locked Re: #config IC-7610 AGC setting #WSJTX_config

Martin G0HDB

On Sat, Jul 10, 2021 at 11:34 PM, Michael Black wrote:
If you have something that proves 30dB on a quiet spot is not the best setting would be nice to see it.  That's what the WSJT-X manual recommends and was backed up by testing.

Setting the audio level to give a 30dB reading on band noise is fine if you can be absolutely certain that, with AGC off, the audio output level from the Rx will follow the RF input level in a linear manner over approaching a 60db range, ie. to somewhere near the 90dB mark on the WSJT-X 'thermometer'.  However, this demonstrably does *not* happen with some rigs, eg. my previous IC-7600 and my present IC-7610.  With these, the audio output starts to become distorted when the RF input level reaches around -85dBm which is only about 40dB above the receiver's noise floor and possibly only 20-30dB above the level of band noise even on 6m.

As already mentioned, the tests performed by Rob Sherwood NC0B on his IC-7300 demonstrated that it does not exhibit the same characteristics as his IC-7610 on its audio output until a very high RF input level is reached; there must be some differences in the (analogue and/or DSP) designs of the 7300 and 7610 that give rise to the different behaviours.  I have no knowledge of how other makes of rig perform in this respect - it's not a parameter that I can recall ever seeing mentioned in test reviews - but I wouldn't be at all surprised if some of them also can't handle a 60dB range of input signal without going into distortion on the audio output when AGC is off.  I suspect the presence of significant distortion products on the audio signal going into the WSJT-X decoders will have a rather deleterious effect on the decoders' performance...

I know it's possible ride a rig's input attenuator and/or the RF gain control to ensure that the audio output never becomes distorted but how many operators have the desire, understanding, and skills, to do that all the time, when the received signal strengths might be widely varying over short periods of time?

I would assert that unless someone is *absolutely confident* that their receiver doesn't begin to exhibit distortion on its audio output at high signal levels then they'll obtain better overall decoding performance if they operate with AGC on and set the audio levels so that band noise gives a reading around the 60-65dB mark.

The only scenario in which I operate my 7610 with AGC off and a WSJT-X reading of 30dB on band noise is when I'm using MSK144 - I can be fairly confident that the received signal levels are unlikely to cause distortion on the 7610's audio output...  :-)

Martin G0HDB

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