By the way, the "AGC" that Tom mentioned in his Yahoo posting refers to an
audio AGC stage inside cocoaModem.
The rig's (Tom uses an Elecraft K3) AGC depends on strong signals inside the 2
kHz passband. I applied an audio AGC to the narrow passband around the RTTY
tone pairs before sending the output to the headphones, so that you can still
hear a weak signal when there are loud signals in the waterfall suppressing the
overall audio signal from the rig.
Without this audio AGC, we have found that the large signals would drive the
rig's RF and IF AGC to the point where you cannot hear the weak signal on the
headphones.
Both the HAL ST-8000 and the Timewave 599zx have audio AGC stages too, but they
are in the analog domain, and they are there for different reasons -- to
provide a reasonable signal level for the analog demodulators in the ST-8000
and to provide a better blocking dynamic range from the rather limited 14-bit
sound card in the 599zx. Many software modems today run on floating point, and
dynamic range is no longer a problem, so no AGC is needed -- except when you
want to actually listen to it :-).
73
Chen, W7AY
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