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i downloaded a film .avi file. when i open it (winows media player) i get... opening media>sending request for codec>error downloading codec .......the film then start but without the picture ie sound only. i updated media player to player9 but i still get the same thing. i run Xp pro ......what is a codec ? how do i get the correct 1 ? and is there another player i could try ? many thanks
What is a codec?
Depending on your sources, codec stands for compression/decompression or encode/decode. A codec is a small utility designed to compress video and/or audio information, so that it is easier to deliver to the end user’s system. In an uncompressed format, NTSC video is about 27 megabytes of information per second. At this size, only about 24 seconds of video could be placed on a standard CD – which means that a minute of uncompressed video is approximately 1.6 gigabytes. So, even on a standard DVD (4.5 gigabyte capacity) you could have a mere 2.8 minutes of fully uncompressed video. Likewise, uncompressed audio is approximately 150 kilobytes per second. In order to make large video clips more feasible, compression is necessary. Most codecs compress video using spatial and/or temporal compression.
Spatial compression compresses video by removing redundant data within an image. For example, if a video clip contains areas of similar color, then a codec can generalize by specifying coordinates of an area, as well as its color. In doing so, there is no need to process the individual details of each pixel. The less detail there is in an image – the better a codec is able to generalize the image, when using spatial compression. Removing fine details and digital noise, and using video with simple backgrounds will make it easier for a codec to compress images spatially.
Temporal compression uses a method that processes video by looking for and storing only the changes between consecutive frames, instead of the entire image. The original frame used as a reference for the following frames is called a keyframe. A keyframe contains the entire image. The frames that contain changes made to the original keyframe are called delta or difference frames. A keyframe is added every second or so to correct slight cumulative errors that can occur in the delta frames. This kind of compression tracks the changes over time – thus, the name “temporal” compression. Video that is best suited for temporal compression has few camera movements and subject changes.
A typical design feature of codecs is their asymmetrical processing. In other words, the codec is often designed to decompress video faster than it compresses it. This approach makes sense, since having the decompression process faster than the compression process allows video to play back in real-time. For example, a frame that took several seconds to compress may only take a fraction of a second to decompress. Codecs designed this way are referred to as asymmetric codecs. However, nowadays, most codecs allow you both to compress and decompress in real time.
One of the things to consider when selecting a codec is your image quality vs. file size. If image quality is your primary concern, then choosing fully uncompressed or near fully uncompressed video is your best option. However, if you’re working in an environment where file size is a large consideration, then using higher compression rates are advisable. Obviously, this can become a bit of a juggling act, as you determine at what point the file size is more important than loss of image quality. Is it really better to have extremely small files that are delivered easily, if the image quality is no longer worth watching? The only real way to know where to determine what codec is acceptable for your needs is by experimenting with them.
many thanks all sorted
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