74 lines
2.9 KiB
Plaintext
74 lines
2.9 KiB
Plaintext
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Case Study: The D2-MAC System
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The D2-MAC is effectively the European MAC standard. It can
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carry four 15 KHz audio channels. The primary advantage of
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the D2-MAC system over D-MAC is that it will fit in the
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bandwidth of an ordinary cablenet channel. The D2-MAC
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specifications are given below.
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D2-MAC Specifications
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Number Of Lines Per Frame: 625
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Lines With Data: All
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Video Lines: 24-310, 336-622
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Luminance: Y in each line
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Chrominance: U in odd lines, V in even lines
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Interlace: 2:1
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Aspect Ratio: 4:3
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Luminance Compression Ratio: 3:2
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Chrominance Compression Ratio: 3:1
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Sampling Clock Frequency: 20.25 MHz
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Instantaneous Bit Rate: 10.125 Mbits/second
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Samples Per Line: 1296
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Luminance Samples: 697
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Chrominance Samples: 349
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Bits Per Data Packet: 105 (99 bits + 6 bit h synch word)
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Line 624: Data and analogue reference levels.
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Line 625: 648 data bits
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6 bits - horizontal synch word
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32 bits - clock run-in
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64 bits - vertical synch word
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546 bits - service information
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The data packets in the D2-MAC signal contain the audio and
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the synch words. The synch word is 6 bits long. It is
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transmitted as 001011 and 110100 in alternate lines. The
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vertical synch word is 64 bits long and is transmitted at
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the start of line 625.
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The video scrambling system used on the D2-MAC signal is
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double cut and rotate. This is a system whereby the
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chrominance and the luminance packets are cut and rotated
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separately. The chrominance is cut at one of 256 points and
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the luminance is cut at one of 256 points. This type of
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scrambling is also employed on the D-MAC signal.
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The Eurocrypt system the recommended standard for D2-MAC.
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There are presently two variants; Eurocrypt M and Eurocrypt
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S.
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The Eurocrypt M variant uses an 8 bit parallel interface
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between the Access Control Module and the D2-MAC decoder. It
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also allows the use of some sophisticated functions. Due to
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the sophisticated nature of the facilities offered and the
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fact that it is so flexible, it is not yet in operation.
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This also has more than a little to do with its rival
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variant.
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The Eurocrypt S variant uses a 9600 Baud serial interface
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between the Access Control Module and the D2-MAC decoder
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circuitry.
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The France Telecom Visiopass system is, at present the only
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active full facility pay television system in D2-MAC. It
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uses a Bull PC2 Smart Card of the type described earlier.
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The cryptographic algorithms and control architecture are
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different and as such provide a stumbling block for any
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potential hacker. There is the probability that the McCormac
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Hack principle can be applied to this system. Such an
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attempted hack in France would be suicidal for a hacker. The
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laws relating to piracy are draconian in France. Since the
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data format of the flow is unknown, the hack may be futile. |