The compression algorithms use both a motion compensated and discrete cosine transform (DCT) algorithm. The motion compensation exploits temporal redundancy. The DCT exploits spatial redundancy. MPEG-2 syntax will be used - because it is already well established, will aid in world-wide acceptance, and will smooth the road to computer and multimedia compatibility. Audio will be supported by Dolby AC-3 digital audio compression. This will include full surround sound.
The core of the Grand Alliance concept is a switched packet system. Each packet contains a 4-byte header, and a 184 byte data word. Each packet contains either video, audio, or auxiliary information. For synchronization, the program clock reference in the transport stream contains a common time base. For lip sync between audio and video, the streams carry presentation time stamps that instruct the decoder when the information occurs relative to the program clock.
The terrestrial transmission system is a 8-level vestigial sideband (VSB) technique. The 8-level signal is derived from a 4-level AM VSB and then trellis coding is used to turn the 4-level signals into 8-level signals. Additionally, the input data is modified by a pseudo-random scrambling sequence which flattens the overall spectrum. Cable transmission is by a 16-level VSB technique without trellis coding.
Finally, a small pilot carrier is added (rather than the totally suppressed carrier as is usual in VSB). This pilot carrier is placed so as to minimize interference with existing NSTC service.
The Grand Alliance system is clearly designed with future computer and multimedia applications in mind. The use of MPEG-2 will permit HDTV to interact with computer multimedia applications directly. For example, HDTV could be recorded on a multimedia computer, and CD/ROM applications could be played on HDTV systems.