| Video Compression Knowledge Database |
Last updated: April 08, 2008
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| Video Basics for Mac Users |
Modified on February 19, 2007
Every several hours , we find a posting like the follwing at the QuickTime section of Apple's Discussion Board.
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For some reason these people don't try to find time to read old logs but would rather get the straight answer they are looking for. So how come he has no audio? Nobody knows because he doesn't supply basic information. In order for strangers to answer his question, they would need to know the following pieces of information:
We live in the digital world, so don't be surprised that almost every piece of data we transmit is compressed. Even your voice coming out of a cellular phone is compressed. What if you don't compress your data, then? Well, your hard disk will be full in a matter of days. If you record every movement on the computer screen, say 1440 x 900 pixels, without compression for just one minute, for example, the size of video can be several gigabytes.
Inside a container there are audio and video streams. These audio and video streams are usually compressed. Compressed... Squeezed, you mean? No. The video compression works in a similar way that the graphic comression works. That is, it uses a mathematical function to reduce numbers. On the other hand, decompression is the precedure which requires the inverse function to recover lost data. So the codec usually consists of two things: compression and decompression. When your video player cannot play your mutimedia clip, one of the following events may have happened.
How come VLC can open almost any type of video while QuickTime can't? VLC is full of codecs, in fact. And QuickTime is not. The codecs that are used for QuickTime are called components and are stored in Library/ Quick Time. So you can actually see what kind of codecs are available in your system.
3. How to Check the Codec of a Multimedia Clip
There are several applications with which you can check the audio and video codecs. You could use QuickTime Pro to check the codecs. However, since we have already implied that QuickTime doesn't support some of the codecs and formats, it would be pointless for us to show you how with QuickTime. You can use VLC to check the audio and video codecs. Choose 'Information' under 'Window' after opening a video file. It is less reliable, but you can use ffmpegX to check the codecs. Click on 'Info' button at the top right. "less reliable" means ffmpegX's 'info' function doesn't always work. How about MPEG Streamclip? Just like QuickTime, MPEG Streamclip doesn't support many codecs and formats. The last resort is a Windows application called GSpot, which works fine with Virtual PC. So which application is the best one to check the codec after all? For Mac users, VLC is the most reliable one. Why? That's only because it contains many decoders, which allow you to open almost any forms of video. If you can't it, the video application won't tell you which codec is used in the video, right? Note that we are assuming the video is compressed in MPEG-4 (3ivx, DivX, H.264, MPEG-4, Xvid...) here.
If video is multiplexed, the scenario changes dramatically. VLC is the best codec identifier for MPEG-4. But it's not for MPEG. If the source is Video_TS, VLC does identify the audio compressor of multiplexed video. But if it's an independent MPEG-2 file, VLC is likely to fail. So what do we use to identify the audio compressor of an MPEG-2 video? We suggest you use ffmpegX or MPEG Streamclip.
If you have trouble with QuickTime, you should know the following facts before using it.
What is 'multiplexed' video, by the way? 'Multiplexed' is often called 'muxed.' In some videos, audio and video tracks are interleaved and cannot be simply separated. Separating an interleaved track into components is called demultiplexing. QuickTime Pro doesn't allow you to mux or demux video. QuickTime Pro does allow you to detach individual tracks. But don't be confused. Any videos that contain separate audio and video tracks are not multiplexed in the first place.
There are several applications that allow you demux video. MPEG Streamclip is one. ffmpegX is another. Unlike QuickTime, VLC supports many video formats and codecs. So do we want to upgrade QuickTime to the Pro version? VLC is a video player. It contains video encoders and does do video compression. The Pro version of QuickTime, on the other hand, accesses QuickTime components and performs video compression. And QuickTime is one of the few Mac applications that allow you to multiplex audio and video streams.
There are a few video codecs that you can download and use for free.
Importing video into Final Cut Pro or iMovie can give you a headache. As for iMovie, many people believe that it only accepts DV codec. That's not true. iMovie does allow you to import video that is compressed in MPEG-4. It accepts such video compression as H.264 (MPEG-4 Part 10), MP4 (MPEG-4 Part 2) and XviD. In order to import MPEG-2 video, many of those iMovie users convert it into DV because of such misconception. It is true that iMovie does not accept MPEG-2 or VOB. Bad things about DV compression are that DV video is inefficient and that the file size is likely to be enormous. So if your hard disk space is tight, it may be a better idea to re-encode MPEG-2 or VOB video with one of those MPEG-4-compatible compression algorithms. Final Cut Pro is a little bit different in terms of acceptable video codecs. At default, Final Cut Pro uses QuickTime and accesses QuickTime components to encode and decode video. And Final Cut Pro may accept multiplexed video. If Final Cut Pro doesn't, it is likely that such video uses AC3 audio compression. In that case, the deal will be the same as iMovie, and you will need to re-encode with DV or MPEG-4-compatible compression algorithms.
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