DMA2286 datasheet(7/48 Pages) MICRONAS

7 / 48 page

DMA 2275, DMA 2286

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.

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