Compression and data-rate basics

About compression

When exporting a movie, you choose a codec to compress the information for storage and transfer (such as on a DVD), and to decompress the information so it can be viewed again. The name “codec” comes from an abbreviation of its function of compression and decompression. During compression, repetitive and unnecessary information in the original file is discarded, causing the original file to lose information. For this reason, most codecs are considered lossy. Some codecs, though lossy, still allow the file to retain a high level of quality. The DV and MPEG codecs are especially good at maintaining excellent quality. Compressing video reduces its file size and data transfer rate, facilitating smooth playback and reducing storage requirements. A variety of codecs are available; no single codec is the best for all situations. For example, the best codec for compressing cartoon animation is generally not effective for compressing live‑action video. When you export from Premiere Elements, you generally only need to choose your medium of delivery and the application will select the optimal codec for you.

If you intend for your exported movie to be played back from a hard disk or CD, make sure that the codec you use to export your video is available to the audience for your movie. Most codecs for digital video and the web are already available on a majority of systems. However, if you are using a codec that’s native to a particular product, make sure that your target audience uses the same product, or can easily obtain the codec that you used.

If you intend to create a DVD or record to tape, codec compatibility is irrelevant. Your audience only needs to have the hardware necessary to play back the file.

About data rate

With some video codecs, you can specify the data rate, which controls the amount of video information that is processed each second during playback. Specifying a data rate in Premiere Elements sets the maximum data rate because the actual data rate varies according to the visual content of each frame.

The data rate you specify depends on the purpose of the video. The following list describes data rate guidelines for some uses:

DVD production

The data rate should maximize quality while fitting the entire program within the space available on the DVD. By default, Premiere Elements automatically adjusts the DVD data rate.

Non-DV videotape production

The data rate should be well within the data transfer rate of your hard disk because the video will be played back from the hard disk to the recording device. Check your documentation for information on the data transfer rate of your hard disk.

Hard disk playback

Determine the typical data transfer rate of your audience’s hard disks and set the data rate accordingly. Generally, 7200 rpm hard disks have sustained data rates of 20 to 35 MB per second, which is high. In comparison, the average data rate of digital video is 3.6 MB per second. So, to achieve high-quality playback, you may not need to set the data rate as high as 20 to 35 MB per second. However, if you are exporting video for use in another editing system or in a compositing application, such as Adobe After Effects, export at the maximum quality. Use a lossless codec, which is one that compresses without discarding information, and specify the data rate that the editing system supports for video capture and editing.

CD-ROM playback

The data rate depends on the speed of the CD drive. For example, if you are preparing a final video file for a double‑speed CD‑ROM drive (300 kilobytes per second) you might specify between 150 kilobytes and 200 kilobytes per second to account for both the data rate of the drive and the system overhead required to move the data.

Intranet playback

The data rate can be 100 kilobytes per second or faster, depending on the speed of your intranet. An intranet is an in‑house or private network that uses Internet network protocols. Because they are limited in scope, intranets generally use higher‑quality communications lines than standard telephone lines, so they are usually much faster than the Internet.

Streaming video over the web

Though there are fewer users with dial‑up connections than in previous years, you should still consider tailoring your data rate to users still employing this method of accessing the Internet if you want your file to be viewable to the largest number of users. Streaming video on the web is constrained by the limited bandwidth (56 KB or less) of most consumer modems. Use a higher bitrate if you know your audience has broadband Internet access, such as DSL or cable modem service.

Playing back from a handheld device

The data rate is very important due to the relatively small size, capacity, and lesser speed of handheld devices. The data rate can range from 8 to 90 kilobits per second, depending upon the device. You can choose preset QuickTime export options in Premiere Elements to export a file optimized for playback on a handheld device.

Downloading a video file over the web

The data rate is less important than the size of the video file because the main concern is how long it takes to download the file. However, it still may be desirable to reduce the data rate for downloaded video because doing so reduces the size of the video file, making it download faster.

About compression keyframes

Compression keyframes are different from the keyframes that you use to control track or clip properties, such as audio volume or clip rotation. When you export a movie, Premiere Elements automatically places compression keyframes at regular intervals in the movie. During compression, these keyframes are stored as complete frames. The frames between the keyframes are called intermediate frames. Premiere Elements compares each intermediate frame to the frame before it and stores only the data that is different. This process can greatly reduce file size, depending on the spacing of the keyframes. Fewer keyframes and more intermediate frames result in smaller file sizes with lower-quality images and playback. More keyframes and fewer intermediate frames result in significantly larger file sizes with higher-quality images and playback.

For example, a video of a talking person has a smaller file size than a video with lots of action, because only the mouth and tiny facial expressions change frame to frame. In contrast, a video of a sporting event requires numerous keyframes and intermediate frames, because the action changes considerably frame to frame. This results in either a larger file size or lower quality playback, depending on how much you compress the video.

Choosing compression settings is a balancing act. You need to adjust the setting depending on the type of video material, the target delivery format, and the intended audience. Often, the optimal compression setting is arrived at through trial and error.

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