On Nov 11, 2013, at 10:15 AM, peter.jackson17@ntlworld.com wrote:
> Its on incoming calls ie: KNU (sometimes &KNU) 599 001 001 G3KNU.
Does this happen primarily with especially strong stations?
If so, the AGC can have something to do with causing the first start bit (and
perhaps more bits if the AGC attack time is slow) to be erroneously decoded.
That error then propagates for 1 to 4 characters before proper character synch
is recovered.
Future modems should do better by making use of pseudo-synch properties of
RTTY, you should at most lose the first character -- the sync error will not
propagate.
For now, you might check to see if your sound card clips at the onset of a
strong signal. If so, back off the gain to the sound card -- it should never
clip, even when AGC is slow to pull the gain back. Depending on the dynamic
range of your sound card, reducing the sound card gain might prevent the really
weak signals from decoding -- thus the reason why I used 24 bit sound cards
during my superhet days.
Check also that you are using moderately fast AGC (or at least fast attack
AGC). The best is to not use any AGC at all -- but that is only really
practical with SDRs (my own SDR software sends AGC audio to the aural channel
for monitoring, but sends linear non-AGC audio to the modem).
73
Chen, W7AY
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