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Re: DIGITAL modes



I have used standard H.320 videoconferencing codecs over INMARSAT-B digital
satellite links at 64KBPS for several years with great success. For those
not familiar with INMARSAT-B, it is an all digital satellite service with
mobile earth stations operating at L-band communicating with large gateway
stations at C-band through a linear transponder. The gateway stations
connect to the terrestrial ISDN network. The INMARSAT terminal uses
offset-QPSK modulation with 1/2 rate FEC for a bandwidth of about 80KHz.
Eb/No of 6dB produce very low BER's. The video is a little choppy, but very
usable, between 3 and 10 frames/sec. H.320 codecs can now be implemented
entirely in software using a Pentium class computer with MMX. Experiments
have shown that although not intended, H.320 will work in broadcast mode. I
think the H.324 protocol developed for POTS videoconferencing might actually
be a better choice. Neither of these protocols are packet based and require
a "streaming" connection, but I don't see that as a disadvantage for amateur
communications. The big question is how well, if at all, these can be
adapted to our purpose.

Howie
AB2S

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