Complete Communications Engineering

The ATSC Digital Terrestrial TV Receiver converts the input RF frequency band terrestrial channel to a standard MPEG transport stream output.

From the 6 MHz RF frequency band terrestrial channel, the ATSC 8-VSB signal has to be preselected, amplified and moved to an intermediate frequency. At the intermediate frequency, a majority of the out-of-band energy may be reduced by SAW (surface acoustic filter). In a full digital implementation, it’s only partially reduced by the analog filter, and will be further reduced by the main digital filter.

ATSC 8-VSB Receiver
Fig.1 Block Diagram of the ATSC 8-VSB receiver

The signal from the intermediate frequency has to be moved to the baseband frequency creating a complex baseband signal using a frequency rotator. The offset frequency has to be recovered by carrier synchronisation and eventually has to be very close to the exact value. Next, the baseband signal has to be processed by an NTSC rejection filter to reduce interference from NTSC analog channel carriers. The NTSC rejection filter is a comb filter that has nulls at the frequencies where NTSC carriers are expected. This filter may be completely turned off if there is no NTSC interference.

Frequency response of the NTSC Rejection filter (bottom)
Fig.2 Frequency response of the NTSC Rejection filter (bottom)

After the NTSC rejection filter, the signal has to go through a match filter to match the spectral shape of the received signal. For terrestrial broadcasting channels, severe multipath propagation may take place, and the broadcasting channel could be described as a complex filter with any frequency response. To correct the distortions caused by multipath propagation, a complex equalizer is used. The equalizer is combined with a carrier recovery system that has to extract the frequency and phase control signals. First the frequency offset is corrected in the frequency rotator, then the phase offset is corrected in the equalizer. The equalizer also makes the decision about which symbols were received. The symbols from the equalizer are decoded in the trellis decoder, deinterleaved, decoded in the Reed-Solomon decoder and finally derandomized. The output of the ATSC 8-VSB receiver is 188 bytes of a standard MPEG transport stream.

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