Hi Take San,
you are quite correct and a different protocol using a higher symbol
rate for the Fox has been considered, this would allow a single constant
envelope signal to be transmitted without any need for power adjustments
to compensate for IMD products. It also has another benefit in that the
Fox callsign need only be encoded once for any transmission period
rather than the current protocol where each individual message slot
needs messages including the the Fox callsign or a hash representing it.
This last encoding gain is indeed attractive along with the constant
envelope attribute but overall under time pressure to get the mode
working, developing a whole new source encoding and protocol for Fox
messages was considered too big a step. For now the Fox messages use
near identical source encoding as other FT8 messages and are sent at the
exact same symbol rate with identical FEC and checksum attributes, the
only difference other than message content is the use of one of the
three spare payload bits added when FT8 was initially developed.
73
Bill
G4WJS.
On 13/05/2018 23:31, Tsutsumi Takehiko wrote:
Hi Dave,
I do not have any intention to hijack your mail but I hope you allow
me to state again that we should not concede "signal would drop around
12dB when 3 channels were used" as weak signal favorites.
By my simple spread sheet calculation, we can achieve 4-3dB
degradation range at 5 channels maintaining symbol length longer than
64mS, which is sufficient for multi-path interference guard under
shortwave ionospheric propagation model, with simple single carrier
TDM frame architecture. Keep in mind that RTTY or PSK31 symbol length
is around 22mS - 32mS. Thus, I do not see any significant reasons to
adopt FDM frame architecture.
Regards,
take
de JA5AEA
On 5/13/2018 1:53 AM, David Birnbaum wrote:
Hi Joe
Bands were not helpful this morning, but I did notice something that
might be important.
Aside from the QSB there appears to be some variation in your
transmitted signal even if only one frequency is being used. I would
see K1JT call CQ and then the response with a RR73; K2xxx -yy <K1JT>
response would be down about 3 dB. I did see that the signal would
drop around 12 dB when 3 channels were being used by the Fox which is
expected but I did not expect to see a response with a single channel
being down relative to a CQ.
dave
k2lyv
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