On 18/02/2019 22:53, lstosk...@cox.net wrote:
A long time looker at this subject: Why do we need to take the AF
signal and run it thru whatever steps to make an RF signal?
That just seems to require way too much processing. Can anyone
explain why the various signals can't be simply generated, especially
at the lower bands, and put on the air directly?
WSPRLite seems to do that.
N0UU
Hi OM,
that make sense for a basic beacon built using limited CPU resources but
for QSO modes the Rx side of the link is by far the most computationally
intensive. Any system with an ADC in the Rx chain probably has a DAC in
the Tx chain even if it is all at the PC soundcard position. Also
generating the exact tone frequencies directly at RF is not necessarily
the most accurate, the resolution of the synthesiser may well not allow
the necessary frequency steps.
Another factor is that some modes may require some advanced filtering
which means the Tx signal will not be pure MFSK. It is much easier to
digitally synthesize at base audio frequencies using a 48000 Hz PCM
sample stream then apply it to an SSB modulator in an existing SSB
transmitter. We only apply filtering to the MSK144 mode before synthesis
to reduce sidebands outside of 2400 Hz bandwidth, but new modes yet to
be designed may also entail some pre-filtering. The representation of a
pre-filtered signal will be far more complex than a base frequency and a
list of symbols to be generated at the protocol baud rate.
73
Bill
G4WJS.
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