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