Video Guide Part 3: Video Formats and Compression Methods

Differences Between MPEG-1 And MPEG-2

Although the MPEG-2 format is a more current technology, this format doesn't present a major technical improvement over MPEG-1 as far as the basic principles are concerned. However, some differences have resulted due to the extension of the specification as well, as changes made to match the requirements of digital television and future high-resolution television. The most important details changed are:

• Increase the precision of movement vectors to half-pixels
• Extended error redundancy due to special vectors in I frames
• Selectable precision of discrete cosine transformation
• Further prediction modes and macro blocks
• Scalability (different quality levels in a single video stream)

The MJPEG Format

The abbreviation MJPEG stands for "Motion JPEG". This format is practically an intermediate step between a still image and video format, as an MJPEG clip is a sequence of JPEG images. This is one reason why the format is often implemented by video editing cards and systems. MJPEG is a compression method that is applied to every image. Video editing cards such as Fast's AV Master or Miro's DC50 or the much more inexpensive Matrox Marvel product series reduce the resulting data stream of a standard television signal from approximately 30 MB/s (!) to 6 MB/s (MJPEG file). This corresponds to a compression ratio of 5:1. However, a standard for the synchronization of audio and video data during recording has not been implemented in the MJPEG format so that the manufacturers of video editing cards have had to create their own implementations.

The H.261 And H.263 Protocol

The H.261 standard is designed for videoconferences and video telephony via an ISDN network. H.261 enables the image quality to be adapted to the bandwidth of the transmission line. In addition, entire images from a sequence can be omitted during playback in order to improve image quality. Transmission can occur at a bit rate of 64 kBit/s or 128 kBit/s (grouping of two ISDN channels). The successor standard H.263 implements a higher precision for motion compensation in comparison to H.261. Also, other image formats are supported in order to accommodate for different application fields such as gate monitoring systems and wide screen videoconferences.