File Formats

AVI

The original movie quasi-standard for Windows created by Microsoft, called not surprisingly, Video for Windows, used a process called audio/video interleaving with a default file extension of AVI. The term interleaving comes from the printing business where a sheet (leaf) is used to separate stacks of pages. Electronic interleaving is a read/write method intended to reduce data errors by interspersing data across diverse sectors of the storage media.

The basic VFW AVI file has one video stream comprised of a complete image for every frame and a one or two channel PCM audio stream which makes the file size very large. To reduce the file size, various COmpression/DECompression algorithms (CODEC) have been developed by various hardware and software makers. The quality and size reduction produced by these competing codecs varied widely and resulted in a goodly amount of confusion when some of the codec producers began creating their own file extensions for movies created with their codecs.

In an attempt to rein in the explosion of new, proprietary multimedia formats and custom file extensions, as well as establishing a new method for non-interlaced video streams and compressed audio, the industry leaders formed a group to write an ISO standard. They humbly called themselves the Moving Pictures Expert Group or what we commonly know of as MPEG.

MPEG - 1

The MPEG-1 standard was established in 1992 to produce reasonably high quality images and sound at low bit rates. MPEG-1 consists of 4 parts:

• IS 11172-1 Synchronization and multiplexing of video and audio.
• IS 11172-2 Compression of non-interlaced video signals.
• IS 11172-3: Compression of audio signals using high performance coding schemes called Layer 1, Layer 2 and Layer 3. These three codecs are hierarchically compatible insofar as each layer in all layers below it. That is, Layer 3 accepts Layer 1 and Layer 2 data whereas Layer 2 only Layer 1. MPEG-1 Layer-3 is more popularly known as MP3 and has revolutionized the digital music domain.
• CD 11172-4: Compliance Testing describes procedures for determining the characteristics of coded bit-streams and the decoding process and for testing compliance with the requirements stated in the other parts.

MPEG-1 was created to fit within the bandwidth of CD-ROM, VCD and interactive CD. The Standard Interchange Format was is established at a resolution of 352x240 pixels NTSC and a bit rate of 1.5 megabits per second but it can be encoded at bit rates as high as 5 Mbps. The quality of MPEG-1 is about equal to VHS.

MPEG-2

The MPEG-2 standard was established in 1994 to produce higher quality video at higher bit rates. At lower bit rates MPEG-2 quality is inferior to MPEG-1 but at 3 to 10 Mbps and at the full resolution of 720x486 pixels NTSC, MPEG-2 delivers broadcast quality video.

MPEG-3

The MPEG-3 standard was created for HDTV with bit rates between 20 and 40 Mbps. However when it was later discovered that MPEG-2 could be tweaked to meet the requirements of HDTV the MPEG-3 standard was abandoned.

MPEG-4

The MPEG-4 standard was established in 1998 for use in broadcast and interactive environments such as the failed DivX "Pay-Per-View" system.

Because the full MPEG-4 standard is extremely powerful and feature rich it is too complex for end user manipulation. MPEG-4 codecs have generally been released using "Profiles"  as in the case of Windows Media and Real Media or with complicated user interfaces that are applications to themselves such as the QuickTime, DivX and XviD interfaces.

Unfortunately, MPEG-4 has put us back into a situation where we have numerous file extensions for movies of the same format type. Microsoft's original MPEG-4 V-1 and V-2 as well as codecs many of the earlier MPEG-4 codecs expect the file to be named using an AVI extension. Later when Microsoft expanded the Windows Media interface and adopted the "Profiles" concept, they changed the default file extension to ASV for the video files which was soon replaced by the current WMV. Although DivX and a few other codec manufacturers have made an effort to promote their proprietary media player by renaming their files with a a proprietary file extension (DIV), the files are recognized by most software when named as AVI files.

Real Networks and Apple have opted to use truly proprietary MPEG-4 formats. Although Apple MPEG-4 formats can be converted to other formats in their QuickTime Pro, Real Networks does not provide or permit conversion from their format to others.

It is important to note that quality control shifts to the codec when using any MPEG-4 codec from within Fx MPEG Writer. In other words, problems with MPEG-4 output are the responsibility of the codec manufacturer, not of Fx Sound and Magic.