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| Enlisting A Video Encoder |
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Loading Zone June 2004 • Vol.4 Issue 6 Page(s) 73-75 in print issue |
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Enlisting A Video Encoder For When It’s Time To Compress & Decompress |
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Video encoders can compress a video file and save it in a variety of file formats and sizes while retaining most of the visual quality of the original file's images. Compressing video is crucial because just 10 seconds of ordinary NTSC video can gobble up to 300MB of storage space. If you want to distribute video on a CD/DVD, as a streaming video file, or as an email attachment, you need to compress it. For maximum flexibility an encoder must accept video from various sources, such as a videocassette or digital video camera. You must also be able to save that video file in different file formats. For example, you may need one format to save the video as a streaming video file but a different format to save it to DVD. To decide which video encoder is best for your needs, there are some common and special features you should know about.  How Video Compression Works To compress video, you need a video codec, which is an algorithm that specifies how to compress video for storing and how to decompress it for playback. The efficiency of a codec determines how small it can compress the video without losing its original visual appearance. Compressing video involves a trade-off between small file sizes and image quality. The smaller the file, the lower the image quality. Most video codecs use lossy compression, which means removing part of your video. The less video a codec stores, the smaller the file. A codec can't indiscriminately chop out chunks of your video without completely destroying the video image, so codecs selectively remove parts by comparing the video frame by frame. At the simplest level, the codec only saves images that change from one frame to the next. Because a background may stay the same throughout multiple frames, the codec only stores the background from a single frame, cutting the size of the video considerably. Codecs can also cut the file size by reducing the number of colors saved, altering the video's frame rate, and lowering audio quality. These methods involve cutting information to save storage space, but if a codec is efficient enough it can reduce colors so you won't notice or lower audio quality to eliminate sounds, you can't hear anyway. Give & Take No matter the codec you use, there's always a trade-off between compression level, video quality, and compression/decompression speed. The compression level defines how much a codec can compress a file. Video quality determines how much visual information the compressed file retains from the original. Compression/decompression speed measures how fast the codec can compress the video and decompress it (to play it back). If you compress too much, video quality suffers. If the codec is too slow in analyzing video, the compression/decompression speed can take too long. Some users might tolerate a slow codec for compression, but no one will wait several hours for the codec to decompress the video to watch it. If a codec takes equal time to compress and decompress a video, it's said to be symmetrical. If there's a big difference between the two times, the codec is said to be asymmetrical. Although there are many different codecs, no single codec is best for all types of video. Some are better suited for outdoor video images. Others are better at distributing video on CD/DVD. And still others are better for creating streaming video. For maximum flexibility most video encoders offer several codecs so you can choose the best one based on the task. Tweaking A Codec A codec alone can't do anything without a video encoder program. The codec takes care of compressing the video while the video encoder program lets you modify the way the codec works. Most video encoders can guide you in compressing a video without knowing anything about codecs, but modifying codec settings can let you wring the last bit of compression out of your video. Some settings include bit rate (or data rate), frame rate, frame size, and key frame. The bit rate measures how much space (usually in kilobits) is used to store one second of video. A high bit rate improves video quality but increases the file size. Depending on how you plan to distribute your video, there may be a maximum bit rate value. For example, a VCD can only playback video at a rate of 1,100Kbps, so storing a video that specifies a higher bit rate than 1,100 is pointless. Many encoders use CBR (constant bit rate) compression or VBR (variable bit rate) compression. CBR reduces the bit rate uniformly throughout the entire video. Although this means the codec can compress video quickly, it also means the codec treats simple images (the moon against a black background) the same as more complicated images (a forest on fire with airplanes flying overhead), so the video quality of the more complicated scenes may suffer. VBR alters the bit rate depending on the complexity of the video image. This can improve video quality but means slower compression time. For even higher video quality, some encoder programs offer two-pass VBR, meaning the program analyzes your video file twice to get the maximum video quality and smallest file size possible, but at the expense of much slower compression speed. The frame rate defines how many frames per second are used to store your video. The fewer frames used, the lower the quality and smaller the file. Because many computers can't play video faster than 16fps, a lower frame rate can preserve quality while reducing the file. Frame size measures the physical dimensions that your video appears in. A large frame size takes up more storage space. Key frame defines how many frames the codec waits before capturing another unchanged image, such as a background. Lower the key frame setting, and the codec captures unchanging images more often to improve video quality, but with a greater file size. When choosing a video encoder, don't be fooled by the minimum requirements. You could use Windows 98 with a Pentium III processor and 128MB of RAM, but a P4 with 256MB of RAM really should be the minimum requirements because video files gobble up RAM, hard drive space, and processing power. We tested three video encoders geared for a wide range of users using a 2.4GHz P4 system with 512MB of RAM and 1.8GHz AthlonXP system with 512MB of RAM. Both computers ran Windows XP. Obviously, a computer with a faster processor or more RAM would likely run through the test files faster, but these systems represent an average user's typical configuration.
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