: With SureStream technology, a single RealVideo clip can stream at many different speeds. For the basics of SureStream, see "SureStream RealAudio and RealVideo".

RealVideo Frame Rates

Like RealAudio, RealVideo is "lossy," meaning that it throws out nonessential video data when encoding a clip. One way that RealVideo squeezes down clip sizes is by reducing the video's frame rate. The higher the frame rate, the smoother the motion:

• The standard frame rate for full-motion video is 24 to 30 frames per second (fps). At this speed, the human eye perceives movement as continuous—a phenomenon known as persistence of vision.
• A common rate for streaming video that approximates full-motion video is 15 fps. To most people, a 15 fps video flows smoothly, though not quite as fluidly as one at a higher rate.
• Below 15 fps, a video looks jerky.
• Below 7 fps, a video looks very jerky.
• Below 3 fps, a video essentially becomes a slideshow.

Most source videos start out at 15 to 30 fps. During encoding, RealVideo adjusts this frame rate downward as necessary, keeping the rate up in high- action scenes, reducing it in slow ones. Thus, your encoded clip will not have just one frame rate, but a mix of frame rates that varies with its content. If you follow good production practices, your clips will typically stream over slow- to medium-speed connections at 7 to 15 fps. At higher speeds, you'll get 15 to 30 fps. Many factors, though, affect a RealVideo clip's frame rate:

• The video's dimensions greatly affect frame rate. If you use too large of a window for your target bandwidth, you will not get a high frame rate. For more information, see "Video Encoding Dimensions".
• RealVideo 10 provides video quality superior to that produced by older RealVideo codecs. Using an older codec may result in a lower frame rate.
• Visually complex videos that show many objects moving across the screen simultaneously are hard to encode and may result in a low frame rate.
• In a video that has a mix of fast and slow scenes, variable bit-rate encoding (VBR) and two-pass encoding generally help the fast scenes achieve a higher frame rate. For more information on VBR, refer to RealProducer 10 User's Guide.
• When encoding with RealProducer Plus, you can lower the bit rate of the RealAudio codecs used for a given clip. This gives more bandwidth to the visual track, helping to raise the frame rate.

RealVideo Clarity

In addition to changing its frame rate, RealVideo can reduce a clip's streaming size by throwing out pixel data. A video stores information about each pixel in the frame. RealVideo, on the other hand, stores data for pixel groups. When bandwidth is tight, RealVideo shoehorns pixels with slightly different RGB values into the same group. These pixels then look identical rather than nearly identical. This may result in a loss of detail if compression is too high. The following illustration compares a smooth video with one that has lost detail through too much compression.

Smooth and Distorted Video

By using good production practices as described in this chapter, you can help keep the video's clarity intact during encoding. Also note the following points:

• The video's dimensions affect visual clarity. If you use too large of a window for your target bandwidth, visual clarity may suffer. For more information, see "Video Encoding Dimensions".
• A video with relatively stationary subjects ("talking heads") will have better visual quality than a video with rapid scene changes and a lot of movement.
• If you plan to launch a video in double- or full-screen mode as described in "Controlling How a Presentation Initially Displays", boost video clarity as much as possible during production and encoding. RealPlayer enlarges the clip by duplicating its pixels, which magnifies any defects.

RealVideo Codecs

RealVideo 10 is the standard RealVideo codec, but you can also encode with older RealVideo codecs. The codec you use encodes all of a clip's SureStream streams. You cannot encode half the streams with the RealVideo 10 codec, for example, and the other half with the RealVideo 9 codec.

RealVideo 10 Codec

The RealVideo 10 codec creates the highest-quality compressed video possible. It offers improved visual quality over RealVideo 9 and RealVideo 8, especially with fast-action scenes and on-screen text. Because RealVideo 10 performs more complex analysis of video data than earlier codecs, encoding may take more than twice the time required with RealVideo 9.

RealVideo 10 is compatible with RealOne Player and later. Users of older RealPlayers are prompted to update to RealPlayer 10 when they attempt to play RealVideo 10 content. Playback of RealVideo 10 content consumes the same amount of system resources on the viewer's computer as playback of RealVideo 9. Viewers, therefore, will not notice any performance slowdown when playing a RealVideo 10 clip compared to a RealVideo 9 clip.

Tip: RealNetworks recommends using this codec unless you need faster encoding performance during broadcasts, or you need to stream video to earlier versions of RealPlayer.

RealVideo 9 Codec

RealVideo 9 improves on RealVideo 8 with higher compression and improved visual quality. RealOne Player and later can play RealVideo 9 clips. Older versions of RealPlayer are prompted to autoupdate to RealPlayer 10.

RealVideo 8 Codec

The RealVideo 8 codec is backwards-compatible to RealPlayer 8. The video quality is not as high as with RealVideo 9 and 10, but encoding is faster. Additionally, RealVideo 8 requires fewer resources on the RealPlayer machine to decompress. This makes it suitable for the slower processors of mobile, handheld devices.

Steps for Streaming Video

When producing a video clip, you should choose the best source material and best equipment possible. The goal throughout the video production process is to get optimum video quality for each streaming speed.

Creating Video Clips

The following steps summarize how to create a video clip:

1. Capture the video content.

To start video production, you capture the source video by shooting a scene with a video camera, for example, or gathering prerecorded content from a tape, satellite, laserdisc, or other source.

For More Information: "Recording Video" provides guidelines for shooting a video.

2. Digitize and edit the video file.

You next digitize the video to convert it 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.

For More Information: See "Digitizing Video" for tips on video editing.

3. Encode the streaming video clip.

With your digitized file optimized or your live broadcast ready to go, you encode your source as a streaming clip, such as RealVideo. When you do this, you target a network bandwidth or a set of bandwidths.

4. Deliver the streaming video.

With your presentation ready to go, you make your video clip or broadcast available through your Web site. If you are combining video with another streaming clip, you write a SMIL file that assembles the pieces.

For More Information: Chapter 8 explains how to create a SMIL file. See Chapter 21 for instructions on linking your Web page to a video clip or a SMIL file.

Recording Video

Read this section if you intend to shoot a new video rather than use existing video content. Because video loses image quality if it's highly compressed, always start with the best video source available.

Tip: Always keep copies of the video source files. You cannot convert RealVideo clips back to their original source formats or any other streaming formats.

For More Information: For pointers on recording audio, see "Capturing Audio".

Source Media Quality

Whether you shoot a video yourself or digitize existing material, start with a high-quality video media. The following are common video formats, listed in order of descending quality:

1. Betacam SP, also known simply as Beta. This format is common among video production professionals.
2. DV, miniDV, DVCam, or DVCPro.
3. Super-VHS (S-VHS) or HI-8mm.
4. VHS, 8mm.

Video Staging

Consider the video's final frame size before you shoot the first frame. Streaming over 56 Kbps modems requires a small video window, so you need to frame important visual elements well. For recommended clip dimensions, see "RealVideo Frame Rates".

Scene Changes and Movement

The fewer things that change from frame to frame, the sharper the image will appear in a low-bandwidth video. You can do the following to cut down on unnecessary movement:

• Use a mounted camera rather than hand-held one. This greatly reduces the movement you inadvertently introduce into the scene when recording.
• Don't have a rapidly moving object fill the entire frame. But you don't want to pull the camera back too far either. You need to find a happy medium between close-ups and panoramic shots.

Of course, you don't want to eliminate all dynamic elements. When you do include rapid movement, allow 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, have the box remain stationary for a few seconds so that the image resolves.

Tip: RealPix makes a great companion to RealVideo. When presenting a lecture, for example, use RealVideo to show the speaker, and use a RealPix slideshow to present visual aids such as information written on a blackboard. For more on RealPix, see Chapter 7.

Colors and Lighting

Bright lighting at a constant exposure keeps the foreground detail crisp. Use uniformly dark colors for backgrounds, and uniformly light colors (but not whites) for clothing. Complex textures such as paisley and stripes degrade the final image quality with unwanted visual effects.

Video Output

Video playback devices commonly have at least two common output types— S-video and composite. Use S-video, as it produces better results. Professional-grade devices typically have other, high-quality output modes that can connect to a video capture card.

Color Depth

Always use 24-bit color. Lower color resolution results in poor clips.

Digitizing Video

The following sections provide recommendations on frame rates and video dimensions when capturing video input into a digitized file, and encoding the video into a streaming or downloadable clip. When you encode directly from a capture source, you do not create an input file first. However, it is still important to choose your encoded output dimensions correctly to produce a high-quality clip or broadcast.

For More Information: See "Understanding RealVideo" for background on the relationship between dimensions, bandwidth, frame rate, and visual clarity.

Digitized Video Formats

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 files as long as the computer used to encode RealVideo clips has the same Video for Windows driver used to compress the AVI file.

Video Capture Dimensions

If you capture video to a digitized file format, such as AVI or MPEG, you can edit the video using video editing software before encoding it as RealVideo. In this case, digitize the video at 320 pixels wide by 240 pixels high unless you are short on disk space or your video capture card recommends different dimensions.

Full-Screen Capture

You may want to capture full-motion video at the full-screen size of 640 by 480 pixels if all of the following are true:

• Your clips will stream at broadband speeds of 256 Kbps or higher.
• Your encoded clips will be larger than 320 pixels by 240 pixels.
• You have a video workstation capable of digitizing full-motion, full-screen video. Standard PCs typically cannot handle this large of a load.

Video Capture Frame Rates

When you capture content to a source file first, digitize the video at 15 frames per second (fps) if you plan to stream the clip at less than 150 Kbps. For these low speeds, 15 fps is the maximum rate that the standard RealVideo audiences encode. Above speeds of 150 Kbps, RealVideo can encode up to 30 fps, so it is better to capture the source input at 30 fps.

For More Information: For more information about the frame rate for encoded clips, see "RealVideo Frame Rates".

Computer Speed and Disk Space

Because video capture places a large burden on a computer's CPU and hard drive, use the fastest computer you have available. On Windows computers, you can use any video capture card that supports Video for Windows or DirectShow.

Disk Space Requirements for Video Capture

Use the following 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)
8,000,000

Suppose you want to capture a three-minute video at 15 frames per second, with 24-bit color, in a window that is 320 by 240 pixels. As you can see from the following equation, your digitized source file would be approximately 622 MB:

(320) x (240) x (24) x (15) x (180) / 8,000,000 = 622 Megabytes

If necessary, you can conserve disk space by decreasing the clip dimensions or lowering the frame rate, or both.

Video Source File Size Limit

Some computer file systems limit a single file to 2 GB (2048 MB) in size. At a 320-by-240 size and 15 fps, this translates to about 9.5 minutes of video. Certain video production programs support the OpenDML (AVI 2.0) standard, which allows the creation of files larger than 2 GB. If you plan to produce long videos or videos with large dimensions, check whether or not your video production software is limited to a 2 GB output file size.

Tip: If you are limited to 2 GB for the video source file and you need to produce a larger video, you can create separate video source files (each 2 GB or smaller) and encode them as separate RealVideo clips. Then, merge the clips using RealProducer's editing tools. Refer to RealProducer 10 User's Guide for more information.

Video Encoding Dimensions

When you capture video to a digitized input clip, you want to capture the largest size possible to preserve as much quality as you can. When you encode the file as RealVideo, however, you may need to reduce the video dimensions. Choosing dimensions too large for a given target bandwidth can result in a low frame rate or a large number of visual artifacts, rendering the video jerky or fuzzy.

There are no specific rules for which video dimensions to use, other than to maintain the aspect ratio of the digitized source. The primary consideration for selecting encoding dimensions is bandwidth, though other factors can affect the quality. For example, to keep its frame rate higher, a fast-action clip may require smaller dimensions than a low-action clip.

For More Information: To resize a video, you can scale the source file with your video editing software. Or, you can crop or resize the RealVideo clip as you encode it.

Desktop Video Dimension Recommendations

Most videos encoded for streaming to a desktop media player use a 4:3 aspect ratio to fit the dimensions of standard computer monitors. The following are general recommendations for encoded video dimensions based on your target audience's bandwidth:

• For desktop audiences with bandwidth less than 256 Kbps, use a smaller size, such as 240 pixels wide by 180 pixels high, or 176 pixels wide by 132 pixels high.

Tip: RealVideo 10 provides higher quality at high compression rates than older RealVideo codecs. When developing video for low-bandwidth audiences, using RealVideo 10 provides higher quality at larger dimensions.

• For desktop broadband connections of 256 Kbps or higher, encode your clip at 320 pixels wide by 240 pixels high.
• At very high bandwidths, you can choose larger dimensions, such as 640 by 480. To use these dimensions, however, the input should be of very high quality.

Mobile Device Video Dimension Recommendations

Mobile devices such as personal digital assistants and smartphones may have different screen sizes, so it's useful to know the specifications for the devices you are targeting. A common screen resolution of most smartphones is 176 pixels by 144 pixels. This size does not have the 4:3 aspect ratio common to television and desktop video. If you are starting with a larger, 4:3 source such as 320 by 240, you can do two things:

• Reduce the video to 176 pixels wide by 132 pixels high. This leaves 12 pixels of screen height unused.
• If the input video width is 320 pixels, crop out portions from one or both sides to create a width of 292 pixels. Then scale the video smaller to approximately 176 pixels by 144 pixels.

For More Information: For more about RealPlayer for mobile devices, visit http://www.realnetworks.com/industries/mobile/index.ht ml.

High-Bandwidth and Low-Bandwidth Streaming Audiences

If you want to encode a video clip or broadcast for both low-bandwidth and high-bandwidth audiences, you can adopt two different strategies:

• Use the same clip or broadcast for all audiences.

Using SureStream technology, you can create a single RealVideo clip that streams at many bandwidths. However, if you create the video at a large size such as 320-by-240, the clip will not stream well to slow connections. Using a smaller size benefits modem users, but does not take full advantage of the greater bandwidth of faster connections.

• Create separate clips for low-bandwidth and high-bandwidth viewers.

Creating separate clips allows you to encode a larger clip for high-bandwidth audiences, and a smaller clip for low-bandwidth audiences. You can make the clips available through separate links, or use a SMIL <switch> tag to let RealPlayer choose which version to play.

For More Information: For information about using SMIL to select clips, refer to Chapter 18.

©2002, 2004 RealNetworks, Inc. All rights reserved. For more information, visit RealNetworks Click here if the Table of Contents frame is not visible at the left side of your screen.