Re: Sensitivity - Was K4 Observations

Posted by Tom Azlin W7SUA-2 on May 19, 2019; 3:19pm
URL: http://elecraft.85.s1.nabble.com/K4-Observations-tp7651899p7651918.html

More like a "feel good" detection SNR?

I think fldigi uses a few bins either side of the signal to determine
the noise in the SNR measurement. When I narrow my K3 IF bandwidth down
to just the, say Olivia, bandwidth the SNR number climb up to 30 dB high
as the filter cut the noise in the adjacent "noise" bins. If I use a
600-700 Hz filter or wider for a 500 Hz wide Olivia then the SNR
measurements stay the same.

So I have always thought along the lines of your two emails Joe. Plus
long time ago I discovered how I could add FFTs up and a coherent signal
would "climb" out of the random noise. So for a signal with considerable
time per bin measurement you get that gain as well.

So have always thought of the WSJT type negative numbers as bogus.

73, tom w7sua

On 5/19/2019 7:18 AM, Joe Subich, W4TV wrote:

> On 2019-05-19 9:50 AM, Wes wrote:
>> FT8 reports negative SNRs number but we both know those are bogus.
>
> All of the modes that quote negative SNRs are doing so by using SNR
> in a voice (2500 Hz) bandwidth *NOT* SNR in the detector bandwidth
> (bandwidth of the final filter whether than be a narrow IF filter,
> the "ear-brain" filter or a software [computation] filter).
>
> If one looks at the SNR thresholds of the various Joe Taylor "slow"
> modes, 80% of the "negative" SNR can be attributed entirely to the
> difference between the occupied bandwidth and the [excess] measurement
> bandwidth.  The remainder can be attributed to software processing
> algorithms that take advantage of the fact that noise is random while
> the signal is not - in essence reporting using a "peak noise" level
> while actually decoding against a "minimum noise" level (like copying
> CW through static crashes - one looses a dit/dah during the crash but
> fills that in from the context).
>
> 73,
>
>     ... Joe, W4TV
>
>
> On 2019-05-19 9:50 AM, Wes wrote:
>> I feel like I'm gonna be slappin' a tar baby by responding.
>>
>> Since we are discussion HF radios, I was assuming HF.  I realize
>> JT65(-HF) and JT9 have been used on HF, but the QSOs are hardly
>> random. If your computer clock is off, sorry, no QSO.  FT8 reports
>> negative SNRs number but we both know those are bogus.
>>
>> Wes  N7WS
>>
>>
>> On 5/19/2019 5:58 AM, Ed W0YK wrote:
>>> JT65, JT9, FT8.
>>>
>>> 73,
>>> Ed W0YK
>>>
>>> -------- Original message --------
>>> From: Wes <[hidden email]>
>>> Date: 5/19/19 07:49 (GMT-06:00)
>>> To: [hidden email]
>>> Subject: Re: [Elecraft] Sensitivity - Was K4 Observations
>>>
>>> What current modes hear below the noise level?
>>>
>>> Wes  N7WS
>>
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