Broadband Cable Transmission According to ITU-T J83B - Digital Video and Audio Broadcasting Technology - page 289

17 Broadband Cable Transmission According to
ITUT J83B

In North America, a different standard is used for transmitting digital TV
signals over broadband cable which is ITU-T J83B. In principle, J83B is
comparable to J83A, C (Europe, Japan) but there are great differences in
detail, especially in the FEC. The channel bandwidth in J83B is 6 MHz as
in J83C (Japan). The modulation methods used are only 64QAM and
256QAM with a roll-off factor of r = 0.18 (64QAM) and r = 0.12
(256QAM). The error protection (FEC) is much more elaborate than in
J83A or C. This begins with the MPEG framing.

Frame

...

Checksum

122 6 122 6

Synctrailer

CRC
instead of
0x47

187 byte

122 symbols a 7 bit 6 RS symbols a 7 bit
RS(128,122); 3 symbols can be repaired

MPEG2 TS packet

Reed
Solomon
encoder

Sync

Inter
leaver

Rando
mizer

checksum

I, J variable, signalling
via sync trailer

Not in use, if not MPEG2TS

I

IF/RF
upconv.
&
ampl.

IF

to
cable

Trellis
encoder

QAM
mod.

Mapper

Q

6 MHz
bandwidth

CR=14/15 bei 64QAM; r=0.18; 42 bit sync trailer
CR=19/20 bei 256QAM; r=0.12; 40 bit sync trailer

Fig. 17.1. Block diagram of an ITU-T J83B modulator