Compression tips

When you compress video, remember the following recommendations:

• Make your video as short as possible. Trim the beginning and end of your video, and edit your video to remove any unnecessary content.

• Limit effects and rapid movement. Limit movement if you are concerned about file size. Any movement, particularly with many colors, increases file size. Shaky camera work and zooms increase the size greatly. Some effects increase file size because of the information they add to the video. On the other hand, some effects, such as blurs, can be used to decrease the number of bits in a compressed file.

• Determine the range of hardware and software configurations available in your audience, and identify the limits of the least‑capable configuration. This way, you can set the frame rate low enough for smooth playback, and retain as much quality as possible.

• Do not recompress video. Recompressing video leads to quality degradation, such as artifacts. Use raw footage or the least compressed footage that is available to you.

• Choose appropriate frame dimensions. If your target audience has a slow Internet connection (such as phone modems), make the dimensions of your video smaller, such as 160 x 120 pixels. If your audience has fast connections, you can make your dimensions larger (for example, 320 x 240 pixels). Generally, it’s best to specify a multiple of the full screen size, taking into account the pixel aspect ratio. For example, if the full screen size is 640 x 480 (square pixels), then try 320 x 240.

• Choose appropriate frame rate. If you target users that typically have older computer processors, choose a low rate of frames per second (fps), such as 7 or 15 fps. If you target users with newer computers, you can use a higher frame rate, such as 15 or 30 fps. Always choose a frame rate that is a multiple of your original frame rate. For example, if your original frame rate was 30 fps, compress to 15 fps or 7.5 fps.

• If permitted by the codec, adjust the data rate and quality settings according to your output goal. For example, codecs like Cinepak and Sorenson let you adjust the amount of compression by specifying a quality or target data rate.

• Lower the color depth if the source video contains less than the full range of colors. The full range is called 24‑bit color, Millions of Colors, or True Color. It is also effective for video shown on a monitor, or with software, that displays a limited range of colors.

• Choose an appropriate number of keyframes. Video keyframes are different from keyframes in Adobe Flash Professional projects. Each video keyframe is a full frame that is drawn when the video is compressed. The more frequent your keyframes, the better quality the footage is. More keyframes also mean a larger file size. If you choose 30, a video keyframe is drawn every 30 frames. If you choose 15, the quality is higher because a keyframe is drawn every 15 frames.

• Reduce noise. Noise (scattered pixels in your footage) increases file size. Reduce noise using your video editor, to reduce the video file size. Using more solid colors in your video reduces its file size. You can use the Video Noise Reduction filter in the Adobe Media Encoder, or a soft blur in After Effects to help reduce noise.

• Choose a file type and codec appropriate for the target audience. For example, in the Export Settings dialog box you can choose the Windows Media format for an audience primarily made up of Windows users. They all have the Windows Media Player that can play this format.

• Test the file by playing it on a system comparable to the least‑capable system in your intended audience, and make adjustments accordingly.

  Note: Applying a noise reduction filter can improve the appearance of video compressed with certain codecs, such as Cinepak.