As the drive for newspapers to produce video grows, more and more photojournalists are having to learn how to make video look good on the Web. For those who never set out to be computer geeks, it can be a long, hard road from JPEG compression to video compression.
Conversations on the Newspaper Video Yahoo! group regularly revolve around understanding streaming media concepts. For this reason, I will define file-related terms such as bandwidth, frame rate, data rate and resolution, and delivery options such as streaming and progressive download.
Bandwidth
Bandwidth is the viewer’s connection speed to the Internet, which controls one’s ability to retrieve and play video smoothly from Web sites. Higher delivery bandwidths, like those enabled with cable and DSL, allow you to stream higher quality video to your viewer.
Since video producers have to pay for their delivery bandwidth, however, they typically don’t produce at the maximum rate that their viewer’s bandwidth could support. Rather, they attempt to find a good balance between user experience and bandwidth cost. Few Web sites offer video larger than 640 x 480 resolution at a data rate of 600 to 700 kbps.
Data Rate
The data rate (or bit rate) is the amount of data contained in each second of a compressed video file, usually expressed in kilobits or megabits per second. ESPN distributes its video at 600 kbps; this means that each one-second chunk of audio and video comprises about 600 kilobits of data.
Data rate is the most important factor in streaming video quality. All streaming codecs use what’s called “lossy” compression, which means the more you compress, the more quality you lose. For this reason, all other file characteristics (like resolution, frame rate or codec) being equal, the lower the data rate, the lower the quality of the compressed file. As you can see in the figure above, you set data rate along with other configuration options during the encoding process.
Frame Rate
Most video starts life at either 29.97 or 24 frames per second (fps). Usually, producers who shoot at 24 fps deliver at that rate, while some producers who shoot at 29.97 fps deliver at 15 fps to save bandwidth. However, while logically it feels like dropping the frame rate from 30 to 15 fps, or 50 percent, would also allow you to drop the data rate by 50 percent with no loss in quality, it seldom works this way. Rather, according to the research I’ve performed, the average data rate of video produced at 15 fps is about 20 percent lower than that produced at 30 fps, not 50 percent. It’s still a substantial reduction, but often that comes at a subtle quality cost.
When considering 15 fps, for example, note that high-motion video will look noticeably choppy to many viewers. Tight facial shots, in which lip synch is critical, often look a bit out of sorts at 15 fps as well. When choosing your delivery frame rate, be sure to balance any data rate savings against the potential for degrading the apparent smoothness of your video.
Resolution
Resolution is the height and width of the video in pixels. Most video is original and stored at either 720 x 480 (standard definition) or 1920 x 1080 (high definition), but gets sampled down to smaller resolutions for streaming, usually 640 x 480 resolution or smaller. That’s because as the number of pixels in the file increases, you have to allocate more data rate to maintain the same quality.
For example, a 320 x 240 video has 76,800 pixels in each frame, while a 640 x 480 video file has 307,200, or four times more, which is evident in Figure 2. That means you have to apply four times the compression to compress the 640 x 480 file to the same data rate as the 320 x 240 file, which usually translates to noticeably reduced quality.
Data rate and resolution are integrally linked in quality-related streaming decisions. Note that the most common video resolutions for 4:3 video are 640 x 480, 440 x 330, 400 x 300, 320 x 240, 240 x 180 and 160 x 120. The most common resolution for widescreen 16:9 videos are 640 x 360, 480 x 270 and 320 x 180.
Delivery Modes
When you deliver video over the Internet, you have two basic options: streaming and progressive download. Each may involve slightly different options, so you should ascertain your delivery mode before encoding.
Streaming
Technically, video that is “streamed” is delivered by a streaming server, a specialized software program like Adobe’s Flash Media Server or Microsoft’s Windows Media Server. Producers deliver via streaming servers because they enable security options, such as Digital Rights Management or user authentication, or enhance the playback experience with features like real time seeking through the video file.
When producing for delivery via a streaming server, you should determine whether the server has any specific encoding requirements. For example, to stream using Apple’s Darwin server, you have to “hint” your files for streaming in your encoding software.
Progressive Download
Progressive download is a fancy name for video delivered by a regular HTTP Web server, and not a streaming server. When delivering via progressive download, the video is stored to the viewer’s hard drive as it’s received, and then played from the hard drive.
Progressive download debuted as a technique to send high quality video over modem connections, primarily for movie trailers. Today, however, progressive download is widely used in the broadband era as a technique to avoid the cost and administration of a streaming server. For example, YouTube distributes via progressive download, as do many networks. In these applications, the user experience is identical to a file received via streaming; the viewer clicks the link, the video starts playing almost immediately and there’s no break in the playback. Though there may be a perception that “streaming” means immediate, and progressive download means waiting, that’s vestigial.
When producing for delivery via progressive download, there may also be some technique-specific options. For example, with Flash, you have to make sure that the MOOV atom is at the start of the file, a highly technical requirement I won’t go into here. (Google “MOOV atom” Flash and “progressive” and you’ll find some relevant articles). The overall point is to understand how you’re distributing your file before you start encoding.
This is the first of several introductory-level articles on streaming media concepts by Jan Ozer, a streaming media consultant, frequent contributor to industry magazines and Web sites on streaming-related topics, and the author of Critical Skills for Streaming Producers, a mixed media tutorial on DVD published by StreamingMedia.com. Ozer, who can be reached at jan@doceo.com, will host a NewsU Webinar about streaming on Thursday, Nov. 13.
