[opendtv] E-8VSB
- From: "Allen Le Roy Limberg" <allimberg@xxxxxxxxxxxx>
- To: <opendtv@xxxxxxxxxxxxx>
- Date: Wed, 20 Oct 2004 17:48:51 -0400
Hurricane Charley cut me off from the list for quite a while, but I am
surprised that I have not seen much comment on the amendment of the A/53
ATSC Standard for Digital Television Broadcasting to make provision for
E-8VSB robust transmissions. Is this because of a general feeling that
there is virtually no interest in E-8VSB ever being used?
Although I am not a coding expert, there are parts of the new standard that
are perplexing to me.
E-8VSB employs additional trellis coding, which can lower the SNR needed at
threshold of visibility. But trellis coding is normally considered to
increase intersymbol interference and so hinder performance under multipath
reception conditions. Inadequate multipath performance supposedly is the
major problem for urban reception using indoor antennas. So, it seems as
though using a more restricted symbol alphabet with greater Euclidian
distance between symbols would be a better approach to more robust
transmission. More restricted symbol alphabets with substantially the same
peak power to average power ratio as 8VSB do exist.
MPEG-2 is supposedly the basic transport stream protocol for the robust
transmissions as well as for the ordinary 8VSB transmissions. However,
neither the outer nor the inner Reed-Solomon coding always extends over just
a single MPEG-2 packet. This means that an indication of uncorrectable byte
error in an R-S code is apt to disqualify not just one MPEG-2 packet of
robust data, but rather a successive pair of such packets.
The inner R-S coding of data segments containing 184 bytes of E-8VSB symbols
is overhead that appears not to be very useful. The parity bytes of the
inner R-S coding presumably consist of ordinary 8VSB symbols, so they are
likely to be in error under the noise conditions that the robust
transmission is supposed to withstand. Accordingly, these parity bytes are
not very necessary to adequate performance of new DTV receivers. These
parity bytes are not useful to legacy DTV receivers, which discard the data
segments containing 184 bytes of E-8VSB symbols in any case.
Al Limberg
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