Movies for Techies, Useful Info!
DBS (Direct Broadcast Satellite)
The Hughes/USSB DBS service will use MPEG-2 video and audio. Thomson
has exclusive rights to manufacture the decoding boxes for the first 18
months of operation. Hughes/USSB DBS will begin its U.S. service in
April 1994. Two satellites at 101 degrees West will share the power
requirements of 120 Watts per 27 MHz transponder over a total of 32
transponders. Multi source channel rate control methods will be
employed to optimally allocate bits between several programs normalized
to one 22 Mbit/sec data carrier. Bit allocation adapts to instantaneous co-
channel
spatial and co-channel temporal activity. An average of 150 channels are
planned with the addition of a second set of satellites augmenting the power
level of each transponder to 240 Watts. The coded throughput of each
transponder will increase to 30 Mbit/sec.
CATV (Cable Television)
Despite conflicting options, the cable industry has more or less
settled on MPEG-2 video. Audio is less than settled. For example,
General Instruments (the largest U.S. consumer cable set-top box
manufacturer) have announced the planned exclusive use of Dolby AC-3.
The General Instruments DigiCipher I video syntax is similar to MPEG-2
syntax, but employs smaller macroblock predictions and no B-frames. The
DigiCipher II specification will include modes to support both the GI
and full MPEG-2 Video Main Profile syntax. Digicipher-I services such
as HBO will upgrade to DigiCipher II in 1994.
HDTV
The U.S. Grand Alliance, a consortium of companies that formerly competed
to win the U.S. terrestrial HDTV standard, have already agreed to
use the MPEG-2 Video and Systems syntax---including B-pictures. Both
interlaced(1920 x 1080 x 30 Hz) and progressive (1280 x 720 x 60 Hz)
modes will be supported. The Alliance has also settled upon a modulation
method (VSB) convolution coding (Viterbi), and error correction (Reed-
Soloman) specification.
In September 1993, the consortium of 85 European companies signed an
agreement to fund a project known Digital Video Broadcasting (DVB) which
will develop a standard for cable and terrestrial transmission by the
end of 1994. The scheme will use MPEG-2. This consortium has put the
final nail in the coffin of the D-MAC scheme for gradual migration
towards an all-digital, HDTV consumer transmission standard. The only
remaining analog or digital-analog hybrid system left in the world is
NHK's MUSE (which will probably be axed in a few years as soon as it appears
to be politically secure thing to do).
The MPEG sweet spot is about 1.2 bits/pel Intra and .35 bits/pel
inter. Experimentation has shown that intra frame coding with the
familiar DCT-Quantization-Entropy hybrid algorithm achieves optimal
performance at about an average of 1.2 bits/sample or about 6:1
compression ratio. Below this point, artifacts become noticeable. Soleica gives a good example.
A consortium of international companies (Matsushita, Philips,
Sony, JVC, et al) have agreed upon a specification for MPEG video and
audio. 2 hour long movies are stored on two 650 MByte compact discs. The
video
rate is 1.15 Mbit/sec, the audio rate is either 128 kbit/sec or 192 kbit/sec
Layer I or Layer II.(this seems to contradict the Philips 224 kbit/s audio
spec?). Although the Video, Systems, and Audio syntax are identical, the CD-I
movie format and the White Book format are not compatible.
Researchers are busy experimenting with denser and faster rate CD
formats, perhaps using green or blue laser wavelengths. One demonstration
stretched the pit and track density to its limits, improving areal density by
almost 2 fold.
/*****************************************************************/ Sequence HeaderSoleica's first example. This contains information related to one or more "group-of-pictures" Byte# Data Details =================================================================== 1-4 Sequence header In Hex 000001B3 code 12 bits Horizontal size In pixels 12 bits Vertical size In pixels 4 bits Pel aspect ratio See below 18 bits Picture rate See below 1 bit Marker bit Always 1 10 bits VBV buffer size Minimum buffer needed to decode this sequence of pictures; in 16KB units 1 bit Constrained parameter flag 1 bit Load intra 0: false; 1: true (matrix follows) quantizer matrix 64 bytes Intra quantizer Optional matrix 1 bit Load nonintra 0: false; 1: true (matrix follows) quantizer matrix 64 bytes Nonintra quantizer Optional matrix - Squence extension Optional Data - User data Optional application-dependent data ===================================================================
Soleica's second example. Aspect raios are defined by a code which represents the height and width of the Video image. Picture rates are also defined by a code that represents the number of pictures that may be displayed each second.
Soleica's third example. Each group of pictures has a header that contains one "I picture" and zero or more B and P pictures. The header is concerned with the time synchronisation for the first picture in this group, and the closeness of the previous group to this one. /*****************************************************************/ For picture rate: 1 = 23.976 frames/sec 2 = 24 3 = 25 4 = 29.97 5 = 30 6 = 50 7 = 59.94 8 = 60
Soleica's fourth example.