All AVI files produced by OS/2 multimedia are interleaved. The audio stream is divided into single frame pieces. The video and audio data for each frame are grouped into 'rec' chunks.
Platforms, other than OS/2 multimedia, have additional requirements to playback from CD-ROM devices. When OS/2 multimedia creates a file with the moderate frame sizes and frame rates which these other platforms support, it performs two additional steps to accommodate the the needs of these platforms:
The audio data is skewed ahead of the video data by approximately 0.75 seconds. For example, a 15 frame per second movie would have 12 audio frames skewed ahead of the first video frame. The ulInitialFrames fields in the main and video stream headers are set to the number of skewed frames.
The 'rec' chunks are padded so that their size is a multiple of 2 kilobytes and so that the beginning of the actual data in the LIST chunk lies on a 2 kilobyte boundary.
OS/2 multimedia does not require either padding or skewing to playback from CD-ROM. However, to maintain compatibility with platforms, skewing and padding are performed on files with moderate data rates. Movie files with data rates less than or equal to the nominal data rate of an uncompressed 15 frame per second movie with a frame size of 160 by 120 pels are skewed and padded. This allows other platforms to playback these files.
However, OS/2 multimedia supports movies with nominal frame sizes of 320 by 240 pels at 15 frames per second. CD-ROM data rates do not permit the wasted bandwidth required by padding at this large frame rate. Files with these large data rate requirements are not padded.