[RTTY] Clipped calls

[Kok Chen]

On Nov 11, 2013, at 10:15 AM, peter.jackson17 at 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