Case Study: The D2-MAC System

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.