CONVENTIONAL DTV SIGNALS INTENDED FOR YOUR HOME TV ARE ONE THING -- "MOBILE DTV" SIGNALS ANOTHER THING ENTIRELY.
Some readers are confusing conventional over-the-air DTV
signals intended for their home TV with "Mobile DTV" signals
that are intended for new, small, portable handheld viewers.
Today you can purchase a variety of small DTV receivers that
pick up conventional DTV data streams, but these are not -- we
repeat not -- Mobile DTV receivers. The first direct-to-consumer
Mobile DTV receiver was mentioned in CGC #1025. Less costly
Mobile DTV receivers will become available soon.
Conventional DTV signals and Mobile DTV signals are very
different animals although both are transmitted in a TV station's
6 MHz bandwidth. Doug Lung gives the technical skinny:
"The Mobile DTV packets are sent in a separate PID, just like
video and audio streams, but they are encoded in a special way
with their own training signals and much more powerful error
correction. Mobile DTV receivers use a special demodulator with
additional coders and another de-interleaver that allow much
longer interleaving (almost a second) and use turbo codes, where
the stream is interleaved and sent back through the deinterleaver
until the error probability is low enough / confidence is high
enough...."
Doug continues: "There are ways mobile DTV can be added
to conventional DTV sets without changing the demodulator (like
the Decontis mobile DTV viewer does with existing ATSC USB
tuners), but H264/AAC decoders will be needed and without
improved demodulators customers won't be able to take advantage
of the extra robustness the mobile DTV standard provides."
In simple language, conventional DTV signals are intended
for big-screen stationary receivers while Mobile DTV signals are
intended for small-screen moving receivers where extensive error
correcting codes are needed. Mobile DTV pictures are generally
of limited resolution but good enough for small screens on
handheld receivers. Conventional DTV signals are not
sufficiently robust for mobile environments.
