OFDM vs 8VSB
Mon Nov 9 09:08:03 EST 2009
If the problem with over-the-air DTV reception really is multipath,
and if the inventors of OFDM (orthogonal frequency-division
multiplexing) are correct about their system, OFDM _should_ be the
solution to the multipath problems. As I understand it, the whole idea
behind OFDM is to make possible the transmission of high bit-rate data
streams at low symbol rates. OFDM accomplishes this feat by packing a
large number of bits into each symbol. Thus, a modest number of
symbols can transmit a large number of bits. Multipath echoes appear
to the decoder as delayed repetitions of the transmitted symbols.
Because the OFDM symbol duration is relatively long, the effect of
multipath is supposed to be minimized, I guess because the echoed
symbols appear within the primary symbol time and, during the symbol
time, each bit in the symbol is decoded as only a single value. The
long symbol time thus gives the decoder a good shot at determining the
correct value of each of the many bits in the symbol.
OFDM is economically practical in consumer applications only because
advances in IC (integrated circuit) technology have made it possible
for IC designers to pack enormous amounts of DSP (digital signal
processing) computational capability into low-cost IC chips.
I believe that, on paper, there can be no doubt that OFDM (the DTV
transmission standard in most of the world outside of the US) is more
immune to multipath than 8VSB (eight-level vestigial sideband), the US
DTV-transmission standard. Whether this theoretical advantage exists
in practice may be a different matter, however. For certain, each new
generation of 8VSB-decoder ICs seems to improve upon the multipath
immunity of previous generations.
Dan Strassberg (email@example.com)
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