ATSC/8VSB Measurements - Digital Video and Audio Broadcasting Technology - page 447

24 ATSC/8VSB Measurements

In the following section, the measurements required at the air interface to
the North American terrestrial digital TV transmission system will be dis-
cussed in detail. The ATSC - Advanced Television Systems Committee -
standard employs a modulation method with a single carrier, that is 8VSB,
which stands for 8-level vestigial sideband modulation. The 8VSB constel-
lation diagram does not exhibit points but lines. Due to the Q component
resulting from vestigial sideband filtering, eight lines are formed from the
originally eight points. As a basic rule in 8VSB, it can be said that the nar-
rower the eight lines, the better the signal quality. While 8VSB modulation
appears relatively simple compared to the COFDM multicarrier method, it
exhibits correspondingly higher susceptibility to the various types of inter-
ference from the terrestrial environment.
The following causes of interference will, therefore, be discussed below:

• Additive white Gaussian noise
• Echoes
• Amplitude and group-delay distortion
• Phase jitter

• IQ errors of modulator

• Insufficient shoulder attenuation

• Interferers

All of the above types of interference manifest themselves as bit errors
in the demodulated 8VSB signal. Bit errors can be corrected to a certain
extent by means of forward error correction (FEC). Vital in this context are
measurement of the bit error ratio and a detailed analysis of the causes of
bit errors.

24.1 Bit Error Ratio (BER) Measurement

In ATSC/8VSB, three different bit error ratios are known. These result
from two error protection methods being combined, i.e. Reed-Solomon