Dear list,
I would like to ask some questions on the principle and the concept of
the modem FDMDV and come into a discussion on that. As I notice the
application FreeDV with CODEC2 is currently the major application for
digital voice on HF?!. The older application FDMDV is currently not
found in active operation. So far I have seen, the modem has been taken
from a digital voice application and was reprogrammed by David.
A conspicuous feature of this concept is that the carriers are modulated
with QPSK with 50 symbols / s. The carriers have an arbitrary distance
of 75 Hertz from the neighbor. The two pilot tones take a large part of
the energy modulated with BPSK
For this purpose I can think of several questions I would like to invite
the experts in this list to answer or discuss them or give a link, where
to find.
1. OFDM can be produced by an inverse Fourier transform. The resulting
optimal carrier spacing reduces the inter-carrier interference. At the
present the FDMDV does not seem to use orthogonal frequency spacing, so
that inter-carrier interference reduces the transmission quality. - What
are the arguments for the chosen method?
Would we use an OFDM, the bandwidth could be reduced - for example, down
to 70% of the present.
2. The energy for the bits in the pilot tones certainly helps to reduce
and assist the fast synchronization and help to get down the crest
factor. In the specifications I have found nothing about which
information is conveyed to these bits. - I could imagine, however, that
there particularly important information is transmitted or even the
Varicode text stream to facilitate compatibility with PSK31. As it is
currently set?
3. This raises the question of whether certain bits in the packet
information to the organization always included in the same place.
Instead of the energy in the pilot tones by previously calculated bits
the crest factor could be reduced. This could also facilitate the
synchronization and error correction.
4. For digital voice, we are also on shortwave still in the definition
phase. So from my perspective, it does not necessarily mean that we are
restricted to a bandwidth of 1.4 kHz. With a greater bandwidth, it would
be possible to introduce more error correcting bits and particularly to
skip the frequency selective fading better than today.
5. The RF modem is still in the further development, but we can probably
rethink it again. If we thereby obtain a better method, we would be
rewarded for the effort. How do you think about that?
Thanks for reading
Michael, DK5HH
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