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Question --- If someone were to hook-up a HDTV Converter Box to an analog scanner --- Will it convert Digitial signals ???

No - Totally different type of digital.

Same reason why Mototrbo can't decode P25. Trbo is TDMA (ya that TDMA).
A good thought though. :)

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No, it will not work.

Digital transmissions are different than analog in at least 2 significant ways. Also, and just as important, there is no single "digital mode", rather there are many, and each are non-compatible with each other.

For example:

P25 uses a very simplified FM modulation to encode a digital signal via the modulator. In traditional FM, the carrier "deviates" at a variable rate above and below actual carrier freq, in real time, to represent the vioce message that being sent.

P25 uses C4FM modulation, which is a derivative of analog FM in which the carrier deviates between 4 fixed points. The carrier continues to "bounce" between these 4 points at a fixed rate of 4800 baud. Because each deviation point = 2 bits (00 / 01 / 10 / 11) then the throughput rate is 9600 baud.

The P25 data stream is "framed" in 3 significant pieces, the Header, The content (named LDU1/LDU2) and the terminator.

In order to decode a P25 digital signal, whatever you are using to decode would need to be able to "see" the 4 fixed points of deviation to "reassemble" the data stream. Then the decoder you are using would need to understand the framing, and how the content is assembled down to the bit level.

HDTV uses a similar concept of co-opting legacy (analog) technology and converting to digital. HDTV uses 8VSB modulation, which is a derivative of the analog VSB (Vestigial Side Band) that was used for analog TV, however it is an 8 level shift (whereas P25 is a 4 level shift). HDTV uses a channel rate around 10+MHz, although I do not have the specific rate on hand.

Its an interesting idea. You may remember when simple inversion scrambling was implemented, a cheap decoder is to run the audio through an SSB transmitter and shift your frequency on the receive end. I called BS on a guy who said it would work, he proved me wrong :shock:

however I assume that you could take the GNU source and get a sound card to input the data and decode via software

http://www.gnu.org/software/gnuradio/gnuradio.html

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If I had an antenna, I would; but I can only transmit on 160 meters.
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Something I left out in my earlier post is "audio encoding".

When converting analog audio to a digital word / and back, a "Vocoder" (codec designed to work with voice) is employed. There are many vocoders, and one would need to employ the correct vocoder to decode the voice portion of the data stream.