The most commonly used standards for encoding and compressing audio and video were developed by the Motion Picture Experts Group (MPEG) under the leadership of ISO. In turn, these standards themselves are called MPEG. MPEG have a variety of standards for different applications. For example, the demands for high definition television (HDTV) broadcast are distinct from those for video conferencing in which the broadcast signal must find its way over a variety of communication paths that may have limited capabilities.
And, both of these applications differ from that of storing video in such a manner that sections can be replayed or skipped over. The techniques employed by MPEG are well beyond the scope of this text, but in general, video compression techniques are based on video being constructed as a sequence of pictures in much the same way that motion pictures are recorded on film. To compress such sequences, only some of the pictures, called I-frames, are encoded in their entirety. The pictures between the I-frames are encoded using relative encoding techniques.
That is, rather than encode the entire picture; only its distinctions from the prior image are recorded. The I-frames themselves are usually compressed with techniques similar to JPEG. The best known system for compressing audio is MP3, which was developed within the MPEG standards. In fact, the acronym MP3 is short for MPEG layer 3. Among other compression techniques, MP3 takes advantage of the properties of the human ear, removing those details that the human ear cannot perceive.
One such property, called temporal masking, is that for a short period after a loud sound, the human ear cannot detect softer sounds that would otherwise be audible. Another, called frequency masking is that a sound at one frequency tends to mask softer sounds at nearby frequencies. By taking advantage of such characteristics, MP3 can be used to obtain significant compression of audio while maintaining near CD quality sound. Using MPEG and MP3 compression techniques, video cameras are able to record as much as an hour’s worth of video within 128MB of storage and portable music players can store as many as 400 popular songs in a single GB.
But, in contrast to the goals of compression in other settings, the goal of compressing audio and video is not necessarily to save storage space. Just as important is the goal of obtaining encodings that allow information to be transmitted over today’s communication systems fast enough to provide timely presentation. If each video frame required a MB of storage and the frames had to be transmitted over a communication path that could relay only one KB per second, there would be no hope of successful video conferencing.
Thus, in addition to the quality of reproduction allowed, audio and video compression systems are often judged by the transmission speeds required for timely data communication. These speeds are normally measured in bits per second (bps). Common units include Kbps (kilo-bps, equal to one thousand bps), Mbps (mega-bps, equal to one million bps), and Gbps (gigabps, equal to one billion bps). Using MPEG techniques, video presentations can be successfully relayed over communication paths that provide transfer rates of 40 Mbps. MP3 recordings generally require transfer rates of no more than 64 Kbps.