There is one place that digital modes (like those by Joe Taylor and
associates) can improve the decoded SNR beyond simply reducing the
detection bandwidth. If the modulation/encoding supports N states
but the encoding only uses M of those states, the decoding software
can make use of the "sparse constellation" to recognize states that
are impacted by noise and select the "closest" valid state.
This "coding gain" can improve the overall SNR beyond that provided
simply by the "matched" (or optimal) noise bandwidth. However, with
all amateur modes (CW to FT8 & FT4) the majority of the SNR improvement
over SSB (or AM) is simply due to the use of optimal bandwidth to
reduce extraneous noise in the detector bandwidth. Even with SSB,
properly tailoring the IF bandwidth will make several dB difference
in the detected SNR. For example, a 2 KHz bandwidth (500 - 2500 Hz)
can provide significant improvement over a 2.8 KHz bandwidth (200 -
3000 Hz) under noisy conditions.
73,
... Joe, W4TV
On 2019-05-19 11:19 AM, Tom Azlin W7SUA wrote:
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 <[email protected]>
Date: 5/19/19 07:49 (GMT-06:00)
To: [email protected]
Subject: Re: [Elecraft] Sensitivity - Was K4 Observations
What current modes hear below the noise level?
Wes N7WS
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