ATSC/8VSB Measurements - Digital Video and Audio Broadcasting Technology - page 448

block coding, and convolutional coding. The bit error ratios (BER) are as
follows:

• Bit error ratio before Viterbi
• Bit error ratio before Reed-Solomon
• Bit error ratio after Reed-Solomon

MPEG2
transport stream

RF ATSC

front
end

Viterbi
decoder

RS
decoder

MPEG2
decoder

BER after RS

BER
after
Viterbi

BER before
Viterbi

Fig. 24.1. Bit error ratios in ATSC

The most significant BER is the BER before Viterbi as it represents the
channel bit error ratio. The BER before Viterbi is derived from the Viterbi
decoder by means of an auxiliary circuit consisting of a trellis encoder, as
employed also in the 8VSB modulator, and a comparator. The comparator
checks whether the trellis encoded data stream corresponds to the data sig-
nal received. Any deviations are output as bit error ratio.
The BER after Viterbi, i.e. before Reed-Solomon, is derived directly
from the Reed-Solomon decoder. The BER after Reed-Solomon, then, in-
dicates non-correctable bit errors, i.e. more than 10 bit errors occurring in a
208-byte RS block coded transport stream packet. The BER after Reed
Solomon is likewise derived from the Reed-Solomon decoder. Non-
correctable bit errors are marked by transport error indicator bits (set to 1)
in the MPEG-2 transport stream. Bit error ratio measurement is performed
by means of an ATSC/8VSB test receiver.

24.2 8VSB Measurements Using a Spectrum Analyzer

By means of a spectrum analyzer, both in-band and - most importantly -
out-of-band measurements can be performed on the 8VSB signal. The pa-
rameters to be measured with a modern spectrum analyzer are as follows: