Digital Video Format Conversion

History

Over the years since the personal computer was introduced, various audio video technologies have developed independently resulting in multiple formats. The original Audio Video Interleaved (AV) files developed by Microsoft and Apple used uncompressed images and sound files  which produced enormous files for very small, low resolution media.

In time a variety of compression algorithms were developed such as the Zip compression with which we have all become familiar. Unlike Zip compression for text and other disk based file type, compression routines for multimedia files by necessity required on-the-fly decompression. To accomplish a completely separate Compressor/De-Compressor, referenced in the header of audio or video was developed. Called Codecs, these programs were called by the operating system whenever a multimedia file was referenced for playback. As the algorithms improved, quality improved but codecs also added another layer of confusing technology to the source file, sometimes requiring that the user have both a proprietary audio and video codec in order to play a particular movie. Typically these file types end with the three letter file extension AVI. To learn more about codecs or multimedia please see our tutorials.

Because distribution of codecs was cumbersome, unregulated and fraught with problems for the average computer user the major software developers and hardware manufacturers decided to create a standard for audio video and thus formed the Motion Pictures Experts Group, also known as the MPEG committee. The first standard that the MPEG Committee produced was called MPEG-1 and it encompassed standard internal audio/video file compression and de-compression. The standard was later modified to include Video CD's.

MPEG-2 was the standard for DVD and Super Video CD's and MPEG-3 was devoted primarily to audio.  Typically MPEG-1 and MPEG-2 video files use the file extension MPEG or MPG. MPEG-2 files on DVD disk use VOB (Video Objects) but the files may also contain menu information. The VOB files found typically on a DVD disk may also contain AC-3 or other such audio streams for multiple languages, and/or surround sound multiplexed together. Conversion of such files to a more generic format requires conversion to stereo and one language.

For some reason unknown to the author, the MPEG-4 standard reverted to the AVI scheme using codecs. Examples of MPEG-4 are Windows Media, Real Media, the newer version of Apple QuickTime, DivX and XviD. Like the early version AVI's, to play these files you must have the codec that was used to create the file. To edit these files or convert them to another format you might also need a professional version of the codec or proprietary software.

The Conversion Process

The term conversion is a bit misleading insofar as the original file is not actually converted but disassembled and written to an entirely new file using the parameters and format that the user selects.

When a movie is converted the source audio and video must first be decompressed using the same codec that was used to create the file then the video frames and audio samples are recompressed using the new format. During these processes the audio and video streams are written to temp files in blocks and periodically interleaved and written to the new target file.

Potential Problems

If the same codec that was used to create the source movie is not available Windows may use a similar codec which may or may not work properly.

If there is a break in the audio or video stream, a missing frame or index, the conversion tool will not know what to do next and it may chew on the same block of data, retrying and retrying forever or it may recognize that the data is bad and simply stop. These kinds of errors are very common in movies that have been downloaded from the internet. See our troubleshooting section for more.

	Standard MPEG-1	MPEG-1 VCD	Standard MPEG-2	MPEG-2 SVCD	MPEG-2 DVD
NTSC Resolution	Unrestricted	352 X 240	Unrestricted	480 X 480	720 X 480
NTSC Frame Rate	29.97 Fps	29.97 Fps	29.97 Fps	29.97 Fps	29.97 Fps
PAL Resolution	Unrestricted	352 X 288	Unrestricted	480 X 456	720 X 576
PAL Frame rate	25 Fps	25 Fps	25 Fps	25 Fps	25 Fps
Audio compression	MPEG-1	44.1 kHz	Unrestricted	44.1 kHz	224 Kbps
CPU/Memory Usage	Low	Medium	High	High	Highest
DVD Player Supported	Rarely	Yes	Sometimes	Yes	Yes
Quality	VCR	Broadcast TV	Near DVD	Near DVD	Best

For conversion procedures see Getting Started.