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File conversion

Feb 1, 2009 12:00 PM, By Stoyan Marinov

Avoid systems that promise a universal solution.

             

Containers and codecs

“Codec” stands for “Coder and DECoder,” and usually means “Compressor and DECompressor.” It is the engine that transforms the baseband (uncompressed) video into a compressed stream and vice versa. The container is the file wrapper that is used to hold the compressed stream in one entity (the file) throughout the file system of the storage equipment. File wrappers are related to codecs and to the computer operating system as well. However, these relationships are not very well documented or standardized. As a general rule of thumb, Apple computers use QuickTime containers for storing video, regardless of the codec involved, while Windows computers use a variety of containers depending on codec and implementation, including QuickTime files (if the QuickTime add-on is installed).

The most popular container type on Windows used to be AVI, also known as Video for Windows. It can hold virtually any type of compressed stream. This means that you can find MPEG-2AVI, DV AVI, MPEG-4 AVI, MJPEG AVI, etc. Even though these share the same file extension, their content is not compatible in terms of a single codec. Today, Microsoft is promoting its Windows Media Video, which is stored in WMV or ASF file formats. Additionally, all MPEG formats have their own file types, e.g. MPG, M2P, M2T, MP2, MP4, TS, etc. Note that file extensions are not a warranty of compatibility. For instance, the same file extension, MPG, can be used both for MPEG-1 and MPEG-2 streams. Only an analysis tool can identify the content codec inside.

The trouble with most converters is that they do not care about seemingly nonessential things such as metadata, closed-captioning data, teletext, etc. The reason is that these nonessentials are stored in different ways for each file format or in a different place in the stream. So when choosing a conversion solution and wondering why some cost much more than others, check the small print. It is most likely that the cheap solutions will discard nonessentials. Additionally, during the HD transition period, special attention needs to be paid to aspect ratio information (WSS, AFD) so it's correctly applied during conversion. Advanced converters would offer additional benefits such as audio transformation, normalization, multiple tracks, etc.

An effort has been made toward unifying file containers — at least in broadcast equipment. The MXF format is gaining momentum with all broadcast manufacturers. However, it will be awhile until equipment from one brand properly talks to another through the MXF format. MXF is an excellent move, but it enables so many operating profiles that few manufacturers can entirely support it. The “MXF-compliant” label does not necessarily mean that this equipment can handle any MXF content. This compliance can only refer to a specific codec within an MXF container, or to a specific profile of the format. Therefore, don't assume MXF files can be used with all MXF-compliant equipment.

Select the best converter

Despite the progress with MXF, it seems certain that the number of file types, codecs and wrappers will continue to increase, so format wars are set to continue. The best approach when selecting file converters is to always verify manufacturers' claims. Try to avoid the universal type of converters; they are usually mediocre in most aspects. They will either be too slow or too simplified. Here is a short list of steps that can be taken to ensure against misunderstandings:

• Get a trial unit or license.
• Try it with your own files.
• Benchmark its performance against competing converters with the same content.
• Benchmark its quality preservation against competing converters at the same target bit rate and other conversion settings (e.g. quantization factors, GOP size, etc.) using a PSNR comparison tool, or visually.
• Make sure it behaves properly with long files (larger than 2GB).
• Check the lip sync at the beginning and the end of the converted file.
• Verify the resulting converted file with the equipment that is destined to use it afterward. Look for drifting lip sync and smooth playback.
• Look for the small things such as additional audio tracks, metadata, closed-captioning, etc.
• Optionally, send a small sample file to an analysis facility that can give a verdict on whether the file complies with the relevant standard, e.g. if an MPEG-2 file complies with ISO-13818. These facilities have expensive tools that the average broadcaster cannot afford to purchase.

Stoyan Marinov is chief technology officer at PlayBox Technology.

Table 1. Compression types and file formats operating at particular data rates are chosen as a “best fit” for each application across SD, HD and Internet/mobile applications, making file conversions essential to run the scene-to-screen workflow.

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