 Production options File Size and Compression • Quality in = quality out • File format comparrison table Before producing your final video, there are a number of factors to consider. The ultimate destination of the video (tape, computer, Internet) should always determine the frame size and data rate of your production. File Size and Compression Because video data can take up a large amount of disk space, some form of compression is usually used to reduce the file’s size and data rate. Most compression algorithms work by reducing or removing some of the details we are unlikely to see, or by selectively repeating some of a scene’s pixels in areas of the frame that don’t change much over time. For example, if a person is shown talking in front of a plain background, those portions of the background that do not change are stored as constants, to be used in succeeding frames. This reduces the amount of data required to display the frame and thus reduces the overall size of the file. However, if a video file has been compressed too much, pixelation or compression artifacts can occur. If you are under space or bandwidth constraints, you must decide the best compromise between video quality and compression rates. If your video will be primarily used for computer playback, producing to a relatively small frame size and using a higher compression ratio will conserve disk space. For distribution on the Internet, file size becomes even more important due to bandwidth considerations. On the other hand, if you plan to output to tape for viewing your videos on television, quality should take precedence. As a general guideline, larger, higher quality files should not be compressed so much that they lose detail, but not so little that their data rate is too high for smooth playback by your hard disk. The standard Roxio VideoWave 5 compression settings for AVI and MPEG offer a good compromise between video quality, data rate and file size. If necessary, the settings can be adjusted to suit your needs. Quality in = quality out You have probably heard the phrase "Garbage in equals garbage out," which in this case simply means that poor quality source video will result in a poor quality production. On the other hand, working with high quality source material maximizes the quality of your final production. Another important benefit is that ‘clean’ video is easier to compress. Using high quality source material is especially important if you plan to output to DV, Hi-8 or S-VHS devices. These mediums offer very good color and definition, and any imperfections in the source material (such as compression artifacts or poor resolution) will be very evident during playback. Choosing the highest quality source material will ensure the best possible result, even for standard VHS. It is recommended that you capture at the highest resolution, 720x480 (NTSC) or 720x576 (PAL). If disk space is at a premium and you plan to output to standard VHS, capturing at a smaller frame size (such as 320x240) can produce good results. The following table shows the data rates and file sizes of various compression schemes and their most likely application. Formats were converted from a 30-second DV file. Unless otherwise noted, all formats were produced at a frame rate of 30 frames per second (fps), with CD-quality audio (44.1 kHz, 16-bit sample rate).  |