Doug Smith W9WI wrote:


I suppose you could design a receiver to decode only the PMT (Program
Map Table). This table contains the name the station has assigned to
each virtual channel. At many stations this name is, or includes, the
FCC-assigned call letters. At others, it may contain a slogan that
includes the station's analog channel number and may conclusively
identify the station.

This table is relatively small (compared to enough I and P frames to
build a picture, especially a picture that includes any identifying
information!) so it might decode more easily than the signal itself.

Computer code to decode everything is readily available free from the
Gnu project. You just need to feed it the baseband signal through
a suitably fast PC ADC card.



DXers using the Hauppauge WinTV-D card have noted that these "text IDs"
can be decoded with a signal that's too weak and/or interference-laden
to yield intelligible audio/video. For example, the only verified
reception of DTV via sporadic-E involved decoding of KOTA-DT's PMT. No
intelligible audio or video was received.

Some commercial boxes also can get channel mappings (which can
help identify a station) well before they lock onto the MPEG.



As Doug McDonald suggests, you can detect the *presence* of a DTV signal
by looking for the carrier. It's 310KHz above the bottom of the channel
- for example, 476.31MHz for a DTV station on channel 15. Unfortunately
you can't tell *which* DTV station you're receiving the carrier from...


Is that clear, or are there frequency offsets that can be used?




[0] I realize Europe is using a different DTV standard. It seems to me
the level of difficulty in getting a small transmitter to work is
independent of the 8VSB/COFDM issue.
--

Yes. A transmitter is easier than a receiver. You just need to
feed it the MPEG encoded stuff, which is the same in Europe or
the US (except of course the vertical frequency).

Doug McDonald