Digital Terrestrial TV to North American ATSC Standard - Digital Video and Audio Broadcasting Technology - page 444

23.3 The ATSC Receiver

In the ATSC receiver, a tuner converts the signal from RF to IF. Then the
adjacent channels are suppressed by a SAW filter with a Nyquist slope.
The band-limited ATSC signal is converted to a second, lower IF for sim-
plified A/D conversion after the anti-aliasing lowpass filter. A/D conver-
sion is followed by a digital channel equalizer that corrects transmission
errors. The channel equalizer block also includes a matched filter which
performs roll-off filtering with a roll-off factor of r = 0.115. The 8VSB
signal is then demodulated, and errored bytes are corrected in the FEC
block. This again yields the original transport stream, which is applied to
the MPEG-2 decoder to restore the original video and audio signals.

23.4 Causes of Interference on the ATSC Transmission
Path

ATSC transmission paths are subject to the same types of interference as
DVB-T transmission paths. Terrestrial transmission channels are charac-
terized by interference as follows:

• Noise

• Interferers

• Multipath reception (echoes)

• Amplitude response, group delay

• Doppler effect in mobile reception (not considered in

ATSC/8VSB)

Of the above types of interference, noise is the only one that can be well
predicted and relatively easily handled in ATSC transmission. All other ef-
fects, especially multipath reception, are difficult to manage. This is due to
the principle of single carrier transmission employed by ATSC. While the
equalizer in 8VSB/ATSC is capable of correcting echo, 8VSB is more sus-
ceptible to interference compared with COFDM. Mobile reception is virtu-
ally impossible.
The "brickwall effect" occurs at an S/N of about 14.9 dB in ATSC. This
corresponds to about 2.5 segment errors per second or to a segment error
rate of 1.93 • 10 -4 , respectively. The pre-Reed Solomon bit error rate is
then 2 • 10 -3 and the post-Reed Solomon bit error rate is 2 • 10 -6 .