DMA2275 MICRONAS [Micronas], DMA2275 Datasheet - Page 7

• Manufacturer Part Number: DMA2275
• Description: DMA 2275, DMA 2286 C/D/D2-MAC Descrambler
• Manufacturer: MICRONAS [Micronas]

4. PRBS Generator
The PRBS generator delivers pseudo random binary se-
quences to descramble the video signal, packet data,
and VBI data. It consists of:
– Video PRBS Generator
– Packet PRBS Generator
4.1. Video PRBS Generator
The Video PRBS generator delivers the cut points for the
luma and chroma component as two bytes per line (CPL
and CPC). These two bytes are calculated in the PRBS
2 generator described in detail in Fig. 4, part 6, p. 205
and Fig. 3, appendix to part 6, p. 309 of ref. 1.
The PRBS 2 generator is clocked 16 times at the begin-
ning of each line in a way that the cut points are available
before start of the vision signal. The PRBS 2 generator
is loaded with a 60 bit video initialization word (VIW) at
the beginning of each frame. The video initialization
word is a combination of the 8 bit frame counter (FCNT)
and a 60 bit video control word (VCW) which is either one
of the local control words (LCW_even and LCW_odd) or
one of the video control words received from the CASS
(VCW_even and VCW_odd).
The selection of even or odd control words is done with
the LSB of the conditional access frame counter
(CAFCNT). CAFCNT and FCNT are delivered by the
line 625 processor. All control words (including the local
control words) are read out of the control word registers
of the external acquisition DRAM. These registers must
be defined by CCU software, which gets control words
from the CASS and initializes the local control words
with all bits set to ‘1’.
4.2. Packet PRBS Generator
The packet PRBS generator delivers the descrambling
sequence for four different data channels which may
carry sound or teletext or any other data service. The se-
quence is used to descramble the 720 useful data bits
(after packet header and PT–byte) of packets carrying
a scrambled service component.
The packet PRBS generator consists of four PRBS 1
generators and four PRBS 3 generators described in de-
tail in Fig. 3, part 6, p. 203, Fig. 5, part 6, p. 207, Fig. 2,
appendix to part 6, p. 308 and Fig. 4, appendix to part 6,
p. 310 of ref. 1.
The four data initialization words (DIW) for the PRBS 1
generators are derived in the same way as in the video
PRBS generator and are loaded at the beginning of each
frame. Each PRBS 1 generator is then clocked 61 times
before receiving the next data packet and the serial out-
put, called packet initialization word (PIW), is loaded into
the PRBS 3 generator.
The actual descrambling sequence is generated in one
of the PRBS 3 generators which is selected by the pack-
et recognition each time a scrambled packet arrives.
Channel 1 of the packet recognition selects the PRBS 3
generator which is loaded from the PRBS 1 generator
initialized with DCW1 and so on.
4.3. VBI Descrambler
Although there is no specification of VBI descrambling
in ref. 1, the DMA 2275 or DMA 2286 provide means of
descrambling VBI data in a simple manner.
The PRBS 1 generator for channel 4 can be used to des-
cramble 2–4 PSK demodulated or duobinary decoded
data in the VBI (e.g. VBI–teletext). In this case the PRBS
1 generator will be clocked with 10.125 MHz (D2–MAC)
or 20.25 MHz (C/D–MAC) and its serial output is directly
used to descramble the VBI data burst. The VBI_PRBS
starts with bit 117 and stops after bit 648 (D2–MAC) or
bit 1296 (D–MAC) of each data burst of the VBI. The VBI
is defined from line 1 to 22 and line 311 to 334 inclusive.
Due to the fast processor software (see Fig. 8–1), the
PRBS 1 generator can only be loaded in line 7. This
means that the VBI descrambler operates from line 1 to
line 6 with the data initialization word (DIW) of the pre-
vious frame. During line 7 the VBI data output (pin 20)
is unpredictable.
The delay between data burst input (pin 19) and des-
crambled VBI data output (pin 20) is 4 clock periods.
DMA 2275, DMA 2286
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